dbcve.org
FreeBSDOperating system

CVE-2023-6534

HIGH · 7.5 CVSS v3.1 Published 2023-12-13
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
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 versions of FreeBSD 14.0-RELEASE before 14-RELEASE-p2, FreeBSD 13.2-RELEASE before 13.2-RELEASE-p7 and FreeBSD 12.4-RELEASE before 12.4-RELEASE-p9, the pf(4) packet filter incorrectly validates TCP sequence numbers.  This could allow a malicious actor to execute a denial-of-service attack against hosts behind the firewall.

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

The pf(4) packet filter in FreeBSD versions 12.4, 13.2, and 14.0 contains an incorrect TCP sequence number validation flaw. This firewall vulnerability can be exploited by a remote attacker to cause a denial-of-service condition against hosts protected by the firewall, likely by bypassing or triggering improper handling of TCP packets.

MitigationApply the appropriate FreeBSD security patches: upgrade to 14-RELEASE-p2 or later, 13.2-RELEASE-p7 or later, or 12.4-RELEASE-p9 or later.

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= 13.2= 14.0

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
None
Availability
High

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

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. Identify FreeBSD version
    Run 'uname -r' or check /etc/os-release to get the installed FreeBSD version number
    Affected if Version is 12.4, 13.2, or 14.0 exactly (not patched versions like 12.4-RELEASE-p9, 13.2-RELEASE-p7, or 14-RELEASE-p2 or later)
  2. Confirm pf is loaded
    Run 'kldstat | grep pf' or check 'pfctl -si' to see if the pf kernel module is loaded
    Affected if pf module is loaded and active
  3. Check pf rules for TCP handling
    Run 'pfctl -sr | grep -i tcp' to list any TCP filter rules in the pf.conf configuration
    Affected if Any TCP pass or block rules exist in the pf configuration
  4. Verify pf is actively filtering traffic
    Run 'pfctl -s all' to display pf status and rules; check if the firewall is in use (rules loaded and active)
    Affected if pf is enabled with loaded rules processing network traffic

You are affected if you are running FreeBSD 12.4, 13.2, or 14.0 (without later security patches) AND have pf enabled with TCP filtering rules actively processing traffic.

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
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply the appropriate FreeBSD security patches: upgrade to 14-RELEASE-p2 or later, 13.2-RELEASE-p7 or later, or 12.4-RELEASE-p9 or later.

Recommended fix High confidence

FreeBSD 14-RELEASE-p2 (or later), 13.2-RELEASE-p7 (or later), or 12.4-RELEASE-p9 (or later)

  1. 1. Check current FreeBSD version using: uname -a
  2. 2. For FreeBSD 14.0: Upgrade to 14-RELEASE-p2 or later using: freebsd-update fetch install
  3. 3. For FreeBSD 13.2: Upgrade to 13.2-RELEASE-p7 or later using: freebsd-update fetch install
  4. 4. For FreeBSD 12.4: Upgrade to 12.4-RELEASE-p9 or later using: freebsd-update fetch install
  5. 5. After upgrade, reboot the system to load the patched kernel
  6. 6. Verify the pf(4) module is loaded and the firewall rules are functioning properly
Caveat FreeBSD patch releases are typically low-risk and contain security fixes without breaking existing functionality

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

Fix this in FreeBSD Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing2.0 h
  • Review / QA1.0 h
7.0 hours of engineering $1,240
Get help mitigating

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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-2023-6534 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