CVE-2026-63913 (CVSS 8.2)

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

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CVSS v3.1 Score

8.2

HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H

EPSS — Exploit Prediction

0.0063

Probability of exploitation

0.46%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

References

Other References

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

Frequently Asked Questions

What is CVE-2026-63913? +

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. It has a CVSS v3.1 base score of 8.2 (HIGH).

How severe is CVE-2026-63913? +

CVE-2026-63913 has a CVSS v3.1 score of 8.2 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching. The EPSS score is 0.0063, placing it in the 0th percentile for exploitation probability.

How do I check if I'm vulnerable to CVE-2026-63913? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

CVE-2026-63913