CVE-2026-52956: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-52956 Overview

CVE-2026-52956 is an out-of-bounds read vulnerability in the Linux kernel's libceph subsystem. The flaw resides in the __ceph_x_decrypt() function, which interprets a portion of a buffer as a ceph_x_encrypt_header struct without validating that the buffer is large enough to contain it. A specially crafted FRAME_TAG_AUTH_REPLY_MORE message with a ciphertext_len of 8 bytes or less can trigger out-of-bounds memory access when the kernel reads the hdr->magic field. The Linux kernel maintainers have resolved the issue by adding a size check before the header is dereferenced.

Critical Impact
An attacker capable of delivering crafted Ceph authentication frames can trigger kernel out-of-bounds memory reads, potentially leading to information disclosure or kernel instability.

Affected Products

Linux kernel versions containing the vulnerable libceph__ceph_x_decrypt() implementation
Systems mounting CephFS or using RBD (RADOS Block Device) clients
Distributions shipping affected upstream kernels prior to the fix commits

Discovery Timeline

2026-06-24 - CVE-2026-52956 published to NVD
2026-06-24 - Last updated in NVD database

Technical Details for CVE-2026-52956

Vulnerability Analysis

The vulnerability resides in the Ceph authentication decryption path inside the kernel's libceph module. When __ceph_x_decrypt() processes an incoming buffer, it casts a region of memory p to a pointer of type struct ceph_x_encrypt_header and reads the magic field for validation. This cast happens without first verifying that the buffer contains enough bytes to represent the full header structure.

The function parameter ciphertext_len describes the length of the ciphertext to be decrypted. While ciphertext_len is bounded by the remaining size of the allocated buffer, it is not bounded by sizeof(struct ceph_x_encrypt_header). When the supplied ciphertext is 8 bytes or smaller, the decrypted plaintext is shorter than the header struct, and dereferencing hdr->magic reads beyond the valid data.

Root Cause

The root cause is a missing length validation before a structure dereference [CWE-125]. The decryption routine trusted the protocol-supplied length field instead of independently verifying that the decoded plaintext could hold the expected header layout.

Attack Vector

An attacker who can deliver malicious Ceph protocol frames to a vulnerable client can craft a FRAME_TAG_AUTH_REPLY_MORE message sized exactly to place a short ciphertext at its end. When the kernel client decrypts and parses the reply, the out-of-bounds read occurs in kernel context. The vulnerability requires the victim to communicate with the attacker-controlled or compromised Ceph endpoint. The fix, applied in kernel commits 821365487aa5 and c7e9b53aebe4, introduces a check ensuring the decrypted plaintext is at least as large as sizeof(struct ceph_x_encrypt_header) before any field is accessed.

Detection Methods for CVE-2026-52956

Indicators of Compromise

Unexpected kernel oops or KASAN reports referencing __ceph_x_decrypt or ceph_x_encrypt_header in dmesg output
Repeated Ceph authentication failures or disconnects from a specific monitor or OSD peer
Anomalous Ceph protocol frames carrying FRAME_TAG_AUTH_REPLY_MORE with truncated ciphertext payloads

Detection Strategies

Enable KASAN (Kernel Address Sanitizer) on test kernels to surface the out-of-bounds read deterministically
Audit installed kernel package versions against distribution advisories that reference the upstream fix commits
Monitor kernel ring buffer logs for libceph warnings and authentication errors correlated with Ceph cluster activity

Monitoring Recommendations

Aggregate syslog and dmesg from Ceph clients into a centralized log platform for anomaly review
Track Ceph monitor and OSD peer identities to detect connections from unexpected endpoints
Alert on repeated __ceph_x_decrypt related stack traces, which indicate either exploitation attempts or unstable peers

How to Mitigate CVE-2026-52956

Immediate Actions Required

Update the Linux kernel to a version that contains the upstream fix from commits 821365487aa5 and c7e9b53aebe4
Restrict Ceph cluster network access to trusted hosts using firewall rules and network segmentation
Verify that CephX authentication is enforced on all cluster endpoints to limit unauthenticated frame delivery

Patch Information

The vulnerability is fixed in the Linux kernel via two stable tree commits: Kernel Git Commit 821365487aa5 and Kernel Git Commit c7e9b53aebe4. Both add a length check to __ceph_x_decrypt() that rejects ciphertext shorter than sizeof(struct ceph_x_encrypt_header) before dereferencing the header. Apply distribution-provided kernel updates that incorporate these commits and reboot affected hosts.

Workarounds

Where patching is delayed, isolate Ceph clients to dedicated VLANs that exclude untrusted peers
Unload the ceph and rbd kernel modules on systems that do not actively require Ceph connectivity
Disable mounting of CephFS shares from untrusted sources until kernels are updated

bash

# Verify running kernel version and check whether libceph module is loaded
uname -r
lsmod | grep -E 'ceph|rbd'

# Temporarily unload Ceph modules on hosts that do not require them
sudo umount -a -t ceph
sudo modprobe -r rbd ceph libceph

# After applying the patched kernel package, reboot to load the fixed module
sudo reboot