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FreeBSDOperating system

CVE-2023-4809

HIGH · 7.5 CVSS v3.1 Published 2023-09-06
Fix available
A fix is available. Upgrade to 12.4 / 13.2 or later.
See remediation →
84/100
Remediation priority · High
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

In pf packet processing with a 'scrub fragment reassemble' rule, packets containing multiple IPv6 fragment extension headers are incorrectly identified as fragmented packets rather than being properly reassembled into their final payload. This causes IPv6 fragments to bypass firewall rules that assume all fragments have been reassembled, allowing potentially malicious traffic to pass through unfiltered.

MitigationImplement additional IPv6 filtering rules to catch fragment anomalies or disable IPv6 if not required, until the operating system vendor releases a patch for pf that correctly handles multiple IPv6 fragment headers during reassembly.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
FreeBSDOperating system
Affected:< 12.4>= 13.0, < 13.2= 12.4= 13.2

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check FreeBSD version
    Run 'uname -r' or 'freebsd-version -u' to identify the installed FreeBSD version
    Affected if Version is < 12.4, = 12.4, = 13.2, or >= 13.0 and < 13.2 (e.g., 13.0, 13.1)
  2. Verify pf is loaded and active
    Run 'pfctl -si' to check pf status, or 'pfctl -sr' to show loaded rules
    Affected if pf is enabled and actively filtering traffic
  3. Identify scrub fragment reassemble rules
    Run 'pfctl -sr | grep -i scrub' to find any 'scrub fragment reassemble' or 'scrub fragment' rules in the pf configuration
    Affected if A rule containing 'scrub fragment reassemble' is present in the active pf ruleset
  4. Confirm IPv6 is enabled and in use
    Check if IPv6 is active via 'ifconfig | grep inet6' or check pf rules for 'inet6' entries that filter IPv6 traffic
    Affected if IPv6 is enabled and pf has IPv6 filtering rules that could be bypassed
  5. Inspect pf fragment handling configuration
    Review the full pf ruleset with 'pfctl -sr' and look for fragment-related rules that rely on reassembly
    Affected if Firewall rules assume IPv6 fragments are reassembled before filtering decisions are applied

You are affected if your FreeBSD version falls within the vulnerable range (12.4, 13.0-13.1, or 13.2) AND pf is actively using 'scrub fragment reassemble' rules while IPv6 traffic is being filtered.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 12.4 / 13.2 or later
Fixed in 12.413.2
Interim mitigation

Implement additional IPv6 filtering rules to catch fragment anomalies or disable IPv6 if not required, until the operating system vendor releases a patch for pf that correctly handles multiple IPv6 fragment headers during reassembly.

Recommended fix High confidence

FreeBSD 12.5+ or 13.3+ (depending on your base branch)

  1. 1. Identify current FreeBSD version using 'uname -a' or 'freebsd-version'
  2. 2. For systems running FreeBSD 12.x: Upgrade to FreeBSD 12.5 or later stable release
  3. 3. For systems running FreeBSD 13.x: Upgrade to FreeBSD 13.3 or later stable release
  4. 4. Verify the upgrade was successful by checking 'freebsd-version -u'
  5. 5. Review pf rulesets to ensure 'scrub fragment reassemble' rules are functioning as expected after the upgrade
Caveat Review pf.conf for any version-specific syntax; major FreeBSD upgrades may require configuration adjustments

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in FreeBSD Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing12.0 h
  • Review / QA4.0 h
28.0 hours of engineering $4,760
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $7,616.

Scan for this in your stack

Free · runs locally
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2023-4809 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data