Unusual way of using java constructor

Unusual way of using java constructor - java

At some part of the code that I've been working on I am tempted to use the constructor in an unusual way. The goal is to have minimal to none sensitive values inside string pool.
public class MyClass{
private String mySafeString;
public MyClass(char[] aCharArray, String salt) {
this.mySafeString = Encrypt.encrypt(new String(aCharArray), salt);
}
public String getMySafeString(){
return this.mySafeString;
}
}
...
public static void main(String[] args){
String salt = getSaltFromSomewhere();
char[] someKey = new char[]{'c', 'x'};
var myObject = new MyClass(someKey, salt);
var encryptedSafeString = myObject.getMySafeString();
}
By creating the String object like that I am avoiding the value's presence inside the string pool. Thereby, intending to leave it to GC to perish the object.
However, I fail to find any resource that forbids the usage of constructor like the way I used it. I am keen to know if it is a bad practice in any way?
Could you please be so kind and advise me on my approach?

That's a standard way to convert a char[] to a String.
You might also want to see if your encryption library can work from a char[] or InputStream, so that you can avoid creating any String.

The goal is to have minimal to none sensitive values inside string pool.
That's a non-goal. Strings only get placed in the string pool if they are either compile time constants OR if some code explicitly calls String.intern to put them in the string pool.
Your real goal is to avoid leaving sensitive information somewhere in memory ... or in a swap file ... where a bad guy may be able to retrieve it.
However, I fail to find any resource that forbids the usage of constructor like the way I used it. I am keen to know if it is a bad practice in any way?
I doubt that you will find anything that forbids this1.
However, it is not clear from the code what it achieves and how you intend to use it.
Is it your intention that the "safe" string be decryptable or not?
If Yes, you have a big problem. In order to decrypt, you need the decryption key in memory at some time. If the cipher algorithm is symmetric, that means that salt is the decryption key. But you have already put that onto the heap as a String object. (Ooops!)
And if you avoid that oops, the next problem is that the bad guy will be able to reverse engineer getSaltFromSomewhere() or the equivalent that generates the decryption key in the case of an asymmetric cipher. They may then be able to recover the key. Note that in order to successfully decrypt, your application must be able to obtain the correct key ... in memory ... to do the decryption.
If No, then it strikes me that the "safe" string could actually be replaced with a cryptographic hash of the original string. You don't necessarily need a salt for that. And if you do need one, you don't need the seed to be generated securely.
Also, it is not sufficient to just use a char[] to avoid storing the unencrypted (etc) secret in a String. The char[] is also in the heap. So it is also imperative that you overwrite the char[] when you are finished with it. A secret in a char[] or byte[] or Buffer, etc can be found just as easily as a secret in a String.
Finally, you probably should be doing a thorough risk assessment to decide whether the effort of protecting strings in the heap is actually going to be worth it. What is the chance that the bad guys can get in to capture a heap dump? If they can get in that way, can they intercept the secrets some other way; e.g. by surreptitiously replacing some of your Java app code, attaching a debugger, stealing keys out of config files, etc.
1 - Normal Java coding standards don't concern themselves with protecting secrets, and security standards don't concern themselves with Java style, etc issues.
In short, there is nothing wrong with the constructor per se, but the chances are that it doesn't solve the real problem.

I think you mean safe means that the string won't be modified outside of the class. As a result, you are making a copy.
You don't need to do this for String because strings are immutable. There's no way to modify the string outside of the class even if there's a reference to the string outside of the class.

Related

Why would it be better to store sensitive data in a char than String? [duplicate]

In Swing, the password field has a getPassword() (returns char[]) method instead of the usual getText() (returns String) method. Similarly, I have come across a suggestion not to use String to handle passwords.
Why does String pose a threat to security when it comes to passwords?
It feels inconvenient to use char[].

Strings are immutable. That means once you've created the String, if another process can dump memory, there's no way (aside from reflection) you can get rid of the data before garbage collection kicks in.
With an array, you can explicitly wipe the data after you're done with it. You can overwrite the array with anything you like, and the password won't be present anywhere in the system, even before garbage collection.
So yes, this is a security concern - but even using char[] only reduces the window of opportunity for an attacker, and it's only for this specific type of attack.
As noted in the comments, it's possible that arrays being moved by the garbage collector will leave stray copies of the data in memory. I believe this is implementation-specific - the garbage collector may clear all memory as it goes, to avoid this sort of thing. Even if it does, there's still the time during which the char[] contains the actual characters as an attack window.

While other suggestions here seem valid, there is one other good reason. With plain String you have much higher chances of accidentally printing the password to logs, monitors or some other insecure place. char[] is less vulnerable.
Consider this:
public static void main(String[] args) {
Object pw = "Password";
System.out.println("String: " + pw);
pw = "Password".toCharArray();
System.out.println("Array: " + pw);
}
Prints:
String: Password
Array: [C#5829428e

To quote an official document, the Java Cryptography Architecture guide says this about char[] vs. String passwords (about password-based encryption, but this is more generally about passwords of course):
It would seem logical to collect and store the password in an object
of type java.lang.String. However, here's the caveat: Objects of
type String are immutable, i.e., there are no methods defined that
allow you to change (overwrite) or zero out the contents of a String
after usage. This feature makes String objects unsuitable for
storing security sensitive information such as user passwords. You
should always collect and store security sensitive information in a
char array instead.
Guideline 2-2 of the Secure Coding Guidelines for the Java Programming Language, Version 4.0 also says something similar (although it is originally in the context of logging):
Guideline 2-2: Do not log highly sensitive information
Some information, such as Social Security numbers (SSNs) and
passwords, is highly sensitive. This information should not be kept
for longer than necessary nor where it may be seen, even by
administrators. For instance, it should not be sent to log files and
its presence should not be detectable through searches. Some transient
data may be kept in mutable data structures, such as char arrays, and
cleared immediately after use. Clearing data structures has reduced
effectiveness on typical Java runtime systems as objects are moved in
memory transparently to the programmer.
This guideline also has implications for implementation and use of
lower-level libraries that do not have semantic knowledge of the data
they are dealing with. As an example, a low-level string parsing
library may log the text it works on. An application may parse an SSN
with the library. This creates a situation where the SSNs are
available to administrators with access to the log files.

