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.
History

Fri, 21 Aug 2026 13:30:00 +0000

Type Values Removed Values Added
First Time appeared Ceph
Ceph ceph
Vendors & Products Ceph
Ceph ceph

Thu, 20 Aug 2026 00:15:00 +0000

Type Values Removed Values Added
Description 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.
Title ceph: ceph: RGW STS session tokens vulnerable to CBC bit-flip attack enabling admin privilege escalation
Weaknesses CWE-327
References
Metrics threat_severity

None

cvssV3_1

{'score': 8.5, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H'}

threat_severity

Important


cve-icon MITRE

No data.

cve-icon Vulnrichment

No data.

cve-icon NVD

No data.

cve-icon Redhat

Severity : Important

Publid Date: 2026-08-19T17:57:46Z

Links: CVE-2026-39944 - Bugzilla

cve-icon OpenCVE Enrichment

Updated: 2026-08-21T12:59:46Z