What lies at fs:[0x0] on windows?

Question

The TEB on 32-bit Windows is located at fs:[0x0018]. What exactly is found in those 24 bytes between fs:0 and fs:0x18? (Yes, I know this undocumented and subject to change, but it'd be interesting to know...)

Answer 1

It is start of Thread Information Block at FS:[0]. The very first field of this structure is Current Structured Exception Handling (SEH) frame.

Thus, at FS:[0] is the pointer to ExceptionList

Source

It is pointer to Exception Callback Functions linked list head.

BTW it is pretty documented and everything from FS:[0] to FS:[0x1C] (excluding) is not part of change, it is very basic structure, MS would not change it in NT OSes.