CVE-2026-63913

/// Technical description

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

netfilter: conntrack: tcp: do not force CLOSE on invalid-seq RST without direction check

An unintended behavior in the TCP conntrack state machine allows a

connection to be forced into the CLOSE state using an RST packet with an

invalid sequence number.

Specifically, after a SYN packet is observed, an RST with an invalid SEQ

can transition the conntrack entry to TCP_CONNTRACK_CLOSE, regardless of

whether the RST corresponds to the expected reply direction. The relevant

code path assumes the RST is a response to an outgoing SYN, but does not

validate packet direction or ensure that a matching SYN was actually sent

in the opposite direction.

As a result, a crafted packet sequence consisting of a SYN followed by an

invalid-sequence RST can prematurely terminate an active NAT entry. This

makes connection teardown easier than intended.

So, tighten the state transition logic to ensure that RST-triggered

CLOSE transition

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References