Detecting user-mode debuggers under Windows

The packer-unpacker game is a game that I disapprove of, however this doesn’t stop me from documenting a trick I remembered after reading the following blog post: Anti-Debugging Series – Part II.

To detect if a user mode debugger is attached to your process, do the following steps:

  1. Allocate some memory with HeapAlloc.
  2. Write something to the allocated memory
  3. Free the allocation with HeapFree.
  4. Check if the pattern you’ve written is still present at the given address. If a debugger is attached, it is overwritten with a fixed pattern when HeapFree gets called.

This is probably because attaching a debugger activates the “debug mode” in the heap allocator.

2 responses to “Detecting user-mode debuggers under Windows”

  1. Whether a debugger is attached or not, the freed memory is modified. Tested with MSCV 2005 SP1/SDK 6.1.

  2. @Anonymous: if I recall correctly I've done the test with Delphi 7 under Windows XP SP2. Maybe Visual C initializes the heap allocator in a different way.

    PS. I just tried this with MSVC 2008 under Windows 7, and it always seems to overwrite the freed region (tried it with malloc/free and HeapAlloc/HeapFree with GetProcessHeap and HeapCreate). Maybe it is a new protection scheme introduced in Vista/7. Also, it is possible that the behavior changes depending on the allocation size/pattern.

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