| Task | Crypto Machine |
|---|---|
| Competition | CONFidence CTF 2014 |
| Location | Krakow, Poland |
| Category | Software exploitation |
| Platform | Linux x64 |
| Scoring | 200 pts (medium difficulty) |
| Number of solves | 0 out of 11 participants |
The crypto machine will hash for you, but will it drop the flag?
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- Exploit the
abs()related logic by specifying operation index0x8000000000000000to get access to the special HMAC functionality (operation #0). - Send only 2 bytes of the input key length (instead of the full 4), which results in bypassing the sanity check of the variable.
- Perform a partial, 2-byte overwrite of the
EVP_md5function pointer on the stack, to redirect it to asystem("/bin/sh")call. Because we haven't defeated ASLR and the most significant nibble (4 bits) of the overwritten memory is randomized, it is necessary to run the exploit in a loop, hoping for it to succeed after a few (or a few dozen) attempts at most.