# CVE-2026-72502: tcp: ipv6: clamp default adverting MSS to avoid GSO_BY_FRAGS (0xFFFF)

> A local attacker can trigger a NULL pointer dereference in the Linux kernel TCP stack by exploiting large MTU values, causing a system crash.

- **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:** Yes
- **Published:** 15 Aug 2026
- **Updated:** 16 Oct 2026
- **Also known as:** BELL-CVE-2026-72502, DEBIAN-CVE-2026-72502, GHSA-cg8c-jghv-84p9, UBUNTU-CVE-2026-72502

## What is CVE-2026-72502?

CVE-2026-72502 is a high-severity vulnerability in Linux, published on 15 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

tcp: ipv6: clamp default adverting MSS to avoid GSO_BY_FRAGS (0xFFFF)

When MTU is large, ip6_default_advmss() can return IPV6_MAXPLEN (65535).
This is interpreted by TCP as mss_clamp, allowing the MSS to reach 65535.

However, 0xFFFF is also used as a magic value GSO_BY_FRAGS in the kernel.
If a TCP packet with gso_size=0xFFFF is passed to skb_segment(), it will
be mistakenly treated as GSO_BY_FRAGS, leading to a NULL pointer
dereference because local TCP packets do not use frag_list.

Fix this by returning min(IPV6_MAXPLEN, GSO_BY_FRAGS - 1) (65534) from
ip6_default_advmss() when MTU is large.

Also update the stale comment in ip6_default_advmss() which suggested
that IPV6_MAXPLEN is returned to mean "any MSS".

## Which versions are affected by CVE-2026-72502?


## How do I fix CVE-2026-72502?

Apply the vendor's patch.

## Is CVE-2026-72502 being exploited?

There is no known exploitation of CVE-2026-72502 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-72502 and fix it if it is affected.

What it is: CVE-2026-72502 is a high-severity vulnerability in Linux, published on 15 Aug 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-2026-72502 in the wild and no public exploit code.

Affected product: Linux. 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-2026-72502/ (Markdown: https://insights.sbomify.com/CVE-2026-72502/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-72502
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-72502
- OSV: https://osv.dev/vulnerability/CVE-2026-72502

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-72502/
Summaries are generated and may contain mistakes; the official description above is the record.