High [CVE-2026-39944] RGW STS session tokens vulnerable to CBC bit-flip attack enabling admin privilege escalation

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. A flaw was found in Ceph RGW's STS (Security Token Service) session token implementation.

The STS tokens use the same unauthenticated AES-128-CBC encryption as CephX, which lacks message authentication and uses a hardcoded initialization vector.

Affected products named by the advisory: Red Hat Ceph Storage 4; Red Hat Ceph Storage 5; Red Hat Ceph Storage 6; Red Hat Ceph Storage 7; and 2 more.