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Date: Fri, 8 Sep 2023 19:48:21 +0200
From: Enrico Bassetti <bassetti@...uniroma1.it>
To: oss-security@...ts.openwall.com
Subject: CVE-2023-4809: FreeBSD pf bypass when using IPv6
Hello *,
A few months ago, as part of our investigations on IPv6 security in the
NetSecurityLab @ Sapienza University, we discovered a vulnerability that
allows attackers to bypass rules in pf-based IPv6 firewalls in
particular conditions.
Vulnerability (TL;DR)
==============
A FreeBSD with `pf` as firewall for IPv6 traffic and `scrub` enabled to
reassemble IPv6 fragments is vulnerable to an attack that uses a crafted
packet posing as IPv6 "atomic" fragment to bypass the rules. After the
fragment is matched against some firewall rules (but not all!), it is
then "corrected" and forwarded to the destination (if no "deny" rule is
matched).
References
==============
This vulnerability has been assigned the ID CVE-2023-4809.
The FreeBSD advisory is at
https://www.freebsd.org/security/advisories/FreeBSD-SA-23:10.pf.asc
We wrote a description with an example at:
https://www.enricobassetti.it/2023/09/cve-2023-4809-freebsd-pf-bypass-when-using-ipv6/
Solution
==============
The solution is to update FreeBSD to the latest version. All FreeBSD
versions up to (but not including) 13.2-STABLE, 13.2-RELEASE-p3,
12.4-STABLE, and 12.4-RELEASE-p5 are affected.
Best regards,
Enrico Bassetti
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