CyberSecBulletin
CVE-2026-68118NVD status: Received

In the Linux kernel, the following vulnerability has been resolved: tcp: challenge ACK for…

Published Updated By the CyberSecBulletin desk

CVE-2026-68118 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 c5dc463cc60eb26079b19607c9dbda76491dad05, 8b0a3a094f4cae2fb92e4d08d4eef7246a9d9c49, 0fe4636665d14a258de70b4f3e8248e6d42038f1 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 0.4% chance of exploitation within 30 days, which is low compared with other CVEs.

Next patch cycle

High severity with no evidence of exploitation so far.

What to do

  1. Update Linux to c5dc463cc60eb26079b19607c9dbda76491dad05 or 8b0a3a094f4cae2fb92e4d08d4eef7246a9d9c49 or 0fe4636665d14a258de70b4f3e8248e6d42038f1 or 234f9ffbd9b2c1b24ec67200ea3cff07401bec48, whichever matches your release line, or a later version.
  2. 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
LinuxLinux282f23c6ee343126156dd41218b22ece96d747e3 up to, but not including, c5dc463cc60eb26079b19607c9dbda76491dad05; 282f23c6ee343126156dd41218b22ece96d747e3 up to, but not including, 8b0a3a094f4cae2fb92e4d08d4eef7246a9d9c49; 282f23c6ee343126156dd41218b22ece96d747e3 up to, but not including, 0fe4636665d14a258de70b4f3e8248e6d42038f1; 282f23c6ee343126156dd41218b22ece96d747e3 up to, but not including, 234f9ffbd9b2c1b24ec67200ea3cff07401bec48; 282f23c6ee343126156dd41218b22ece96d747e3 up to, but not including, 22cec809b048495310f206d9abbcdbbfbdce3ae3; 282f23c6ee343126156dd41218b22ece96d747e3 up to, but not including, a28c4fcbf774e23b4779cae468e3497a5ad1f4a1; 86791bbfe5ed7b275be040cfeff049a1624af1b7; 61f69dc4e40e41b0018f00fa4aeb23d3239556fb; and 5 more rangesc5dc463cc60eb26079b19607c9dbda76491dad05, 8b0a3a094f4cae2fb92e4d08d4eef7246a9d9c49, 0fe4636665d14a258de70b4f3e8248e6d42038f1, 234f9ffbd9b2c1b24ec67200ea3cff07401bec48, 22cec809b048495310f206d9abbcdbbfbdce3ae3, a28c4fcbf774e23b4779cae468e3497a5ad1f4a1

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: tcp: challenge ACK for non-exact RST in SYN-RECEIVED The SYN-RECEIVED request-socket path in tcp_check_req() accepts an in-window RST without requiring SEG.SEQ to exactly match RCV.NXT. A non-exact RST therefore removes the request instead of eliciting a challenge ACK. RFC 9293 section 3.10.7.4 applies the RFC 5961 reset check in SYN-RECEIVED: an exact RST resets the connection, while a non-exact in-window RST must trigger a challenge ACK and be dropped. Apply that check before the ACK-field validation, following the RFC sequence-number, RST, then ACK processing order. Factor the per-netns challenge ACK quota out of tcp_send_challenge_ack() so request sockets can share it. Use the request socket's send_ack() callback and its own out-of-window ACK timestamp to send and rate-limit the response.

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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