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Vulnerabilities

Vulnerability · CVE-2026-4247

When a challenge ACK is to be sent tcp_respond() constructs and sends the challenge ACK and consumes the mbuf that is passed in.

Published Mar 26, 2026 · catalog updated Jun 17, 2026 · When a challenge ACK

Check if you run it

No confirmed exploitation is on record for CVE-2026-4247. EPSS is 1.40%. Act only if a listed version of When a challenge ACK is installed.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (When a challenge ACK).
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
Severity
High7.5
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
1.40%
Remediation
Advisory link
Affected products
cpe freebsd/freebsd

When a challenge ACK is to be sent tcp_respond() constructs and sends the challenge ACK and consumes the mbuf that is passed in. When no challenge ACK should be sent the function returns and leaks the mbuf. If an attacker is either on path with an established TCP connection, or can themselves establish a TCP connection, to an affected FreeBSD machine, they can easily craft and send packets which meet the challenge ACK criteria and cause the FreeBSD host to leak an mbuf for each crafted packet in excess of the configured rate limit settings i.e. with default settings, crafted packets in excess of the first 5 sent within a 1s period will leak an mbuf. Technically, off-path attackers can also exploit this problem by guessing the IP addresses, TCP port numbers and in some cases the sequence numbers of established connections and spoofing packets towards a FreeBSD machine, but this is harder to do effectively.

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • cpe freebsd/freebsd14.3

How to fix CVE-2026-4247

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

No confirmed exploitation is on record for CVE-2026-4247. EPSS is 1.40%. Act only if a listed version of When a challenge ACK is installed.

Which versions are affected?

Compare what you have installed with 14.3. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

CWE-401: Missing Release of Memory after Effective Lifetime. EPSS 1.40%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

No public source repository is identified for this product.

Timeline

Detection, updates, and resolution for this record.

  1. Mar 26, 2026 · NIST NVD

    Detected

    When a challenge ACK is to be sent tcp_respond() constructs and sends the challenge ACK and consumes the mbuf that is passed in. When no challenge ACK should be sent the function returns and leaks the mbuf. If an attacker is either on path with an established TCP connection, or can themselves establish a TCP connection, to an affected FreeBSD machine, they can easily craft and send packets which m

  2. Jun 17, 2026 · NIST NVD

    Updated

    When a challenge ACK is to be sent tcp_respond() constructs and sends the challenge ACK and consumes the mbuf that is passed in. When no challenge ACK should be sent the function returns and leaks the mbuf. If an attacker is either on path with an established TCP connection, or can themselves establish a TCP connection, to an affected FreeBSD machine, they can easily craft and send packets which m

Sources

  • CVE.orgVulnerability recordCVE-2026-4247

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2026-4247

    30-day exploitation probability 1.4%

    first.org
  • NIST NVDVulnerability recordCVE-2026-4247

    When a challenge ACK is to be sent tcp_respond() constructs and sends the challenge ACK and consumes the mbuf that is passed in. When no challenge ACK should be sent the function returns and leaks the mbuf. If an attacker is either on path with an established TCP connection, or

    nvd.nist.gov

References

Freshness

Last verified in this catalog Oct 18, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.