xmlseclibs PHP library for XML Encryption and Signatures contains a critical vulnerability in versions prior to 3.1.5. The vulnerability affects XML nodes encrypted with AES-GCM algorithms (aes-128-gcm, aes-192-gcm, aes-256-gcm) due to lack of authentication tag length validation. Attackers can exploit this to brute-force authentication tags, recover GHASH keys, decrypt encrypted nodes, and forge arbitrary ciphertexts without knowing the encryption key. The vulnerability has been patched in version 3.1.5.