Adobe Flash - 'IExternalizable.writeExternal' Type Confusion








If IExternalizable.writeExternal is overridden with a value that is not a function, Flash assumes it is a function even though it is not one. This leads to execution of a 'method' outside of the ActionScript object's ActionScript vtable, leading to memory corruption.

A sample swf is attached. ActionScript code is also attached, but it does not compile to the needed to swf. To get the PoC, decompress the swf using flasm -x myswf, and then search for "triteExternal" and change it to "writeExternal".

This bug is in the AVM serializer (, and is type confusion when calling the method writeExternal, which is implemented when a class extends IExternalizable ( The method is resolved on line 1437 of AvmSerializer.cpp by calling toplevel->getBinding, which does not guarantee that the binding is a method binding. It then gets cast to a method on line 773 and called, which is type confusion.

One challenge with the bug is actually creating a SWF which can hit this code, as usually overriding a defined method will lead to an illegal override exception. The 0-day author did this differently than I did. The code where all class properties (methods, internal classes, variables, etc.) are resolved is in You can see on line 813 that a check that no two properties of a class have the same name is commented out due to some legitimate SWFs doing that. This means that a SWF can have a variable with the same name as a method (overriding a method with less restrictive method is still illegal), which is how my PoC overrode the method. The 0-day did something slightly different, it put the redefinition of writeExternal in a different public namespace than the original definition of writeExternal. This has the benefit that the ActionScript will compile and hit the bug without modification. 

Proof of Concept: