The vulnerability stemmed from IV reuse in AES encryption. The commit diff shows the AESCipher class previously allowed static IV storage via init and reused it in build_cipher/encrypt. The patched version removed IV storage and enforced fresh IV generation. The IDP server's use of a persistent AESCipher instance with a fixed IV (via symkey/iv initialization in server.py and authn.py) would trigger this reuse. The functions directly handling IV management (init, build_cipher, encrypt) are the root cause.