# 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.

- **Severity:** High 8.2 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 19 Jul 2026
- **Updated:** 19 Oct 2026
- **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

## 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 (Improper Input Validation)

## Which versions are affected by CVE-2026-63913?


## How do I fix CVE-2026-63913?

Apply the vendor's patch.

## Is CVE-2026-63913 being exploited?

There is no known exploitation of CVE-2026-63913 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63913 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-63913 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-63913/ (Markdown: https://insights.sbomify.com/CVE-2026-63913/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63913
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63913
- OSV: https://osv.dev/vulnerability/CVE-2026-63913

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63913/
Summaries are generated and may contain mistakes; the official description above is the record.