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

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. The Red Hat Product Security team has assessed the severity of this vulnerability as Important, given that it can be exploited over the network by any holder of a valid STS token without user interaction.

Successful exploitation allows an attacker to escalate from any unprivileged STS session to full RGW admin, gaining complete read, write, and delete access to all S3 objects and buckets. The vulnerability's root cause is the use of unauthenticated AES-128-CBC encryption in STS session tokens, sharing the same cryptographic weakness as CVE-2025-30156.

Weakness: CWE-327.