CyberSecBulletin
CVE-2026-63913NVD status: Analyzed

In the Linux kernel, the following vulnerability has been resolved: netfilter: conntrack: tcp…

Published Updated By the CyberSecBulletin desk

CVE-2026-63913 is a high-severity improper input validation vulnerability in Linux, with a CVSS 3.1 base score of 8.2 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. The record lists versions 2006979a15af5404bf932a325357683c0bac1656, 6476c17d536dbd321c073242e762ddb2713a1238, f5547bebc416d56f56fb5b86dc20aabfa42165a0 as the first without the flaw, so updating Linux to one of those releases or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.97% chance of exploitation within 30 days, higher than 62% of scored CVEs.

Next patch cycle

High severity with no evidence of exploitation so far.

What to do

  1. Update Linux to 2006979a15af5404bf932a325357683c0bac1656 or 6476c17d536dbd321c073242e762ddb2713a1238 or f5547bebc416d56f56fb5b86dc20aabfa42165a0 or 2bb6d82b586ea5a4cb73bbdd6b7432e96096bc77, whichever matches your release line, or a later version.
  2. Update Linux Kernel to 5.10.259 or 5.15.210 or 6.1.176 or 6.6.143, whichever matches your release line, or a later version.
  3. Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.

Affected products and versions

VendorProductAffected versionsFixed in
LinuxLinux9fb9cbb1082d6b31fb45aa1a14432449a0df6cf1 up to, but not including, 2006979a15af5404bf932a325357683c0bac1656; 9fb9cbb1082d6b31fb45aa1a14432449a0df6cf1 up to, but not including, 6476c17d536dbd321c073242e762ddb2713a1238; 9fb9cbb1082d6b31fb45aa1a14432449a0df6cf1 up to, but not including, f5547bebc416d56f56fb5b86dc20aabfa42165a0; 9fb9cbb1082d6b31fb45aa1a14432449a0df6cf1 up to, but not including, 2bb6d82b586ea5a4cb73bbdd6b7432e96096bc77; 9fb9cbb1082d6b31fb45aa1a14432449a0df6cf1 up to, but not including, f206def4e86d810f927ba1d8e322ea72b29bce58; 9fb9cbb1082d6b31fb45aa1a14432449a0df6cf1 up to, but not including, b98ab51c45c5608a1c19ce7fd17a3032469bb83f; 9fb9cbb1082d6b31fb45aa1a14432449a0df6cf1 up to, but not including, d67c6adee8d1b65330d0174c4c367faba14e80a8; 9fb9cbb1082d6b31fb45aa1a14432449a0df6cf1 up to, but not including, bed6e04be8e6b9133d8b16d5a42d0e0ce674fa9a; and 1 more ranges2006979a15af5404bf932a325357683c0bac1656, 6476c17d536dbd321c073242e762ddb2713a1238, f5547bebc416d56f56fb5b86dc20aabfa42165a0, 2bb6d82b586ea5a4cb73bbdd6b7432e96096bc77, f206def4e86d810f927ba1d8e322ea72b29bce58, b98ab51c45c5608a1c19ce7fd17a3032469bb83f
LinuxLinux Kernel2.6.15 up to, but not including, 5.10.259; 5.11 up to, but not including, 5.15.210; 5.16 up to, but not including, 6.1.176; 6.2 up to, but not including, 6.6.143; 6.7 up to, but not including, 6.12.93; 6.13 up to, but not including, 6.18.35; 6.19 up to, but not including, 7.0.12; 7.15.10.259, 5.15.210, 6.1.176, 6.6.143, 6.12.93, 6.18.35

Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.

How the severity score breaks down

Attack vector
NetworkCan be exploited remotely over a network, including the internet.
Attack complexity
LowNo special conditions are needed; the attack works reliably.
Privileges required
NoneNo account or login is needed.
User interaction
NoneNo action by a victim is required.
Scope
UnchangedImpact stays within the vulnerable component.
Confidentiality
NoneNo data is disclosed.
Integrity
LowLimited modification of data is possible.
Availability
HighThe component can be taken down completely.

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H. Scored by CNA.

Official 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.

Source: the CVE record as published by its numbering authority, via NVD and CVE.org.

The underlying weakness: Improper Input Validation

The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.

How developers prevent this class of flaw

  • Consider using language-theoretic security (LangSec) techniques that characterize inputs using a formal language and build recognizers for that language. This effectively requires parsing to be a distinct layer that effectively enforces a boundary between raw input and internal data representations, instead of allowing parser code to be scattered throughout the program, where it could be subject to errors or inconsistencies that create weaknesses.
  • Use an input validation framework such as Struts or the OWASP ESAPI Validation API. Note that using a framework does not automatically address all input validation problems; be mindful of weaknesses that could arise from misusing the framework itself (CWE-1173).
  • Understand all the potential areas where untrusted inputs can enter the product, including but not limited to: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide data to the application. Remember that such inputs may be obtained indirectly through API calls.

More Improper Input Validation vulnerabilities (CWE-20)

Timeline

  1. CVE record published.
  2. CISA published its SSVC assessment (exploitation: none, automatable: yes, technical impact: partial).
  3. Record last changed materially (score, affected products, status or references).

References

Recent Improper Input Validation vulnerabilities

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