CVE-2026-31416: Linux Kernel Netfilter Header Vulnerability

CVE-2026-31416 Overview

CVE-2026-31416 is a Linux kernel vulnerability in the netfilter subsystem, specifically within the nfnetlink_log module. The flaw is a followup to an earlier bug fix where NLMSG_DONE did not account for the netlink header size when calculating message length. The previous fix accounted only for attribute size, missing the netlink header itself. Triggering the condition produces a WARN splat and causes the netlink message to be dropped. The Linux kernel maintainers have resolved the issue across multiple stable branches.

Critical Impact
The condition produces a kernel WARN splat and drops the affected netlink message. There are no other documented ill effects such as memory corruption or privilege escalation.

Affected Products

Linux kernel netfilter subsystem (nfnetlink_log module)
Multiple stable kernel branches receiving backported fixes
Distributions packaging the affected kernel versions

Discovery Timeline

2026-04-13 - CVE-2026-31416 published to NVD
2026-04-18 - Last updated in NVD database

Technical Details for CVE-2026-31416

Vulnerability Analysis

The vulnerability resides in the nfnetlink_log component of the Linux kernel netfilter subsystem. nfnetlink_log delivers packet log events to userspace through netlink sockets. When the kernel composes a multipart netlink response and emits the trailing NLMSG_DONE marker, the size calculation must reserve space for the netlink message header (nlmsghdr) in addition to any attribute payload.

The original fix corrected attribute size accounting but did not include the header size. As a result, when NLMSG_DONE is appended to certain log dumps, the buffer accounting can fall short of the required allocation. The mismatch is detected by sanity checks in the netlink code path, which emit a WARN and discard the message rather than returning it to userspace.

The defect is a logic error in size arithmetic rather than a memory safety issue. There is no out-of-bounds write or information disclosure documented in the upstream commit description.

Root Cause

The root cause is incomplete length accounting in the nfnetlink_log dump path. The code reserved space for attribute data but omitted the nlmsghdr octets required by NLMSG_DONE. The fix updates the size computation to include both the header and attribute lengths so that the netlink buffer is sized correctly.

Attack Vector

No remote or unprivileged attack vector has been documented. Triggering the path requires the use of nfnetlink_log for packet logging, which is typically configured by an administrator. The observed outcome is limited to a kernel warning and a dropped log message, which can degrade visibility for security monitoring that depends on netfilter logs.

No exploitation in the wild has been reported, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.

Detection Methods for CVE-2026-31416

Indicators of Compromise

Kernel ring buffer entries containing WARN splats originating from netlink message construction paths associated with nfnetlink_log.
Gaps or missing entries in userspace consumers of nfnetlink_log such as ulogd2 where expected packet log records are absent.

Detection Strategies

Monitor dmesg and /var/log/kern.log for new WARN_ON traces referencing netlink or nfnetlink_log functions.
Correlate dropped netlink messages with the version of the running kernel using uname -r to identify hosts on unpatched branches.
Track package inventory against vendor advisories that reference the upstream commits listed in the Kernel Git Commit 09883bf and related backports.

Monitoring Recommendations

Forward kernel logs to a centralized logging system and alert on WARN traces from netfilter components.
Validate that downstream tooling consuming nfnetlink_log records reports continuous event flow without unexplained drops.
Subscribe to distribution security feeds to track backport status for affected stable kernel branches.

How to Mitigate CVE-2026-31416

Immediate Actions Required

Identify hosts running kernels that include nfnetlink_log and have not received the upstream fix.
Apply vendor-supplied kernel updates that incorporate the netlink header size accounting fix.
Reboot affected systems after kernel package upgrades to load the patched image.

Patch Information

The issue is resolved across multiple Linux stable branches. Reference commits include Kernel Git Commit 09883bf, Kernel Git Commit 4ec2164, Kernel Git Commit 607245c, Kernel Git Commit 6b41970, Kernel Git Commit 6d52a4a, Kernel Git Commit 761b45c, Kernel Git Commit 88a8f56, and Kernel Git Commit f08ffa3. Apply your distribution's kernel update that includes the corresponding backport.

Workarounds

If nfnetlink_log is not required for operations, avoid configuring NFLOG targets in iptables or nftables rules until the kernel is patched.
Where packet logging is required, consider temporarily routing logs through alternative mechanisms such as nfnetlink_queue consumers, validating that they meet operational requirements.
Track kernel WARN events and treat repeated splats as a signal to prioritize the kernel update.

bash

# Check running kernel version and identify nfnetlink_log usage
uname -r
lsmod | grep nfnetlink_log
dmesg | grep -i -E 'nfnetlink|WARN'