Character arrays (char[]) can be cleared after use by setting each character to zero and Strings not. If someone can somehow see the memory image, they can see a password in plain text if Strings are used, but if char[] is used, after purging data with 0's, the password is secure.

Some people believe that you have to overwrite the memory used to store the password once you no longer need it. This reduces the time window an attacker has to read the password from your system and completely ignores the fact that the attacker already needs enough access to hijack the JVM memory to do this. An attacker with that much access can catch your key events making this completely useless (AFAIK, so please correct me if I am wrong).
Update
Thanks to the comments I have to update my answer. Apparently there are two cases where this can add a (very) minor security improvement as it reduces the time a password could land on the hard drive. Still I think it's overkill for most use cases.
Your target system may be badly configured or you have to assume it is and you have to be paranoid about core dumps (can be valid if the systems are not managed by an administrator).
Your software has to be overly paranoid to prevent data leaks with the attacker gaining access to the hardware - using things like TrueCrypt (discontinued), VeraCrypt, or CipherShed.
If possible, disabling core dumps and the swap file would take care of both problems. However, they would require administrator rights and may reduce functionality (less memory to use) and pulling RAM from a running system would still be a valid concern.

I don't think this is a valid suggestion, but, I can at least guess at the reason.
I think the motivation is wanting to make sure that you can erase all trace of the password in memory promptly and with certainty after it is used. With a char[] you could overwrite each element of the array with a blank or something for sure. You can't edit the internal value of a String that way.
But that alone isn't a good answer; why not just make sure a reference to the char[] or String doesn't escape? Then there's no security issue. But the thing is that String objects can be intern()ed in theory and kept alive inside the constant pool. I suppose using char[] forbids this possibility.

The answer has already been given, but I'd like to share an issue that I discovered lately with Java standard libraries. While they take great care now of replacing password strings with char[] everywhere (which of course is a good thing), other security-critical data seems to be overlooked when it comes to clearing it from memory.
I'm thinking of e.g. the PrivateKey class. Consider a scenario where you would load a private RSA key from a PKCS#12 file, using it to perform some operation. Now in this case, sniffing the password alone wouldn't help you much as long as physical access to the key file is properly restricted. As an attacker, you would be much better off if you obtained the key directly instead of the password. The desired information can be leaked manifold, core dumps, a debugger session or swap files are just some examples.
And as it turns out, there is nothing that lets you clear the private information of a PrivateKey from memory, because there's no API that lets you wipe the bytes that form the corresponding information.
This is a bad situation, as this paper describes how this circumstance could be potentially exploited.
The OpenSSL library for example overwrites critical memory sections before private keys are freed. Since Java is garbage-collected, we would need explicit methods to wipe and invalidate private information for Java keys, which are to be applied immediately after using the key.

As Jon Skeet states, there is no way except by using reflection.
However, if reflection is an option for you, you can do this.
public static void main(String[] args) {
System.out.println("please enter a password");
// don't actually do this, this is an example only.
Scanner in = new Scanner(System.in);
String password = in.nextLine();
usePassword(password);
clearString(password);
System.out.println("password: '" + password + "'");
}
private static void usePassword(String password) {
}
private static void clearString(String password) {
try {
Field value = String.class.getDeclaredField("value");
value.setAccessible(true);
char[] chars = (char[]) value.get(password);
Arrays.fill(chars, '*');
} catch (Exception e) {
throw new AssertionError(e);
}
}
when run
please enter a password
hello world
password: '***********'
Note: if the String's char[] has been copied as a part of a GC cycle, there is a chance the previous copy is somewhere in memory.
This old copy wouldn't appear in a heap dump, but if you have direct access to the raw memory of the process you could see it. In general you should avoid anyone having such access.

There is nothing that char array gives you vs String unless you clean it up manually after use, and I haven't seen anyone actually doing that. So to me the preference of char[] vs String is a bit exaggerated.
Take a look at the widely used Spring Security library here and ask yourself - are Spring Security guys incompetent or char[] passwords just don't make much sense. When some nasty hacker grabs memory dumps of your RAM be sure s/he'll get all the passwords even if you use sophisticated ways to hide them.
However, Java changes all the time, and some scary features like String Deduplication feature of Java 8 might intern String objects without your knowledge. But that's a different conversation.

Edit: Coming back to this answer after a year of security research, I realize it makes the rather unfortunate implication that you would ever actually compare plaintext passwords. Please don't. Use a secure one-way hash with a salt and a reasonable number of iterations. Consider using a library: this stuff is hard to get right!
Original answer: What about the fact that String.equals() uses short-circuit evaluation, and is therefore vulnerable to a timing attack? It may be unlikely, but you could theoretically time the password comparison in order to determine the correct sequence of characters.
public boolean equals(Object anObject) {
if (this == anObject) {
return true;
}
if (anObject instanceof String) {
String anotherString = (String)anObject;
int n = value.length;
// Quits here if Strings are different lengths.
if (n == anotherString.value.length) {
char v1[] = value;
char v2[] = anotherString.value;
int i = 0;
// Quits here at first different character.
while (n-- != 0) {
if (v1[i] != v2[i])
return false;
i++;
}
return true;
}
}
return false;
}
Some more resources on timing attacks:
A Lesson In Timing Attacks
A discussion about timing attacks over on Information Security Stack Exchange
And of course, the Timing Attack Wikipedia page

Strings are immutable and cannot be altered once they have been created. Creating a password as a string will leave stray references to the password on the heap or on the String pool. Now if someone takes a heap dump of the Java process and carefully scans through he might be able to guess the passwords. Of course these non used strings will be garbage collected but that depends on when the GC kicks in.
On the other side char[] are mutable as soon as the authentication is done you can overwrite them with any character like all M's or backslashes. Now even if someone takes a heap dump he might not be able to get the passwords which are not currently in use. This gives you more control in the sense like clearing the Object content yourself vs waiting for the GC to do it.

String is immutable and it goes to the string pool. Once written, it cannot be overwritten.
char[] is an array which you should overwrite once you used the password and this is how it should be done:
char[] passw = request.getPassword().toCharArray()
if (comparePasswords(dbPassword, passw) {
allowUser = true;
cleanPassword(passw);
cleanPassword(dbPassword);
passw = null;
}
private static void cleanPassword (char[] pass) {
Arrays.fill(pass, '0');
}
One scenario where the attacker could use it is a crashdump—when the JVM crashes and generates a memory dump—you will be able to see the password.
That is not necessarily a malicious external attacker. This could be a support user that has access to the server for monitoring purposes. He/she could peek into a crashdump and find the passwords.

