# CVE-2026-4247: TCP: remotely exploitable DoS vector (mbuf leak)

> Remote attackers can cause a denial of service on FreeBSD systems by leaking memory through crafted TCP packets that trigger a challenge ACK logic error.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 26 Mar 2026
- **Updated:** 26 Mar 2026
- **Also known as:** GHSA-374j-38xh-4wr2

## What is CVE-2026-4247?

CVE-2026-4247 is a high-severity denial of service vulnerability in Freebsd Freebsd, published on 26 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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.

Weakness: CWE-401 (Missing Release of Memory after Effective Lifetime)

## Which versions are affected by CVE-2026-4247?

It affects 1 product.


Products: Freebsd Freebsd.

## How do I fix CVE-2026-4247?

No fix has been published for CVE-2026-4247 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-4247 being exploited?

There is no known exploitation of CVE-2026-4247 in the wild and no public exploit code.

## Other vulnerabilities in Freebsd Freebsd

- [CVE-2018-17160](https://insights.sbomify.com/CVE-2018-17160/): Memory corruption in FreeBSD (critical 10.0)
- [CVE-2014-3954](https://insights.sbomify.com/CVE-2014-3954/): Memory corruption in n/a (critical 10.0)
- [CVE-2006-4304](https://insights.sbomify.com/CVE-2006-4304/): Memory corruption in n/a (critical 10.0)
- [CVE-2006-0226](https://insights.sbomify.com/CVE-2006-0226/): Integer overflow in n/a (critical 10.0)
- [CVE-2004-0002](https://insights.sbomify.com/CVE-2004-0002/): Denial of service in n/a (critical 10.0)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-4247 and fix it if it is affected.

What it is: CVE-2026-4247 is a high-severity denial of service vulnerability in Freebsd Freebsd, published on 26 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-4247 in the wild and no public exploit code.

Affected product: Freebsd Freebsd. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-4247/ (Markdown: https://insights.sbomify.com/CVE-2026-4247/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-4247
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-4247
- OSV: https://osv.dev/vulnerability/CVE-2026-4247

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-4247/
Summaries are generated and may contain mistakes; the official description above is the record.