CVE-2026-63913 netfilter: conntrack: tcp: do not force CLOSE on invalid-seq RST without direction check
Attackers can prematurely terminate active NAT connections by sending crafted RST packets, causing denial of service in the Linux kernel.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-63913?
CVE-2026-63913 is a high-severity denial of service vulnerability in Linux, published on 19 Jul 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:
netfilter: conntrack: tcp: do not force CLOSE on invalid-seq RST without direction check
An unintended behavior in the TCP conntrack state machine allows a connection to be forced into the CLOSE state using an RST packet with an invalid sequence number.
Specifically, after a SYN packet is observed, an RST with an invalid SEQ can transition the conntrack entry to TCP_CONNTRACK_CLOSE, regardless of whether the RST corresponds to the expected reply direction. The relevant code path assumes the RST is a response to an outgoing SYN, but does not validate packet direction or ensure that a matching SYN was actually sent in the opposite direction.
As a result, a crafted packet sequence consisting of a SYN followed by an invalid-sequence RST can prematurely terminate an active NAT entry. This makes connection teardown easier than intended.
So, tighten the state transition logic to ensure that RST-triggered CLOSE transitions only occur when the RST is a valid response to a previously observed SYN in the correct direction.
Weakness: CWE-20
Which versions are affected by CVE-2026-63913?
How do I fix CVE-2026-63913?
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-2026-63913 being exploited?
There is no known exploitation of CVE-2026-63913 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2026-63913, DEBIAN-CVE-2026-63913, GHSA-8577-pmm3-chq2, ROOT-OS-UBUNTU-2204-CVE-2026-63913, UBUNTU-CVE-2026-63913.