The short and straightforward answer would be because char[] is mutable while String objects are not.
Strings in Java are immutable objects. That is why they can't be modified once created, and therefore the only way for their contents to be removed from memory is to have them garbage collected. It will be only then when the memory freed by the object can be overwritten, and the data will be gone.
Now garbage collection in Java doesn't happen at any guaranteed interval. The String can thus persist in memory for a long time, and if a process crashes during this time, the contents of the string may end up in a memory dump or some log.
With a character array, you can read the password, finish working with it as soon as you can, and then immediately change the contents.

Case String:
String password = "ill stay in StringPool after Death !!!";
// some long code goes
// ...Now I want to remove traces of password
password = null;
password = "";
// above attempts wil change value of password
// but the actual password can be traced from String pool through memory dump, if not garbage collected
Case CHAR ARRAY:
char[] passArray = {'p','a','s','s','w','o','r','d'};
// some long code goes
// ...Now I want to remove traces of password
for (int i=0; i<passArray.length;i++){
passArray[i] = 'x';
}
// Now you ACTUALLY DESTROYED traces of password form memory

A string in Java is immutable. So whenever a string is created, it will remain in memory until it is garbage collected. So anyone who has access to the memory can read the value of the string.
If the value of the string is modified then it will end up creating a new string. So both the original value and the modified value stay in the memory until it is garbage collected.
With the character array, the contents of the array can be modified or erased once the purpose of the password is served. The original contents of the array will not be found in memory after it is modified and even before the garbage collection kicks in.
Because of the security concern it is better to store password as a character array.

It is debatable as to whether you should use String or use Char[] for this purpose because both have their advantages and disadvantages. It depends on what the user needs.
Since Strings in Java are immutable, whenever some tries to manipulate your string it creates a new Object and the existing String remains unaffected. This could be seen as an advantage for storing a password as a String, but the object remains in memory even after use. So if anyone somehow got the memory location of the object, that person can easily trace your password stored at that location.
Char[] is mutable, but it has the advantage that after its usage the programmer can explicitly clean the array or override values. So when it's done being used it is cleaned and no one could ever know about the information you had stored.
Based on the above circumstances, one can get an idea whether to go with String or to go with Char[] for their requirements.

A lot of the previous answers are great. There is another point which I am assuming (please correct me if I am wrong).
By default Java uses UTF-16 for storing strings. Using character arrays, char[] array, facilitates use of Unicode, regional characters, etc. This technique allows all character set to be respected equally for storing the passwords and henceforth will not initiate certain crypto issues due to character set confusion. Finally, using the character array, we can convert the password array to our desired character set string.

Secure way to perform basic authorization [duplicate]

In Swing, the password field has a getPassword() (returns char[]) method instead of the usual getText() (returns String) method. Similarly, I have come across a suggestion not to use String to handle passwords.
Why does String pose a threat to security when it comes to passwords?
It feels inconvenient to use char[].

Strings are immutable. That means once you've created the String, if another process can dump memory, there's no way (aside from reflection) you can get rid of the data before garbage collection kicks in.
With an array, you can explicitly wipe the data after you're done with it. You can overwrite the array with anything you like, and the password won't be present anywhere in the system, even before garbage collection.
So yes, this is a security concern - but even using char[] only reduces the window of opportunity for an attacker, and it's only for this specific type of attack.
As noted in the comments, it's possible that arrays being moved by the garbage collector will leave stray copies of the data in memory. I believe this is implementation-specific - the garbage collector may clear all memory as it goes, to avoid this sort of thing. Even if it does, there's still the time during which the char[] contains the actual characters as an attack window.

While other suggestions here seem valid, there is one other good reason. With plain String you have much higher chances of accidentally printing the password to logs, monitors or some other insecure place. char[] is less vulnerable.
Consider this:
public static void main(String[] args) {
Object pw = "Password";
System.out.println("String: " + pw);
pw = "Password".toCharArray();
System.out.println("Array: " + pw);
}
Prints:
String: Password
Array: [C#5829428e

Character arrays (char[]) can be cleared after use by setting each character to zero and Strings not. If someone can somehow see the memory image, they can see a password in plain text if Strings are used, but if char[] is used, after purging data with 0's, the password is secure.

Some people believe that you have to overwrite the memory used to store the password once you no longer need it. This reduces the time window an attacker has to read the password from your system and completely ignores the fact that the attacker already needs enough access to hijack the JVM memory to do this. An attacker with that much access can catch your key events making this completely useless (AFAIK, so please correct me if I am wrong).
Update
Thanks to the comments I have to update my answer. Apparently there are two cases where this can add a (very) minor security improvement as it reduces the time a password could land on the hard drive. Still I think it's overkill for most use cases.
Your target system may be badly configured or you have to assume it is and you have to be paranoid about core dumps (can be valid if the systems are not managed by an administrator).
Your software has to be overly paranoid to prevent data leaks with the attacker gaining access to the hardware - using things like TrueCrypt (discontinued), VeraCrypt, or CipherShed.
If possible, disabling core dumps and the swap file would take care of both problems. However, they would require administrator rights and may reduce functionality (less memory to use) and pulling RAM from a running system would still be a valid concern.

I don't think this is a valid suggestion, but, I can at least guess at the reason.
I think the motivation is wanting to make sure that you can erase all trace of the password in memory promptly and with certainty after it is used. With a char[] you could overwrite each element of the array with a blank or something for sure. You can't edit the internal value of a String that way.
But that alone isn't a good answer; why not just make sure a reference to the char[] or String doesn't escape? Then there's no security issue. But the thing is that String objects can be intern()ed in theory and kept alive inside the constant pool. I suppose using char[] forbids this possibility.

