Source: https://code.google.com/p/google-security-research/issues/detail?id=548

If IExternalizable.readExternal 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 "teadExternal" and change it to "readExternal".


Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/38969.zip