Type narrowing security leak

[Rob Spoor]

This isn't a security leak in Java (because that would mean it would be 
a security leak in any language that supports narrowing). This is a 
security leak in the application that does the narrowing. Developers 
should be aware that narrowing can change values. And furthermore, I 
don't think there are many developers that would perform this bounds 
check before the narrowing. Well, at least I wouldn't.

The fix you suggest could break many, many applications that use 
narrowing that actually *want* the value to be changed. An example is 
reading input streams byte-by-byte: the result is an int between 0 and 
255 (inclusive), or -1 for EOF. If the result is not -1, the result is 
almost always cast to a byte. The suggested fix would break for all 
values between 128 and 255 (inclusive) - half the available values.

I would rather have developers fix this in their code. There's already a 
method for "checked narrowing" from long to int: 
https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/Math.html#toIntExact(long).


Rob


On 29/12/2021 07:43, Fabrice Tiercelin wrote:
> Greetings,
> Any Java application may be concerned by a hacker attack using a type narrowing leak. If a program does the following things in this order:
>   - Assert that a numerical id is allowed - Do a type narrowing among other things, even followed by a type widening - Do an action with the numerical id
> ...the hacker can do forbidden actions. Let's say that a given user doesn't have rights to change an amount for the id 63:
> public void changeAmount(long userId, double newAmount) throws IllegalArgumentException {  isUserIdAllowedOrThrowException(userId); // userId = 4294967359
> 
>     int theUserId = (int) userId; // theUserId = 63    userId = theUserId; // userId = 63
>    doChangeAmount(userId, newAmount); // userId = 63
> }
> It will fail passing 63 but it will success passing 4294967359 because 4_294_967_359 is narrowed into 63. Let's call 4_294_967_359  a rebound of 63. 4294967359 can be retrieved in few seconds by a basic program like this:
> public class MyClass {
>      public static void main(String args[]) {
>        long targettedNumber = 63;
>        
>         for (long rebound = Integer.MAX_VALUE + 1; true; rebound++) {          int typeNarrowing = (int) rebound;
>            long typeWidening = typeNarrowing;
>            
>            if (typeWidening == targettedNumber) {
>                System.out.println("Rebound for " + targettedNumber + " found: " + rebound);
>                return;
>            }
>        }
>      }
> }
> And it can be optimized. It works for any type narrowing. It not only works for numerical id but also for flags. If a numerical value should contain or not several flags, you can search a rebound among billions of rebounds until you find one with the perfect features. All the Java versions are concerned. The security layer can even be coded in another programming language.
> 
> To fix it, I suggest to add a test just before the type narrowing in the bytecode. The test verifies if the type narrowing will alter the numerical value. If true, it throws an unchecked exception or an error. Otherwise, it continues as currently. Note that it changes the behavior but the current behavior is dangerous, useless and is a failing case. You can add a compiler option to restore the original behavior but the new behavior should be the default.
> 
> Best regards,Fabrice TIERCELIN
>