Proof of Concept or GTFO issue 0x14 is a PDF document file that can also be run as a NES ROM. The file will display its own MD5 hash in a PDF viewer, and also displays its own MD5 hash in a NES emulator (only first 40KB+16 bytes are actually loaded there)
Not only is MD5 broken as shown here, if you have a modern CPU it's also quite slow compared to good, non-broken alternatives. See for example this comparison[1] (post says JavaScript but it's actually OpenSSL's implementation that's actually tested).
honestly, normal.php is not a valid php file. i do understand that it might bypass some checks if say normal.php was somehow flagged as a valid / benign file but in all honesty that would be really bad sec product u wanna swap with something that more intelligently classifies files... additionally, most products these days also use sha1, sha2 and sometimes things like ssdeep to have multiple hash variants to check. this ensures that any collisions will be mitigated as it's not known yet to make 1 file match on all of these different types of hashes, despite collisions being possible in a number of them for sure.
if normal.php had actual php code in there, being really 'normal' as the name implies, this would be much more severe / interesting because it might be more easy to convince modern security products it's actually a benign file.
Currently if it would be analysed, it would be flagged as suspicious simply because its not a valid file. and really, it dont need to be php, it could be any valid file format as long as it's an actually file that has benign behavior or contents.
plaintext might be easier to generate, but you'd need it to be 'executable' format or something interpretable like a script to have it actually stored in databases marking files as malicious or benign. matching filetype with the malicious file, in a valid form that does actual benign behavior would be 'best'.
don't take me wrong tho. still fun to see these things and honestly props, if it bypasses anything that's always a 'nice result' :)
I made https://github.com/DavidBuchanan314/monomorph, which packs up to 4KB of shellcode into an executable that always has the same hash. So you're not just limited to a good/evil pair, you can arbitrarily change the behaviour in future without changing the hash.
Also, a more recent innovation in MD5 collisions is textcoll, which creates colliding blocks that are completely plaintext. This would allow for colliding PHP source files like in OP but without any obvious binary artefacts (although this requires identical prefixes).
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[ 82.3 ms ] story [ 673 ms ] threadhttps://github.com/angea/pocorgtfo#0x14
And yes, documents are not normally supposed to be able to display their own MD5 hash.
[1]: https://lemire.me/blog/2025/01/11/javascript-hashing-speed-c...
if normal.php had actual php code in there, being really 'normal' as the name implies, this would be much more severe / interesting because it might be more easy to convince modern security products it's actually a benign file.
Currently if it would be analysed, it would be flagged as suspicious simply because its not a valid file. and really, it dont need to be php, it could be any valid file format as long as it's an actually file that has benign behavior or contents.
plaintext might be easier to generate, but you'd need it to be 'executable' format or something interpretable like a script to have it actually stored in databases marking files as malicious or benign. matching filetype with the malicious file, in a valid form that does actual benign behavior would be 'best'.
don't take me wrong tho. still fun to see these things and honestly props, if it bypasses anything that's always a 'nice result' :)
Also, a more recent innovation in MD5 collisions is textcoll, which creates colliding blocks that are completely plaintext. This would allow for colliding PHP source files like in OP but without any obvious binary artefacts (although this requires identical prefixes).
https://github.com/cr-marcstevens/hashclash?tab=readme-ov-fi...