Why is char[] preferred over String for passwords?

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.