The answer has already been given, but I'd like to share an issue that I discovered lately with Java standard libraries. While they take great care now of replacing password strings with char[] everywhere (which of course is a good thing), other security-critical data seems to be overlooked when it comes to clearing it from memory.
I'm thinking of e.g. the PrivateKey class. Consider a scenario where you would load a private RSA key from a PKCS#12 file, using it to perform some operation. Now in this case, sniffing the password alone wouldn't help you much as long as physical access to the key file is properly restricted. As an attacker, you would be much better off if you obtained the key directly instead of the password. The desired information can be leaked manifold, core dumps, a debugger session or swap files are just some examples.
And as it turns out, there is nothing that lets you clear the private information of a PrivateKey from memory, because there's no API that lets you wipe the bytes that form the corresponding information.
This is a bad situation, as this paper describes how this circumstance could be potentially exploited.
The OpenSSL library for example overwrites critical memory sections before private keys are freed. Since Java is garbage-collected, we would need explicit methods to wipe and invalidate private information for Java keys, which are to be applied immediately after using the key.

As Jon Skeet states, there is no way except by using reflection.
However, if reflection is an option for you, you can do this.
public static void main(String[] args) {
System.out.println("please enter a password");
// don't actually do this, this is an example only.
Scanner in = new Scanner(System.in);
String password = in.nextLine();
usePassword(password);
clearString(password);
System.out.println("password: '" + password + "'");
}
private static void usePassword(String password) {
}
private static void clearString(String password) {
try {
Field value = String.class.getDeclaredField("value");
value.setAccessible(true);
char[] chars = (char[]) value.get(password);
Arrays.fill(chars, '*');
} catch (Exception e) {
throw new AssertionError(e);
}
}
when run
please enter a password
hello world
password: '***********'
Note: if the String's char[] has been copied as a part of a GC cycle, there is a chance the previous copy is somewhere in memory.
This old copy wouldn't appear in a heap dump, but if you have direct access to the raw memory of the process you could see it. In general you should avoid anyone having such access.

There is nothing that char array gives you vs String unless you clean it up manually after use, and I haven't seen anyone actually doing that. So to me the preference of char[] vs String is a bit exaggerated.
Take a look at the widely used Spring Security library here and ask yourself - are Spring Security guys incompetent or char[] passwords just don't make much sense. When some nasty hacker grabs memory dumps of your RAM be sure s/he'll get all the passwords even if you use sophisticated ways to hide them.
However, Java changes all the time, and some scary features like String Deduplication feature of Java 8 might intern String objects without your knowledge. But that's a different conversation.

Edit: Coming back to this answer after a year of security research, I realize it makes the rather unfortunate implication that you would ever actually compare plaintext passwords. Please don't. Use a secure one-way hash with a salt and a reasonable number of iterations. Consider using a library: this stuff is hard to get right!
Original answer: What about the fact that String.equals() uses short-circuit evaluation, and is therefore vulnerable to a timing attack? It may be unlikely, but you could theoretically time the password comparison in order to determine the correct sequence of characters.
public boolean equals(Object anObject) {
if (this == anObject) {
return true;
}
if (anObject instanceof String) {
String anotherString = (String)anObject;
int n = value.length;
// Quits here if Strings are different lengths.
if (n == anotherString.value.length) {
char v1[] = value;
char v2[] = anotherString.value;
int i = 0;
// Quits here at first different character.
while (n-- != 0) {
if (v1[i] != v2[i])
return false;
i++;
}
return true;
}
}
return false;
}
Some more resources on timing attacks:
A Lesson In Timing Attacks
A discussion about timing attacks over on Information Security Stack Exchange
And of course, the Timing Attack Wikipedia page

Strings are immutable and cannot be altered once they have been created. Creating a password as a string will leave stray references to the password on the heap or on the String pool. Now if someone takes a heap dump of the Java process and carefully scans through he might be able to guess the passwords. Of course these non used strings will be garbage collected but that depends on when the GC kicks in.
On the other side char[] are mutable as soon as the authentication is done you can overwrite them with any character like all M's or backslashes. Now even if someone takes a heap dump he might not be able to get the passwords which are not currently in use. This gives you more control in the sense like clearing the Object content yourself vs waiting for the GC to do it.

String is immutable and it goes to the string pool. Once written, it cannot be overwritten.
char[] is an array which you should overwrite once you used the password and this is how it should be done:
char[] passw = request.getPassword().toCharArray()
if (comparePasswords(dbPassword, passw) {
allowUser = true;
cleanPassword(passw);
cleanPassword(dbPassword);
passw = null;
}
private static void cleanPassword (char[] pass) {
Arrays.fill(pass, '0');
}
One scenario where the attacker could use it is a crashdump—when the JVM crashes and generates a memory dump—you will be able to see the password.
That is not necessarily a malicious external attacker. This could be a support user that has access to the server for monitoring purposes. He/she could peek into a crashdump and find the passwords.

The short and straightforward answer would be because char[] is mutable while String objects are not.
Strings in Java are immutable objects. That is why they can't be modified once created, and therefore the only way for their contents to be removed from memory is to have them garbage collected. It will be only then when the memory freed by the object can be overwritten, and the data will be gone.
Now garbage collection in Java doesn't happen at any guaranteed interval. The String can thus persist in memory for a long time, and if a process crashes during this time, the contents of the string may end up in a memory dump or some log.
With a character array, you can read the password, finish working with it as soon as you can, and then immediately change the contents.

Case String:
String password = "ill stay in StringPool after Death !!!";
// some long code goes
// ...Now I want to remove traces of password
password = null;
password = "";
// above attempts wil change value of password
// but the actual password can be traced from String pool through memory dump, if not garbage collected
Case CHAR ARRAY:
char[] passArray = {'p','a','s','s','w','o','r','d'};
// some long code goes
// ...Now I want to remove traces of password
for (int i=0; i<passArray.length;i++){
passArray[i] = 'x';
}
// Now you ACTUALLY DESTROYED traces of password form memory

A string in Java is immutable. So whenever a string is created, it will remain in memory until it is garbage collected. So anyone who has access to the memory can read the value of the string.
If the value of the string is modified then it will end up creating a new string. So both the original value and the modified value stay in the memory until it is garbage collected.
With the character array, the contents of the array can be modified or erased once the purpose of the password is served. The original contents of the array will not be found in memory after it is modified and even before the garbage collection kicks in.
Because of the security concern it is better to store password as a character array.

