Hi,
if I understand correctly, you are supposing that method changeAmount()
is part of a security sensitive application under attack.
Its implementation first attempts a data validation by invoking
isUserIdAllowedOrThrowException(), then transforms validated data in a
lossy way (narrowing, then widening) for further usage (invoking
doChangeAmount()).
A seriously conducted review of security related code, whether by
experienced engineers or by means of tools, would reject this code as
fatally flawed in the first place, code that should never become productive.
(BTW, (1L << Integer.SIZE) + 63 is a more efficient way to produce e
rebound of 63 for your example.)
Yours is just one example of the more general class of integer overflow
attacks that potentially affects modular integer arithmetic in Java and
many other languages. To prevent such attacks, operations on integer
values should throw on overflow. Modifying the Java spec to mandate
throwing on overflow would be a substantial change with major
compatibility issues and preventing many legitimate usages of modular
arithmetic as it is currently specified.
Security sensitive code can (and does) make good use of
j.l.Math.*Exact() methods (addExact(), toIntExact(), etc.)
Greetings
Raffaello
On 2021-12-29 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
>