CVE-2026-39944

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. Because there is no integrity protection on the tokens, an attacker who holds any valid unprivileged STS token can perform a CBC bit-flip attack to modify the token contents and escalate to full RGW admin privileges. This requires only that STS is enabled (rgw_s3_auth_use_sts = true) and that the attacker has a single valid STS token. Successful exploitation grants complete control over the RGW service, including reading, writing, and deleting all objects and buckets.