CVE-2026-68118
HighCVSS 8.2Exploitation Probability (EPSS)
Low risk32th percentile - higher than 32% of all known CVEs
Summary
In the Linux kernel, in tcp_check_req() in the SYN-RECEIVED state, a non-exact RST (not matching RCV.NXT) is accepted and removes the request instead of triggering a challenge ACK as per RFC 9293. The fix adds the RFC 5961 reset check before ACK-field validation to enforce challenge ACK for non-exact RSTs.
Risk Assessment
The vulnerability may allow an attacker to more easily terminate TCP connections in SYN-RECEIVED state, potentially leading to DoS attacks. RFC compliance improves resilience against such attacks.
Recommendation
Apply the official Linux kernel patch that implements correct challenge ACK behavior for non-exact RSTs in SYN-RECEIVED state.
Original NVD description (English source)
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.