Ceph CephX CVE-2025-30156

CephX is Ceph's native challenge-response authentication protocol, conceptually analogous to Kerberos, governing all daemon-to-daemon and client-to-cluster authentication over the internal Ceph messenger network. The cryptographic flaw mirrors the vulnerability class documented in the MIT 'Perils of Kerberos 4' paper (NDSS 2004): AES-CBC operated with a static, hardcoded IV and without a MAC or HMAC. A fixed IV eliminates semantic security - identical plaintext blocks always encrypt to identical ciphertext blocks, enabling traffic analysis. The absence of authenticated encryption (AE/AEAD) enables CBC malleability: an attacker can XOR a targeted byte in a known ciphertext block to produce a predictable, controlled change in the next decrypted plaintext block, permitting credential scope field manipulation without detection. The root cause maps to CWE-327 (Use of a Broken or Risky Cryptographic Algorithm) and CWE-329 (Not Using a Random IV with CBC Mode). The remediation - AES-256-CTS-HMAC-SHA384-192 - provides both confidentiality (CTS mode) and integrity (HMAC-SHA384) and required coordinated changes across Ceph userspace daemons and upstream Linux kernel support.