It is debatable as to whether you should use String or use Char[] for this purpose because both have their advantages and disadvantages. It depends on what the user needs.
Since Strings in Java are immutable, whenever some tries to manipulate your string it creates a new Object and the existing String remains unaffected. This could be seen as an advantage for storing a password as a String, but the object remains in memory even after use. So if anyone somehow got the memory location of the object, that person can easily trace your password stored at that location.
Char[] is mutable, but it has the advantage that after its usage the programmer can explicitly clean the array or override values. So when it's done being used it is cleaned and no one could ever know about the information you had stored.
Based on the above circumstances, one can get an idea whether to go with String or to go with Char[] for their requirements.

A lot of the previous answers are great. There is another point which I am assuming (please correct me if I am wrong).
By default Java uses UTF-16 for storing strings. Using character arrays, char[] array, facilitates use of Unicode, regional characters, etc. This technique allows all character set to be respected equally for storing the passwords and henceforth will not initiate certain crypto issues due to character set confusion. Finally, using the character array, we can convert the password array to our desired character set string.

Java Fortify issue For Storing password in String object [duplicate]

In Swing, the password field has a getPassword() (returns char[]) method instead of the usual getText() (returns String) method. Similarly, I have come across a suggestion not to use String to handle passwords.
Why does String pose a threat to security when it comes to passwords?
It feels inconvenient to use char[].

Strings are immutable. That means once you've created the String, if another process can dump memory, there's no way (aside from reflection) you can get rid of the data before garbage collection kicks in.
With an array, you can explicitly wipe the data after you're done with it. You can overwrite the array with anything you like, and the password won't be present anywhere in the system, even before garbage collection.
So yes, this is a security concern - but even using char[] only reduces the window of opportunity for an attacker, and it's only for this specific type of attack.
As noted in the comments, it's possible that arrays being moved by the garbage collector will leave stray copies of the data in memory. I believe this is implementation-specific - the garbage collector may clear all memory as it goes, to avoid this sort of thing. Even if it does, there's still the time during which the char[] contains the actual characters as an attack window.

While other suggestions here seem valid, there is one other good reason. With plain String you have much higher chances of accidentally printing the password to logs, monitors or some other insecure place. char[] is less vulnerable.
Consider this:
public static void main(String[] args) {
Object pw = "Password";
System.out.println("String: " + pw);
pw = "Password".toCharArray();
System.out.println("Array: " + pw);
}
Prints:
String: Password
Array: [C#5829428e

Character arrays (char[]) can be cleared after use by setting each character to zero and Strings not. If someone can somehow see the memory image, they can see a password in plain text if Strings are used, but if char[] is used, after purging data with 0's, the password is secure.

Some people believe that you have to overwrite the memory used to store the password once you no longer need it. This reduces the time window an attacker has to read the password from your system and completely ignores the fact that the attacker already needs enough access to hijack the JVM memory to do this. An attacker with that much access can catch your key events making this completely useless (AFAIK, so please correct me if I am wrong).
Update
Thanks to the comments I have to update my answer. Apparently there are two cases where this can add a (very) minor security improvement as it reduces the time a password could land on the hard drive. Still I think it's overkill for most use cases.
Your target system may be badly configured or you have to assume it is and you have to be paranoid about core dumps (can be valid if the systems are not managed by an administrator).
Your software has to be overly paranoid to prevent data leaks with the attacker gaining access to the hardware - using things like TrueCrypt (discontinued), VeraCrypt, or CipherShed.
If possible, disabling core dumps and the swap file would take care of both problems. However, they would require administrator rights and may reduce functionality (less memory to use) and pulling RAM from a running system would still be a valid concern.

I don't think this is a valid suggestion, but, I can at least guess at the reason.
I think the motivation is wanting to make sure that you can erase all trace of the password in memory promptly and with certainty after it is used. With a char[] you could overwrite each element of the array with a blank or something for sure. You can't edit the internal value of a String that way.
But that alone isn't a good answer; why not just make sure a reference to the char[] or String doesn't escape? Then there's no security issue. But the thing is that String objects can be intern()ed in theory and kept alive inside the constant pool. I suppose using char[] forbids this possibility.

The answer has already been given, but I'd like to share an issue that I discovered lately with Java standard libraries. While they take great care now of replacing password strings with char[] everywhere (which of course is a good thing), other security-critical data seems to be overlooked when it comes to clearing it from memory.
I'm thinking of e.g. the PrivateKey class. Consider a scenario where you would load a private RSA key from a PKCS#12 file, using it to perform some operation. Now in this case, sniffing the password alone wouldn't help you much as long as physical access to the key file is properly restricted. As an attacker, you would be much better off if you obtained the key directly instead of the password. The desired information can be leaked manifold, core dumps, a debugger session or swap files are just some examples.
And as it turns out, there is nothing that lets you clear the private information of a PrivateKey from memory, because there's no API that lets you wipe the bytes that form the corresponding information.
This is a bad situation, as this paper describes how this circumstance could be potentially exploited.
The OpenSSL library for example overwrites critical memory sections before private keys are freed. Since Java is garbage-collected, we would need explicit methods to wipe and invalidate private information for Java keys, which are to be applied immediately after using the key.

As Jon Skeet states, there is no way except by using reflection.
However, if reflection is an option for you, you can do this.
public static void main(String[] args) {
System.out.println("please enter a password");
// don't actually do this, this is an example only.
Scanner in = new Scanner(System.in);
String password = in.nextLine();
usePassword(password);
clearString(password);
System.out.println("password: '" + password + "'");
}
private static void usePassword(String password) {
}
private static void clearString(String password) {
try {
Field value = String.class.getDeclaredField("value");
value.setAccessible(true);
char[] chars = (char[]) value.get(password);
Arrays.fill(chars, '*');
} catch (Exception e) {
throw new AssertionError(e);
}
}
when run
please enter a password
hello world
password: '***********'
Note: if the String's char[] has been copied as a part of a GC cycle, there is a chance the previous copy is somewhere in memory.
This old copy wouldn't appear in a heap dump, but if you have direct access to the raw memory of the process you could see it. In general you should avoid anyone having such access.

There is nothing that char array gives you vs String unless you clean it up manually after use, and I haven't seen anyone actually doing that. So to me the preference of char[] vs String is a bit exaggerated.
Take a look at the widely used Spring Security library here and ask yourself - are Spring Security guys incompetent or char[] passwords just don't make much sense. When some nasty hacker grabs memory dumps of your RAM be sure s/he'll get all the passwords even if you use sophisticated ways to hide them.
However, Java changes all the time, and some scary features like String Deduplication feature of Java 8 might intern String objects without your knowledge. But that's a different conversation.

