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 directio...

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.

Metrics

CVSS 3.1

8.2/10

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

EPSS Probability

0.62%

46.0th percentile

Probability of exploitation in the next 30 days. Learn more

Affected Software

Source: CNA advisory (CVE.org). NVD analysis pending.

Vendor	Product	Versions
Linux	Linux	>= 9fb9cbb1082d6b31fb45aa1a14432449a0df6cf1, < 2006979a15af5404bf932a325357683c0bac1656; >= 9fb9cbb1082d6b31fb45aa1a14432449a0df6cf1, < 6476c17d536dbd321c073242e762ddb2713a1238; >= 9fb9cbb1082d6b31fb45aa1a14432449a0df6cf1, < f5547bebc416d56f56fb5b86dc20aabfa42165a0; >= 9fb9cbb1082d6b31fb45aa1a14432449a0df6cf1, < 2bb6d82b586ea5a4cb73bbdd6b7432e96096bc77; >= 9fb9cbb1082d6b31fb45aa1a14432449a0df6cf1, < f206def4e86d810f927ba1d8e322ea72b29bce58; >= 9fb9cbb1082d6b31fb45aa1a14432449a0df6cf1, < b98ab51c45c5608a1c19ce7fd17a3032469bb83f; >= 9fb9cbb1082d6b31fb45aa1a14432449a0df6cf1, < d67c6adee8d1b65330d0174c4c367faba14e80a8; >= 9fb9cbb1082d6b31fb45aa1a14432449a0df6cf1, < bed6e04be8e6b9133d8b16d5a42d0e0ce674fa9a
Linux	Linux	2.6.15

References

Timeline

Published: Jul 19, 2026
Last Modified: Jul 27, 2026
Status: Awaiting Analysis

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.

How severe is CVE-2026-63913?

CVE-2026-63913 has a CVSS score of 8.2/10 (HIGH severity). The EPSS model estimates a 0.62% probability of exploitation in the next 30 days.

How do I fix CVE-2026-63913?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

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