CVE-2023-4809Network attack vector

Firewall bypass via nested IPv6 fragment misclassification

Published Sep 6, 2023 · Updated Feb 13, 2025

Firewall bypass in FreeBSD 12.4-RELEASE before p5 and 13.2-RELEASE before p3 allows remote attackers to evade IPv6 pf rules. With scrub fragment reassemble enabled, pf resumes processing after removing the first fragment header and misclassifies a second fragment header instead of the ultimate payload. The attack requires IPv6, pf, and that scrub rule; crafted fragments can then reach a protected destination or be processed by the host despite payload-specific rules.

CVSS severity7.5
High
EPSS probability0.80%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Firewall bypass in FreeBSD 12.4-RELEASE before p5 and 13.2-RELEASE before p3 allows remote attackers to evade IPv6 pf rules. With scrub fragment reassemble enabled, pf resumes processing after removing the first fragment header and misclassifies a second fragment header instead of the ultimate payload. The attack requires IPv6, pf, and that scrub rule; crafted fragments can then reach a protected destination or be processed by the host despite payload-specific rules.

The record

CVE
CVE-2023-4809
Published
Sep 6, 2023
Updated
Feb 13, 2025
Vendor
The FreeBSD Project
Product
FreeBSD
Classifications
CWE-167
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 6, 2023CVE publishedPublication date reported by the CVE source.
  2. Feb 13, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=12.4-RELEASE <p5
  2. Affected versionversion=13.2-RELEASE <p3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The FreeBSD Project · FreeBSDversion=12.4-RELEASE <p5; version=13.2-RELEASE <p3

Published CVSS scores

7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-167Improper Handling of Additional Special Element

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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