Edit: Coming back to this answer after a year of security research, I realize it makes the rather unfortunate implication that you would ever actually compare plaintext passwords. Please don't. Use a secure one-way hash with a salt and a reasonable number of iterations. Consider using a library: this stuff is hard to get right!
Original answer: What about the fact that String.equals() uses short-circuit evaluation, and is therefore vulnerable to a timing attack? It may be unlikely, but you could theoretically time the password comparison in order to determine the correct sequence of characters.
public boolean equals(Object anObject) {
if (this == anObject) {
return true;
}
if (anObject instanceof String) {
String anotherString = (String)anObject;
int n = value.length;
// Quits here if Strings are different lengths.
if (n == anotherString.value.length) {
char v1[] = value;
char v2[] = anotherString.value;
int i = 0;
// Quits here at first different character.
while (n-- != 0) {
if (v1[i] != v2[i])
return false;
i++;
}
return true;
}
}
return false;
}
Some more resources on timing attacks:
A Lesson In Timing Attacks
A discussion about timing attacks over on Information Security Stack Exchange
And of course, the Timing Attack Wikipedia page

Strings are immutable and cannot be altered once they have been created. Creating a password as a string will leave stray references to the password on the heap or on the String pool. Now if someone takes a heap dump of the Java process and carefully scans through he might be able to guess the passwords. Of course these non used strings will be garbage collected but that depends on when the GC kicks in.
On the other side char[] are mutable as soon as the authentication is done you can overwrite them with any character like all M's or backslashes. Now even if someone takes a heap dump he might not be able to get the passwords which are not currently in use. This gives you more control in the sense like clearing the Object content yourself vs waiting for the GC to do it.

String is immutable and it goes to the string pool. Once written, it cannot be overwritten.
char[] is an array which you should overwrite once you used the password and this is how it should be done:
char[] passw = request.getPassword().toCharArray()
if (comparePasswords(dbPassword, passw) {
allowUser = true;
cleanPassword(passw);
cleanPassword(dbPassword);
passw = null;
}
private static void cleanPassword (char[] pass) {
Arrays.fill(pass, '0');
}
One scenario where the attacker could use it is a crashdump—when the JVM crashes and generates a memory dump—you will be able to see the password.
That is not necessarily a malicious external attacker. This could be a support user that has access to the server for monitoring purposes. He/she could peek into a crashdump and find the passwords.

The short and straightforward answer would be because char[] is mutable while String objects are not.
Strings in Java are immutable objects. That is why they can't be modified once created, and therefore the only way for their contents to be removed from memory is to have them garbage collected. It will be only then when the memory freed by the object can be overwritten, and the data will be gone.
Now garbage collection in Java doesn't happen at any guaranteed interval. The String can thus persist in memory for a long time, and if a process crashes during this time, the contents of the string may end up in a memory dump or some log.
With a character array, you can read the password, finish working with it as soon as you can, and then immediately change the contents.

Case String:
String password = "ill stay in StringPool after Death !!!";
// some long code goes
// ...Now I want to remove traces of password
password = null;
password = "";
// above attempts wil change value of password
// but the actual password can be traced from String pool through memory dump, if not garbage collected
Case CHAR ARRAY:
char[] passArray = {'p','a','s','s','w','o','r','d'};
// some long code goes
// ...Now I want to remove traces of password
for (int i=0; i<passArray.length;i++){
passArray[i] = 'x';
}
// Now you ACTUALLY DESTROYED traces of password form memory

A string in Java is immutable. So whenever a string is created, it will remain in memory until it is garbage collected. So anyone who has access to the memory can read the value of the string.
If the value of the string is modified then it will end up creating a new string. So both the original value and the modified value stay in the memory until it is garbage collected.
With the character array, the contents of the array can be modified or erased once the purpose of the password is served. The original contents of the array will not be found in memory after it is modified and even before the garbage collection kicks in.
Because of the security concern it is better to store password as a character array.

It is debatable as to whether you should use String or use Char[] for this purpose because both have their advantages and disadvantages. It depends on what the user needs.
Since Strings in Java are immutable, whenever some tries to manipulate your string it creates a new Object and the existing String remains unaffected. This could be seen as an advantage for storing a password as a String, but the object remains in memory even after use. So if anyone somehow got the memory location of the object, that person can easily trace your password stored at that location.
Char[] is mutable, but it has the advantage that after its usage the programmer can explicitly clean the array or override values. So when it's done being used it is cleaned and no one could ever know about the information you had stored.
Based on the above circumstances, one can get an idea whether to go with String or to go with Char[] for their requirements.

A lot of the previous answers are great. There is another point which I am assuming (please correct me if I am wrong).
By default Java uses UTF-16 for storing strings. Using character arrays, char[] array, facilitates use of Unicode, regional characters, etc. This technique allows all character set to be respected equally for storing the passwords and henceforth will not initiate certain crypto issues due to character set confusion. Finally, using the character array, we can convert the password array to our desired character set string.

Safely using String for passwords by using reflection to scrub contents prior to garbage collection

Does using reflection to scrub a String make using String as safe as using char[] for passwords?
From a security aspect, it is generally considered best practice to use char[] for storing/passing passwords, because one can zero-out its contents as soon as possible in code, which may be significantly before garbage collection cleans it up and the memory is reused (wiping all trace), limiting the window of time for a memory attack.
However, char[] is not as convenient as String, so it would be handy if one could "scrub" a String if needed, thus making String as safe as char[].
Below is a method that uses reflection to zero-out the fields of String.
Is this method "OK", and does it achieve the goal of making String as safe as char[] for passwords?
public static void scrub(String str) throws NoSuchFieldException, IllegalAccessException {
Field valueField = String.class.getDeclaredField("value");
Field offsetField = String.class.getDeclaredField("offset");
Field countField = String.class.getDeclaredField("count");
Field hashField = String.class.getDeclaredField("hash");
valueField.setAccessible(true);
offsetField.setAccessible(true);
countField.setAccessible(true);
hashField.setAccessible(true);
char[] value = (char[]) valueField.get(str);
// overwrite the relevant array contents with null chars
Arrays.fill(value, offsetField.getInt(str), countField.getInt(str), '\0');
countField.set(str, 0); // scrub password length too
hashField.set(str, 0); // the hash could be used to crack a password
valueField.setAccessible(false);
offsetField.setAccessible(false);
countField.setAccessible(false);
hashField.setAccessible(false);
}
Here's a simple test:
String str = "password";
scrub(str);
System.out.println('"' + str + '"');
Output:
""
Note: You may assume that passwords are not String constants and thus calling this method will have no adverse effect on interned Strings.
Also, I have left the method is a fairly "raw" state for simplicity's sake. If I were to use it, I would not declare exceptions thrown (try/catch/ignoring them) and refactor repeated code.

