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When Hearts Collide writeup TryHackMe

Shubham
Shubham
Cybersecurity Enthusiast

initial observations​

reading through the problem statement for "when hearts collide," the application's underlying logic became clear. the web app allows you to upload an image, calculates its md5 hash, and then compares that hash against its database to find a "match."

the vulnerability​

the critical flaw here is the application's reliance on md5 to determine file uniqueness. md5 is a deprecated and broken cryptographic hash function that is highly vulnerable to collision attacks. this means an attacker can intentionally generate two entirely different files that produce the exact same md5 hash output.

(for a deeper understanding of how this works, check out this official exploit-db reference: md5 collision of these 2 images is now() trivial and instant)

the exploit​

to exploit this logic flaw, i needed to create an md5 collision using a standard image file. rather than dealing with compiling collision tools locally, i opted to use a pre-built docker container for fastcoll.

  • grabbed a random standard jpeg image to use as my base (e.g., fly.jpg).
  • spun up the brimstone/fastcoll docker container and passed my image in as a prefix block to generate two new, distinct images (f1.jpg and f2.jpg):

docker run --rm --platform linux/amd64 -v $PWD:/work -w /work brimstone/fastcoll --prefixfile fly.jpg -o f1.jpg f2.jpg

(note: tweak the --platform flag if your local machine's architecture requires it).

the result​

with the two newly generated images in hand, i returned to the web app.

  • uploaded f1.jpg to the server.
  • immediately followed up by uploading f2.jpg.

because f2.jpg had the exact same md5 hash as f1.jpg but contained different file data, the application's matching logic broke perfectly. boom. the flag popped up on the screen.