A security vulnerability report arrived that said that if you passed a carefully-malformed value to the Call­Window­Proc function, then it would call an unexpected function.

Recall that when you call Get­Window­Long­Ptr(GWLP_WNDPROC) and the window procedure’s character set is different from the character set of th Get­Window­Long­Ptr, then the window manager returns a magic cookie as the pretend window procedure. This magic cookie is meaningful only to the Call­Window­Proc function, and it indicates that the message parameters need to be changed from one character set to another before calling the real window procedure.

The finder wrote, “I haven’t looked into it further to see any other possible security implications.”

What are the security implications of letting people guess the magic cookies?

Nothing, really. Because you’re already on the other side of the airtight hatchway.

Which made me kind of confused by that statement about “other possible security implications,” since I couldn’t even see the first one.

Remember, when looking at a potential security issue, you have to identify who the attacker is, who the victim is, and what the attacker has gained.

One possible attacker is “the process that passed an artificial magic cookie to the Call­Window­Proc function.” But all you’re doing is attacking yourself. Even if the parameter happens to match an actual magic cookie, all you did was call a function in your own process. The WPARAM and LPARAM parameters might be transformed as part of the character set conversion, but really, what you found was a way to call a function in your own process in an extremely convoluted way.

Another attacker might be “an external entity which tricked a process into passing a crafted magic cookie to the Call­Window­Proc function.” But that means that the attacker found a way to trick a process into passing a value of its choosing to the Call­Window­Proc function. If an attacker has that much power over the process, then what’s it doing wasting its time with magic cookies? It can just trick the app into passing arbitrary function pointers to the Call­Window­Proc function! No need to limit yourself to functions that are callable via magic cookies; you can just call any function you like. In other words, the attacker gained nothing they didn’t already have.