CVE-2025-30156: 
Linux Kernel vulnerability analysis and mitigation

CVE-2025-30156 is an authentication bypass vulnerability in Ceph's CephX authentication implementation, stemming from AES-CBC misuse. The flaw allows an attacker with a low-privilege CephX key and network visibility to forge credentials for privileged cluster entities including Manager, MDS, and OSD components. It affects Ceph versions prior to 19.2.6 and 20.2.4, with patched versions 19.2.6 and 20.2.4 available. The vulnerability carries a CVSS v3.1 base score of 8.9 (High) (GitHub Advisory).

The root cause is classified as CWE-327 (Use of a Broken or Risky Cryptographic Algorithm). CephX uses AES-128-CBC encryption with a hard-coded initialization vector (IV) and no HMAC or message authentication, meaning ciphertexts are malleable and unauthenticated. This mirrors the weakness documented in MIT's 2004 PERILS paper on Kerberos 4: the fixed IV causes identical plaintexts to produce identical ciphertexts (leaking equality), and the absence of authentication allows an attacker to flip bits in ciphertext to alter underlying data without detection. Two attack vectors exist: (1) using the monitor as an encryption oracle by requesting tickets for crafted entity names and splicing resulting ciphertext blocks into forged credentials for privileged entities; and (2) a single-bit flip in a CephX service ticket to set the allow_all field to true, granting full cluster access — discovered by David Mohren of CLYSO (GitHub Advisory).

Successful exploitation grants an attacker cluster-wide access to the Ceph storage cluster, including the ability to impersonate Manager, MDS, and OSD daemons. This results in high confidentiality and integrity impact — an attacker can read and modify all stored data — with low availability impact. The scope change (S:C) in the CVSS score reflects that a compromised client credential can be leveraged to compromise the entire cluster infrastructure beyond the initial access point (GitHub Advisory).

Exploitation requires an adjacent network position (AV:A), a low-privilege CephX credential, and the ability to observe CephX ciphertext on the network — no user interaction is needed. A Nessus detection plugin (ID 338279) has been published, indicating active scanner coverage (Tenable). The vulnerability was discussed on oss-sec mailing lists and the Proxmox community forum, suggesting public awareness. No confirmed in-the-wild exploitation or CISA KEV listing has been reported at this time (oss-sec, Proxmox Forum).

1. Reconnaissance: Identify Ceph clusters running vulnerable versions (< 19.2.6 or < 20.2.4) with CephX authentication enabled. Gain a position on the network adjacent to the Ceph monitor traffic.
2. Obtain a low-privilege credential: Acquire any valid CephX client key (e.g., a standard client credential with limited permissions).
3. Observe CephX ciphertext: Capture CephX authentication traffic on the network to observe encrypted ticket data.
4. Oracle-based credential forgery (Attack Vector 1): Create entities with chosen names and request tickets for them from the Ceph monitor. The monitor encrypts these names using the fixed IV and unauthenticated AES-CBC, acting as an encryption oracle. Splice the resulting ciphertext blocks to forge credentials for privileged entities (Manager, MDS, OSD).
5. Bit-flip privilege escalation (Attack Vector 2): Request a legitimate CephX service ticket from the monitor, then perform a single-bit flip in the ciphertext to set the allow_all field to true in the decrypted ticket, granting unrestricted cluster access.
6. Achieve cluster-wide access: Present the forged or manipulated credential to gain full access to the Ceph cluster, including read/write access to all stored data (GitHub Advisory).

• Network: Unusual or repeated CephX ticket requests from a single client to the Ceph monitor for multiple entity names (especially names resembling internal daemon identities like mgr, mds, osd); unexpected authentication attempts using credentials for privileged daemon roles from non-daemon hosts.
• Logs: Ceph monitor logs showing authentication successes for privileged entities (Manager, MDS, OSD) from unexpected source IPs or client identities; anomalous allow_all permissions appearing in service ticket grants.
• Process/Behavior: Unexpected client connections to OSD or MDS services using credentials that should belong to daemon-level entities; unusual data access patterns consistent with bulk read or modification across the cluster.

Upgrade Ceph to patched versions 19.2.6 or 20.2.4, which address the AES-CBC misuse in CephX. As a network-level workaround, restrict access to Ceph monitor and cluster networks to trusted hosts only, minimizing the attacker's ability to observe CephX ciphertext or reach the monitor. Rotate all CephX credentials after upgrading to invalidate any potentially compromised keys. The OpenStack community has also issued guidance (OSSN-0108) for deployments using Ceph as a backend (GitHub Advisory, OpenStack OSSN).

The vulnerability was disclosed on the oss-sec mailing list and generated discussion in the Proxmox community forum, where administrators sought guidance on affected deployments (oss-sec, Proxmox Forum). The OpenStack project published security note OSSN-0108 to address implications for OpenStack deployments using Ceph storage backends (OpenStack OSSN). The flaw drew comparisons to the well-known Kerberos 4 PERILS vulnerability from 2004, highlighting the long-standing risks of unauthenticated CBC encryption in authentication protocols.