CVE-2026-63913

Summary

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 transitions only occur when the RST is a valid response to a previously observed SYN in the correct direction.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/2006979a15af5404bf932a325357683c0bac1656
• https://git.kernel.org/stable/c/6476c17d536dbd321c073242e762ddb2713a1238
• https://git.kernel.org/stable/c/f5547bebc416d56f56fb5b86dc20aabfa42165a0
• https://git.kernel.org/stable/c/2bb6d82b586ea5a4cb73bbdd6b7432e96096bc77
• https://git.kernel.org/stable/c/f206def4e86d810f927ba1d8e322ea72b29bce58
• https://git.kernel.org/stable/c/b98ab51c45c5608a1c19ce7fd17a3032469bb83f
• https://git.kernel.org/stable/c/d67c6adee8d1b65330d0174c4c367faba14e80a8
• https://git.kernel.org/stable/c/bed6e04be8e6b9133d8b16d5a42d0e0ce674fa9a

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