CVE-2026-31693: Linux Kernel CIFS Replay Vulnerability

CVE-2026-31693 Overview

CVE-2026-31693 is a Linux kernel vulnerability in the Common Internet File System (CIFS) client code. The flaw stems from missing reinitialization of local variables at replay labels used when a request must be retried. When execution jumps back to these labels, stale variable state can be reused, leading to undefined behavior categorized as use of uninitialized resource [CWE-908].

The issue affects Linux kernel 6.8 release candidates and related stable branches. Local users on systems mounting CIFS/SMB shares can trigger the replay path and influence kernel memory state.

Critical Impact

A local attacker with low privileges can leverage uninitialized variables in the CIFS replay path to achieve high impact on confidentiality, integrity, and availability of the affected kernel.

Affected Products

• Linux Kernel 6.8 (including release candidates rc2 through rc7)
• Linux Kernel stable branches containing the unpatched CIFS client code
• Distributions shipping the affected kernel versions with CIFS support enabled

Discovery Timeline

• 2026-04-30 - CVE-2026-31693 published to the National Vulnerability Database
• 2026-05-07 - Last updated in NVD database

Technical Details for CVE-2026-31693

Vulnerability Analysis

The vulnerability resides in the CIFS client implementation within the Linux kernel. Several functions use a goto label to mark the start of code that can be replayed if an SMB request fails and must be retried. The original code did not reinitialize certain local variables before re-executing this block.

When the replay path is taken, these variables retain values from the prior iteration. Depending on which variables are reused, the kernel may operate on stale pointers, incorrect length fields, or partially initialized structures. This is classified as a use of uninitialized resource weakness [CWE-908].

The attack vector is local. Successful exploitation requires the ability to interact with a CIFS mount and induce conditions that trigger request replay, such as transient network errors or session reconnects. Because the affected code runs in kernel context, exploitation can affect kernel memory integrity.

Root Cause

The root cause is incomplete state reset at replay labels in the CIFS client. Developers added the replay mechanism but did not reset every local variable whose value depends on a fresh request context. The fix moves the initialization of these variables to occur after the replay label so that each replay iteration starts from a clean state.

Attack Vector

Exploitation requires local access and an active or attacker-controlled CIFS mount. The attacker triggers a condition that forces the kernel to replay an SMB request through the affected code paths. Stale variable contents from the prior attempt are then consumed during the replay, producing the unsafe state. See the patch commits referenced in the Linux stable git tree for the specific code locations.

Detection Methods for CVE-2026-31693

Indicators of Compromise

• Unexpected kernel oops, BUG, or KASAN reports referencing fs/smb/client/ or CIFS function names following SMB session reconnects
• Kernel log entries showing repeated SMB request replays followed by memory corruption warnings
• Local processes with CIFS mount access performing abnormal patterns of session disruption

Detection Strategies

• Audit installed kernel versions against the fixed commits listed in the kernel.org advisories to identify exposed hosts
• Monitor dmesg and /var/log/kern.log for CIFS-related warnings, slab corruption alerts, and KASAN reports
• Correlate local user activity with CIFS mount events and unexpected kernel state transitions through endpoint telemetry

Monitoring Recommendations

• Enable kernel address sanitizer (KASAN) in test environments to surface uninitialized-memory conditions before production deployment
• Collect kernel ring buffer logs centrally and alert on cifs subsystem panics or warnings
• Track CIFS mount usage on multi-user hosts where local privilege boundaries are security-relevant

How to Mitigate CVE-2026-31693

Immediate Actions Required

• Apply the upstream Linux kernel patches referenced in the kernel.org commits, including commit 14f66f44, commit 1d731e51, and the related stable backports
• Update to a distribution kernel that includes the CIFS replay initialization fixes
• Inventory hosts using cifs.ko and prioritize patching multi-user systems and servers with local accounts

Patch Information

The fix ensures that local variables required by the replay code path are reinitialized after the replay label. Patches are available in the Linux stable tree at commits 14f66f44, 1d731e51, 7c9ce681, c854ab48, and c99e1609. Refer to the Linux kernel stable releases for backport guidance.

Workarounds

• Unload the cifs kernel module on systems that do not require SMB client functionality using modprobe -r cifs
• Restrict CIFS mount and unmount privileges to trusted administrative users only
• Avoid mounting untrusted or unstable CIFS shares on multi-tenant systems until patches are deployed

# Verify kernel version and CIFS module status
uname -r
lsmod | grep cifs

# Disable the CIFS module if not required
sudo modprobe -r cifs
echo "blacklist cifs" | sudo tee /etc/modprobe.d/blacklist-cifs.conf