CVE-2026-68118 tcp: challenge ACK for non-exact RST in SYN-RECEIVED
Attackers can disrupt TCP connections by sending non-exact RST packets, causing the kernel to drop requests instead of sending challenge ACKs.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-68118?
CVE-2026-68118 is a high-severity denial of service vulnerability in Linux, published on 10 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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.
Weakness: CWE-20
Which versions are affected by CVE-2026-68118?
How do I fix CVE-2026-68118?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-68118 being exploited?
There is no known exploitation of CVE-2026-68118 in the wild and no public exploit code.
Sources
Also known as AZL-95003, BELL-CVE-2026-68118, DEBIAN-CVE-2026-68118, GHSA-wr69-pfwc-72wx, UBUNTU-CVE-2026-68118.