High · published August 10, 2026
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.