CVE-2026-89651
CriticalCVSS 9.8Exploitation Probability (EPSS)
Low risk10th percentile - higher than 10% of all known CVEs
Summary
In the Linux kernel, handle_session() in the ceph module lacked bounds checks when decoding the match.path and match.fs_name fields of MDSCapAuth records in a CEPH_SESSION_OPEN message. A malicious or compromised MDS can supply an oversized length, causing an out-of-bounds read (up to 4 GiB) and crashing the client. The fix switches both copies to ceph_decode_copy_safe().
Risk Assessment
A malicious or compromised MDS can crash the Ceph client on the first post-connect message during mount, with no client-side user interaction, resulting in denial of service. Under KASAN it is reported as a slab-out-of-bounds read.
Recommendation
Update the Linux kernel to a version containing the fix for CVE-2026-89651. Until patched, restrict Ceph clients to connecting only to trusted MDS servers.
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Original NVD description (English source)
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.