CVE-2026-89650
Critical · 9.1CVSS
9.1
EPSS
0.5%
Published
Sep 11, 2026
CISA due date
—
Cloudanix analysis
A bug in the Linux kernel's handling of configuration messages from Ceph storage servers can let a bad server read memory it should not, which may cause client machines to crash.
- Exploitation
- Remote Unauthenticated
- Attacker needs
- None
- Complexity
- Low
- Impact
- Denial Of Service
- Component
- OS kernel
- Class
- Memory Corruption
- Remediation
- Upgrade
- Effort
- Medium
- Patch risk
- Low
What it actually does
Out-of-bounds read in ceph_mdsmap_decode() when num_export_targets for MDS info v2/v3 exceeds remaining buffer space; the cursor advances without a preceding ceph_decode_need() check, landing decoded values in export_targets[] and reported as slab-out-of-bounds on x86_64 with KASAN.
Why it matters to the business
CephFS client kernels on Linux may crash or become unstable when receiving a malicious MDS map from a compromised or impersonated monitor, interrupting storage access for any workloads depending on Ceph.
How an attack unfolds
A malicious or compromised Ceph monitor (or an on-path attacker on an unauthenticated messenger session) crafts a CEPH_MSG_MDS_MAP containing per-MDS info with version 2 or 3 and an oversized num_export_targets value; the client decodes past the front-buffer bound, triggering the unchecked read.
How to fix it
Apply a kernel update that adds a ceph_decode_need() guard and uses size_mul() before advancing the cursor for every info_v >= 2; rebuild and deploy the patched kernel on all CephFS clients.
How to confirm the fix worked
Confirm the patched ceph_mdsmap_decode() contains the size_mul() + ceph_decode_need() sequence before the export-targets loop, then replay a crafted MDS map under KASAN to verify the out-of-bounds condition is no longer reported.
Typical attack path
- Obtain control of a Ceph monitor or position on the network path to an unauthenticated monitor session
- Send a CEPH_MSG_MDS_MAP with info_v in {2,3} and num_export_targets large enough to exceed the message front buffer
- Trigger ceph_mdsmap_decode() to perform out-of-bounds memory read while populating export_targets[]
How to detect it
Search kernel logs and KASAN reports for slab-out-of-bounds read originating from ceph_mdsmap_decode after receipt of CEPH_MSG_MDS_MAP messages.
What your security team should do
- Prioritize kernel updates on all CephFS clients before the next scheduled maintenance window
- Restrict monitor-to-client messenger sessions to authenticated and encrypted channels where possible
- Add kernel OOPS and KASAN alerts mentioning ceph_mdsmap_decode to high-priority alerting
The record
In the Linux kernel, the following vulnerability has been resolved: ceph: bound num_export_targets array for mds info v2/v3 ceph_mdsmap_decode() in fs/ceph/mdsmap.c reads num_export_targets from each per-mds info record and advances the decode cursor by num_export_targets * sizeof(u32) without first checking that many bytes remain. The only upper-bound check that catches a runaway cursor (*p > info_end) is gated on info_v >= 4, because info_end is left NULL for info_v 2 and 3. When the monitor sends an MDS map whose per-mds info version is 2 or 3 with an oversized num_export_targets, the cursor moves past the message front buffer and the later export-targets loop calls the unchecked ceph_decode_32() on out-of-bounds memory. A kernel client processes CEPH_MSG_MDS_MAP from its monitor session (net/ceph/mon_client.c dispatches it; fs/ceph/super.c routes it to ceph_mdsc_handle_mdsmap(), which sets end to the front buffer bound and calls ceph_mdsmap_decode()). A malicious or compromised monitor, or an on-path attacker on an unsigned/unencrypted messenger session, can therefore drive an out-of-bounds read in the client kernel; on x86_64 with KASAN it is reported as a slab-out-of-bounds read in ceph_mdsmap_decode(). The decoded values land in the internal info->export_targets[] array, so the consequence is a kernel out-of-bounds read, not an information leak to the attacker. Impact: a malicious or compromised Ceph monitor sending an MDS map with a per-mds info version of 2 or 3 and an oversized num_export_targets field triggers an out-of-bounds read in the CephFS client kernel. Add a ceph_decode_need() for the export-targets array before advancing the cursor, so the bound is enforced for every info_v >= 2, not only info_v >= 4. This mirrors the count-then-need idiom already used for m_data_pg_pools later in the same function. Compute the export-targets byte count with size_mul() and reuse that checked length when advancing the cursor, so the attacker-controlled num_export_targets multiplication fails closed on overflow rather than relying on the later kcalloc() guard.
Affected packages
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