How does Java strings being immutable increase security?

Question

I just learned that a String is immutable. When I was reading the reason behind it a few reasons came up, for example performance increases, or since its value cannot be modified it can be shared by multiple threads. These reasons I do understand.

But I don't get how it is related to security. How does a String being immutable help in Java security?

Answer 1

A very common practice in writing class libraries is storing the parameters passed into your API, say, in a constructor, like this:

public class MyApi {
    final String myUrl;
    public MyApi(String urlString) {
        // Verify that urlString points to an approved server
        if (!checkApprovedUrl(urlString)) throw new IllegalArgumentException();
        myUrl = urlString;
    }
}

Were String mutable, this would lead to a subtle exploit: an attacker would pass a good URL, wait for a few microseconds, and then set the URL to point to an attack site.

Since storing without copying is a reasonably common practice, and because strings are among the most commonly used data types, leaving strings mutable would open up many APIs that are not written yet open to a serious security problem. Making strings immutable closes this particular security hole for all APIs, including the ones that are not written yet.