There are two potential safety concerns:
The String may share its backing array with other Strings; e.g. if the String was created by calling substring on a larger String. So when you zero the entire value array you could be overwriting the state of other strings ... that don't contain passwords.
The cure is to only zero the part of the backing array that is used by the password string.
The JLS (17.5.3) warns that the effects of using reflection to change final variables is undefined.
However, the context for this is the Java Memory Model, and the fact that the compiler is allowed to aggressively cache final variables. In this case:
you would expect the String to be thread-confined, and
you shouldn't be using any of those variables again.
I wouldn't expect either of these to be real problems ... modulo fixing the over-aggressive zeroing of value.
But the real concern is Velociraptors. :-)
I'm puzzled that you would actually bothering to zap passwords like this. When you think about it, what you are protecting against is the possibility that someone can read process memory ... or a core dump or swap file ... to retrieve passwords. But if someone can do that, your system security has to have already been compromised ... cos' those things most likely require root access (or equivalent). And if they have root access they can "debug" your program and catch the passwords before your application zaps them.

One argument I have against String is that it's just too easy to inadvertently make a copy. Using strings safely is possible in theory, but the whole library ecosystem is based on the assumption that it's perfectly OK to copy strings. In the end, considering all the restrictions, strings may not be as convenient for this use case as they generally are.

Why is char[] preferred over String for passwords?

In Swing, the password field has a getPassword() (returns char[]) method instead of the usual getText() (returns String) method. Similarly, I have come across a suggestion not to use String to handle passwords.
Why does String pose a threat to security when it comes to passwords?
It feels inconvenient to use char[].

Strings are immutable. That means once you've created the String, if another process can dump memory, there's no way (aside from reflection) you can get rid of the data before garbage collection kicks in.
With an array, you can explicitly wipe the data after you're done with it. You can overwrite the array with anything you like, and the password won't be present anywhere in the system, even before garbage collection.
So yes, this is a security concern - but even using char[] only reduces the window of opportunity for an attacker, and it's only for this specific type of attack.
As noted in the comments, it's possible that arrays being moved by the garbage collector will leave stray copies of the data in memory. I believe this is implementation-specific - the garbage collector may clear all memory as it goes, to avoid this sort of thing. Even if it does, there's still the time during which the char[] contains the actual characters as an attack window.

While other suggestions here seem valid, there is one other good reason. With plain String you have much higher chances of accidentally printing the password to logs, monitors or some other insecure place. char[] is less vulnerable.
Consider this:
public static void main(String[] args) {
Object pw = "Password";
System.out.println("String: " + pw);
pw = "Password".toCharArray();
System.out.println("Array: " + pw);
}
Prints:
String: Password
Array: [C#5829428e

Character arrays (char[]) can be cleared after use by setting each character to zero and Strings not. If someone can somehow see the memory image, they can see a password in plain text if Strings are used, but if char[] is used, after purging data with 0's, the password is secure.

Some people believe that you have to overwrite the memory used to store the password once you no longer need it. This reduces the time window an attacker has to read the password from your system and completely ignores the fact that the attacker already needs enough access to hijack the JVM memory to do this. An attacker with that much access can catch your key events making this completely useless (AFAIK, so please correct me if I am wrong).
Update
Thanks to the comments I have to update my answer. Apparently there are two cases where this can add a (very) minor security improvement as it reduces the time a password could land on the hard drive. Still I think it's overkill for most use cases.
Your target system may be badly configured or you have to assume it is and you have to be paranoid about core dumps (can be valid if the systems are not managed by an administrator).
Your software has to be overly paranoid to prevent data leaks with the attacker gaining access to the hardware - using things like TrueCrypt (discontinued), VeraCrypt, or CipherShed.
If possible, disabling core dumps and the swap file would take care of both problems. However, they would require administrator rights and may reduce functionality (less memory to use) and pulling RAM from a running system would still be a valid concern.

I don't think this is a valid suggestion, but, I can at least guess at the reason.
I think the motivation is wanting to make sure that you can erase all trace of the password in memory promptly and with certainty after it is used. With a char[] you could overwrite each element of the array with a blank or something for sure. You can't edit the internal value of a String that way.
But that alone isn't a good answer; why not just make sure a reference to the char[] or String doesn't escape? Then there's no security issue. But the thing is that String objects can be intern()ed in theory and kept alive inside the constant pool. I suppose using char[] forbids this possibility.

The answer has already been given, but I'd like to share an issue that I discovered lately with Java standard libraries. While they take great care now of replacing password strings with char[] everywhere (which of course is a good thing), other security-critical data seems to be overlooked when it comes to clearing it from memory.
I'm thinking of e.g. the PrivateKey class. Consider a scenario where you would load a private RSA key from a PKCS#12 file, using it to perform some operation. Now in this case, sniffing the password alone wouldn't help you much as long as physical access to the key file is properly restricted. As an attacker, you would be much better off if you obtained the key directly instead of the password. The desired information can be leaked manifold, core dumps, a debugger session or swap files are just some examples.
And as it turns out, there is nothing that lets you clear the private information of a PrivateKey from memory, because there's no API that lets you wipe the bytes that form the corresponding information.
This is a bad situation, as this paper describes how this circumstance could be potentially exploited.
The OpenSSL library for example overwrites critical memory sections before private keys are freed. Since Java is garbage-collected, we would need explicit methods to wipe and invalidate private information for Java keys, which are to be applied immediately after using the key.

