Sometimes people ask, “So I know how to get my return address [use the _ReturnAddress() intrinsic]; how do I figure out what DLL that return address belongs to?”
Even if you figure out which DLL the return address belongs to
A common trick is to search through a “trusted” DLL for some code bytes that coincidentally match ones you (the attacker) want to execute. This can be something as simple as a “retd” instruction, which are quite abundant. The attacker then builds a stack frame that looks like this, for, say, a function that takes two parameters.
trusted_retd hacked parameter 1 hacked parameter 2 hacker_code_addr
After building this stack frame, the attacker then jumps to the start of the function being attacked.
The function being attacked looks
at the return address and sees
which resides in a trusted DLL. It then foolishly trusts the
caller and allows some unsafe operation to occur, using
hacked parameters 1 and 2. The function being attacked then
does a “retd 8″ to return and clean the parameters.
This transfers control to the
which performs a simple
retd, which now gives
control to the
hacker_code_addr, and the hacker
can use the result to continue his nefarious work.
This is why you should be concerned if somebody says, “This code verifies that its caller is trusted…” How do they know who the caller really is?