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

Severity: high (CVSS 8.2 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-08-10 · updated 2026-10-30
Weakness: CWE-20

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.

Affected products

Linux: from 3.0.58 before 3.1; from 3.2.37 before 3.3; from 3.4.25 before 3.5 — fixed in 3.1, 3.3, 3.5

How to fix

Upgrade to a fixed version: Linux 3.1, 3.3, 3.5. Check the vendor advisory for mitigations.

References

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