As Jon Skeet states, there is no way except by using reflection.
However, if reflection is an option for you, you can do this.
public static void main(String[] args) {
System.out.println("please enter a password");
// don't actually do this, this is an example only.
Scanner in = new Scanner(System.in);
String password = in.nextLine();
usePassword(password);
clearString(password);
System.out.println("password: '" + password + "'");
}
private static void usePassword(String password) {
}
private static void clearString(String password) {
try {
Field value = String.class.getDeclaredField("value");
value.setAccessible(true);
char[] chars = (char[]) value.get(password);
Arrays.fill(chars, '*');
} catch (Exception e) {
throw new AssertionError(e);
}
}
when run
please enter a password
hello world
password: '***********'
Note: if the String's char[] has been copied as a part of a GC cycle, there is a chance the previous copy is somewhere in memory.
This old copy wouldn't appear in a heap dump, but if you have direct access to the raw memory of the process you could see it. In general you should avoid anyone having such access.

There is nothing that char array gives you vs String unless you clean it up manually after use, and I haven't seen anyone actually doing that. So to me the preference of char[] vs String is a bit exaggerated.
Take a look at the widely used Spring Security library here and ask yourself - are Spring Security guys incompetent or char[] passwords just don't make much sense. When some nasty hacker grabs memory dumps of your RAM be sure s/he'll get all the passwords even if you use sophisticated ways to hide them.
However, Java changes all the time, and some scary features like String Deduplication feature of Java 8 might intern String objects without your knowledge. But that's a different conversation.

Edit: Coming back to this answer after a year of security research, I realize it makes the rather unfortunate implication that you would ever actually compare plaintext passwords. Please don't. Use a secure one-way hash with a salt and a reasonable number of iterations. Consider using a library: this stuff is hard to get right!
Original answer: What about the fact that String.equals() uses short-circuit evaluation, and is therefore vulnerable to a timing attack? It may be unlikely, but you could theoretically time the password comparison in order to determine the correct sequence of characters.
public boolean equals(Object anObject) {
if (this == anObject) {
return true;
}
if (anObject instanceof String) {
String anotherString = (String)anObject;
int n = value.length;
// Quits here if Strings are different lengths.
if (n == anotherString.value.length) {
char v1[] = value;
char v2[] = anotherString.value;
int i = 0;
// Quits here at first different character.
while (n-- != 0) {
if (v1[i] != v2[i])
return false;
i++;
}
return true;
}
}
return false;
}
Some more resources on timing attacks:
A Lesson In Timing Attacks
A discussion about timing attacks over on Information Security Stack Exchange
And of course, the Timing Attack Wikipedia page

Strings are immutable and cannot be altered once they have been created. Creating a password as a string will leave stray references to the password on the heap or on the String pool. Now if someone takes a heap dump of the Java process and carefully scans through he might be able to guess the passwords. Of course these non used strings will be garbage collected but that depends on when the GC kicks in.
On the other side char[] are mutable as soon as the authentication is done you can overwrite them with any character like all M's or backslashes. Now even if someone takes a heap dump he might not be able to get the passwords which are not currently in use. This gives you more control in the sense like clearing the Object content yourself vs waiting for the GC to do it.

String is immutable and it goes to the string pool. Once written, it cannot be overwritten.
char[] is an array which you should overwrite once you used the password and this is how it should be done:
char[] passw = request.getPassword().toCharArray()
if (comparePasswords(dbPassword, passw) {
allowUser = true;
cleanPassword(passw);
cleanPassword(dbPassword);
passw = null;
}
private static void cleanPassword (char[] pass) {
Arrays.fill(pass, '0');
}
One scenario where the attacker could use it is a crashdump—when the JVM crashes and generates a memory dump—you will be able to see the password.
That is not necessarily a malicious external attacker. This could be a support user that has access to the server for monitoring purposes. He/she could peek into a crashdump and find the passwords.

The short and straightforward answer would be because char[] is mutable while String objects are not.
Strings in Java are immutable objects. That is why they can't be modified once created, and therefore the only way for their contents to be removed from memory is to have them garbage collected. It will be only then when the memory freed by the object can be overwritten, and the data will be gone.
Now garbage collection in Java doesn't happen at any guaranteed interval. The String can thus persist in memory for a long time, and if a process crashes during this time, the contents of the string may end up in a memory dump or some log.
With a character array, you can read the password, finish working with it as soon as you can, and then immediately change the contents.

Case String:
String password = "ill stay in StringPool after Death !!!";
// some long code goes
// ...Now I want to remove traces of password
password = null;
password = "";
// above attempts wil change value of password
// but the actual password can be traced from String pool through memory dump, if not garbage collected
Case CHAR ARRAY:
char[] passArray = {'p','a','s','s','w','o','r','d'};
// some long code goes
// ...Now I want to remove traces of password
for (int i=0; i<passArray.length;i++){
passArray[i] = 'x';
}
// Now you ACTUALLY DESTROYED traces of password form memory

A string in Java is immutable. So whenever a string is created, it will remain in memory until it is garbage collected. So anyone who has access to the memory can read the value of the string.
If the value of the string is modified then it will end up creating a new string. So both the original value and the modified value stay in the memory until it is garbage collected.
With the character array, the contents of the array can be modified or erased once the purpose of the password is served. The original contents of the array will not be found in memory after it is modified and even before the garbage collection kicks in.
Because of the security concern it is better to store password as a character array.

It is debatable as to whether you should use String or use Char[] for this purpose because both have their advantages and disadvantages. It depends on what the user needs.
Since Strings in Java are immutable, whenever some tries to manipulate your string it creates a new Object and the existing String remains unaffected. This could be seen as an advantage for storing a password as a String, but the object remains in memory even after use. So if anyone somehow got the memory location of the object, that person can easily trace your password stored at that location.
Char[] is mutable, but it has the advantage that after its usage the programmer can explicitly clean the array or override values. So when it's done being used it is cleaned and no one could ever know about the information you had stored.
Based on the above circumstances, one can get an idea whether to go with String or to go with Char[] for their requirements.

A lot of the previous answers are great. There is another point which I am assuming (please correct me if I am wrong).
By default Java uses UTF-16 for storing strings. Using character arrays, char[] array, facilitates use of Unicode, regional characters, etc. This technique allows all character set to be respected equally for storing the passwords and henceforth will not initiate certain crypto issues due to character set confusion. Finally, using the character array, we can convert the password array to our desired character set string.

Develop Reference

Java is a programming language and computing platform first released by Sun Microsystems in 1995.