CVE-2026-31681 netfilter: xt_multiport: validate range encoding in checkentry

Medium 5.5 Published 25 Apr 2026 Updated 15 Oct 2026

A missing validation in the Linux kernel netfilter module allows malformed rules to cause out-of-bounds reads, potentially leading to a denial of service.

Exploited in the wild: Not known
Login needed: Any signed-in user
Attackable: Local or same network only
Victim action needed: Not needed
Fix: Available

What is CVE-2026-31681?

CVE-2026-31681 is a medium-severity denial of service vulnerability in Linux kernel, published on 25 Apr 2026. It can only be exploited from the same machine or local network, with no action from the victim.

Official description

In the Linux kernel, the following vulnerability has been resolved:

netfilter: xt_multiport: validate range encoding in checkentry

ports_match_v1() treats any non-zero pflags entry as the start of a port range and unconditionally consumes the next ports[] element as the range end.

The checkentry path currently validates protocol, flags and count, but it does not validate the range encoding itself. As a result, malformed rules can mark the last slot as a range start or place two range starts back to back, leaving ports_match_v1() to step past the last valid ports[] element while interpreting the rule.

Reject malformed multiport v1 rules in checkentry by validating that each range start has a following element and that the following element is not itself marked as another range start.

Which versions are affected by CVE-2026-31681?

It affects 1 product.

Products include Linux kernel.

How do I fix CVE-2026-31681?

Apply the vendor's patch.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2026-31681 being exploited?

There is no known exploitation of CVE-2026-31681 in the wild and no public exploit code.

All Linux kernel vulnerabilities

Sources

Also known as AZL-84219, BELL-CVE-2026-31681, DEBIAN-CVE-2026-31681, GHSA-f822-mpgr-7cqx, ROOT-OS-DEBIAN-11-CVE-2026-31681, ROOT-OS-DEBIAN-12-CVE-2026-31681 and 3 more.