CVE-2026-39944
moderateUnauthenticated AES-CBC STS token tampering allows admin takeover in Ceph RADOS Gateway
Ceph's RADOS Gateway (RGW) protects STS session tokens with AES-128-CBC without any message authentication (CWE-327), so tokens can be modified without detection. An attacker who already holds any valid STS token — even one with no elevated privileges — can remotely tamper with their own token via CBC bit-flips on the acct_type, perm_type, and is_admin fields over the RGW S3 endpoint; a forged is_admin value triggers a global administrative override that bypasses all capability checks. Successful exploitation grants full RGW administrative access, with high impact on confidentiality, integrity, and availability. Any Ceph deployment running RGW with STS enabled on versions prior to 20.2.4 or 19.2.6 is affected. The flaw is not yet known to be exploited: EPSS is low at 0.2%, it is not in CISA KEV, and no public proof-of-concept is known.
What to do: Upgrade Ceph to 20.2.4 or 19.2.6 depending on your release branch. As interim mitigation, consider disabling STS or restricting STS token issuance to trusted users, and check whether STS is enabled on any internet-exposed RGW S3 endpoints. Review RGW logs for unexpected administrative activity from STS-authenticated sessions.
| Ceph RADOS Gateway (RGW) — 20.x releases | All versions prior to 20.2.4; fixed in 20.2.4 |
| Ceph RADOS Gateway (RGW) — 19.x releases | All versions prior to 19.2.6; fixed in 19.2.6 |
Order-of-magnitude estimate by the model from install counts, market share and public scan data it knows; verify before quoting.
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.
- Weakness
- CWE-327
- Vector
- CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
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