# CVE-2023-4809: pf incorrectly handles multiple IPv6 fragment headers

> Attackers can bypass pf firewall rules by sending IPv6 packets with multiple fragment headers, allowing unauthorized traffic to reach the host.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 6 Sep 2023
- **Updated:** 13 Feb 2025
- **Also known as:** GHSA-g6hh-x63c-83gf

## What is CVE-2023-4809?

CVE-2023-4809 is a high-severity vulnerability in Freebsd Freebsd, published on 6 Sep 2023. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

In pf packet processing with a 'scrub fragment reassemble' rule, a packet containing multiple IPv6 fragment headers would be reassembled, and then immediately processed.  That is, a packet with multiple fragment extension headers would not be recognized as the correct ultimate payload. Instead a packet with multiple IPv6 fragment headers would unexpectedly be interpreted as a fragmented packet, rather than as whatever the real payload is.

As a result, IPv6 fragments may bypass pf firewall rules written on the assumption all fragments have been reassembled and, as a result, be forwarded or processed by the host.

Weakness: CWE-167 (Improper Handling of Additional Special Element)

## Which versions are affected by CVE-2023-4809?

It affects 1 product.


Products: Freebsd Freebsd.

## How do I fix CVE-2023-4809?

No fix has been published for CVE-2023-4809 yet. Limit exposure of the affected software until one is available.

## Is CVE-2023-4809 being exploited?

There is no known exploitation of CVE-2023-4809 in the wild and no public exploit code.

## Other vulnerabilities in Freebsd Freebsd

- [CVE-2018-17160](https://insights.sbomify.com/CVE-2018-17160/): Memory corruption in FreeBSD (critical 10.0)
- [CVE-2014-3954](https://insights.sbomify.com/CVE-2014-3954/): Memory corruption in n/a (critical 10.0)
- [CVE-2006-4304](https://insights.sbomify.com/CVE-2006-4304/): Memory corruption in n/a (critical 10.0)
- [CVE-2006-0226](https://insights.sbomify.com/CVE-2006-0226/): Integer overflow in n/a (critical 10.0)
- [CVE-2004-0002](https://insights.sbomify.com/CVE-2004-0002/): Denial of service in n/a (critical 10.0)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2023-4809 and fix it if it is affected.

What it is: CVE-2023-4809 is a high-severity vulnerability in Freebsd Freebsd, published on 6 Sep 2023. 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-2023-4809 in the wild and no public exploit code.

Affected product: Freebsd Freebsd. 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-2023-4809/ (Markdown: https://insights.sbomify.com/CVE-2023-4809/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2023-4809
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2023-4809
- OSV: https://osv.dev/vulnerability/CVE-2023-4809

---
From sbomify Insights: https://insights.sbomify.com/CVE-2023-4809/
Summaries are generated and may contain mistakes; the official description above is the record.