CVE-2026-4247: When a challenge ACK is to be sent tcp_respond() constructs and sends the challenge ACK and consumes the mbuf

This month

HIGH 7.5REMOTEMITIGATE

CVE-2026-4247 is a high-severity flaw (CVSS 7.5) affecting freebsd. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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 Fre

Why this priority

High severity
Remotely exploitable, no login, no user action

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.1% (top 34%)
CISA exploited list	No
Weakness	CWE-401
Published	2026-03-26
Last modified	2026-06-17

Publisher (CNA): FreeBSD · Vendor: freebsd

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-03-26 07:16 UTCPublished CVE record published
2026-06-17 10:56 UTCUpdated Record last modified

freebsd freebsd MITIGATE

Affected: 14.3 · 14.3 p1 · 14.3 p2 · 14.3 p3 · 14.3 p4 · 14.3 p5 · 14.3 p6 · 14.3 p7

Fixed in: no fixed release known

Safe to downgrade to: a release before 14.3

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-4247?

freebsd: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-4247 being exploited?

CVE-2026-4247 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-4247 affect?

CVE-2026-4247 affects freebsd. Severity is high (CVSS 7.5). 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 pat

How urgent is CVE-2026-4247?

Priority 48/100. High severity; Remotely exploitable, no login, no user action

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References

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