Cloudanix VulnDB

CVE-2026-89651

Critical · 9.8

CVSS

9.8

EPSS

0.6%

Published

Sep 11, 2026

CISA due date

—

Cloudanix analysis

Next patch cycle SSVC: Track

A bug in the Linux kernel's Ceph client code can let a malicious server crash client machines simply by sending a specially crafted message when the client first connects.

Exploitation
Remote Unauthenticated
Attacker needs
None
Complexity
Low
Impact
Denial Of Service
Component
OS kernel
Class
Memory Corruption
Remediation
Patch
Effort
Medium
Patch risk
Low

What it actually does

Missing bounds check before ceph_decode_copy allows an oversized length field in MDSCapAuth match.path or match.fs_name records to trigger a slab-out-of-bounds read of up to 4 GiB past the dedicated message-front allocation in handle_session().

Why it matters to the business

Linux Ceph clients can be forced offline during initial mount by a malicious or compromised MDS, causing immediate service interruption and potential data-access loss for any workloads relying on Ceph filesystems.

How an attack unfolds

An attacker operating a malicious Ceph MDS sends a CEPH_SESSION_OPEN message (msg_version >= 6) containing MDSCapAuth records whose match.path or match.fs_name length fields exceed remaining message bytes; the client performs the unchecked copy on first post-connect message, resulting in an out-of-bounds read that crashes the kernel client.

How to fix it

Upgrade the Linux kernel to a version containing the commit that replaces ceph_decode_copy with ceph_decode_copy_safe for the two affected fields; rebuild and reboot all Ceph client nodes.

How to confirm the fix worked

Confirm the patch is present by inspecting the ceph_decode_copy_safe calls in fs/ceph/mds_client.c around handle_session(); recompile with KASAN and attempt mount against a test MDS that sends maximum-length fields to verify no out-of-bounds access occurs.

Typical attack path

  1. Attacker positions a malicious or compromised MDS reachable by the target client.
  2. Client initiates mount and establishes TCP connection to the MDS.
  3. MDS replies with a crafted CEPH_SESSION_OPEN message carrying oversized length values.
  4. handle_session() decodes the record without ceph_decode_need, invoking ceph_decode_copy on attacker-controlled lengths.
  5. Kernel performs slab-out-of-bounds read, triggering KASAN report or immediate crash.

How to detect it

Inspect kernel logs and dmesg for "slab-out-of-bounds read" or "handle_session" stack traces immediately after Ceph mount attempts; correlate with Ceph MDS connection events from auditd or system call tracing.

What your security team should do

  • Prioritize kernel upgrades on all Ceph clients within the next maintenance window given the 43-day publication window and network-reachable attack vector.
  • Restrict Ceph client connectivity to only trusted MDS endpoints using network ACLs or mutual authentication.
  • Enable kernel crash-dump collection and monitor for slab-out-of-bounds read signatures in dmesg or kernel logs.
  • Test the patched kernel in a staging Ceph cluster before broad rollout.

The record

In the Linux kernel, the following vulnerability has been resolved: ceph: bound MDSCapAuth path and fs_name decode in handle_session() handle_session() decodes the MDSCapAuth records carried by a CEPH_SESSION_OPEN message (msg_version >= 6). For each record the match.path and match.fs_name byte strings are read by first decoding a 32-bit length and then copying that many bytes with the bare ceph_decode_copy(). Unlike the surrounding fields, which all use the _safe decode variants, these two copies are not preceded by a ceph_decode_need() bounds check, and the enclosing MDSCapAuth and MDSCapMatch struct_len fields are skipped rather than enforced as an upper bound. A length larger than the bytes remaining in the message front makes ceph_decode_copy() read past the end of the front buffer. The message front is a dedicated allocation (ceph_msg_new2() -> kvmalloc), so the over-read runs off that object. A malicious or compromised MDS can trigger this with the first post-connect message on mount, with no client-side user interaction; under KASAN it is reported as a slab-out-of-bounds read in handle_session(). Impact: a malicious MDS can force the kernel client to read up to 4 GiB past the message front allocation during session setup, crashing the client (out-of-bounds read). Switch both copies to ceph_decode_copy_safe(), which performs the ceph_decode_need() bounds check before the copy and branches to the existing bad label, matching the rest of the decoder and the error path that frees the partially decoded cap_auths array.

Affected packages

Ecosystem Package Introduced Fixed