CVE-2026-39944: Ceph: CephX AES Authentication error

Vulnerability summary

CVE-2026-39944 is a high (CVSS 8.8) vulnerability in ceph. EPSS estimates a 0.29% chance of exploitation in the next 30 days, higher than 21% of scored vulnerabilities.

Timeline

1. 27-08-2026
   PublishedVulnerability first disclosed.
2. 01-09-2026
   Last modifiedVulnerability information updated.

Description

Ceph is an open-source distributed storage platform providing object, block, and file storage. In versions prior to 20.2.4 and 19.2.6, the RADOS Gateway (RGW) protects STS session tokens with an AES-128-CBC handler that provides no message authentication, allowing an attacker who holds any valid STS token to tamper with it undetected and escalate to full RGW administrative access. Because the ciphertext is unauthenticated, the attacker can perform a CBC bit-flip on the acct_type, perm_type, and is_admin fields of their own token, and a forged is_admin value triggers a global administrative override that bypasses all capability checks. The attack is reachable remotely over the RGW S3 endpoint and is a self-contained modification of a token the attacker already possesses, requiring no encryption oracle and no network observation. It requires only a single valid STS token, which need not carry any elevated privileges, with STS enabled. This issue is fixed in versions 20.2.4 and 19.2.6.

• CWE-327Use of a Broken or Risky Cryptographic AlgorithmThe product uses a broken or risky cryptographic algorithm or protocol.