# CVE-2026-80725: net: gro: properly validate BIG TCP aggregation criteria

> A flaw in Linux kernel GRO allows crafted frames to cause out-of-bounds writes, potentially leading to code execution or denial of service.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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:** 29 Aug 2026
- **Updated:** 16 Oct 2026
- **Also known as:** AZL-98223, BELL-CVE-2026-80725, DEBIAN-CVE-2026-80725, GHSA-gvcm-7jx2-fwf2, UBUNTU-CVE-2026-80725

## What is CVE-2026-80725?

CVE-2026-80725 is a critical memory corruption vulnerability in Linux, published on 29 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:

net: gro: properly validate BIG TCP aggregation criteria

When GRO attempts to aggregate packets beyond GRO_LEGACY_MAX_SIZE (64KB),
BIG TCP should only be permitted for plain IPv4 TCP and plain IPv6 TCP
(with sufficient MAC header room to insert the temporary HBH jumbo header).

However, commit b1a78b9b9886 ("net: add support for ipv4 big tcp")
loosened the check in skb_gro_receive(), leading to several issues:

1. skb_gro_receive() checked skb_headroom(p) instead of the actual space
   before the MAC header (p->mac_header). Because skb_headroom(p) includes
   mac_len, crafted frames (e.g. injected via AF_PACKET) can pass the check
   with p->mac_header < 8 bytes. When ipv6_gro_complete() inserts the
   temporary HBH jumbo header, the memmove() starts before skb->head,
   causing an out-of-bounds write and wrapping skb->mac_header.
2. It allowed non-IP protocols such as software VLAN (ETH_P_8021Q /
   ETH_P_8021AD) to aggregate beyond 64KB because
   p->protocol != ETH_P_IPV6 was true.
3. It checked p->encapsulation instead of NAPI_GRO_CB(skb)->encap_mark,
   allowing encapsulated flows (e.g. SIT / IPv6-in-IPv4) to aggregate
   beyond 64KB.

Fix skb_gro_receive() to strictly enforce:
- NAPI_GRO_CB(skb)->proto == IPPROTO_TCP
- Not encapsulated (!NAPI_GRO_CB(skb)->encap_mark && !p->encapsulation)
- Protocol must be either ETH_P_IP or ETH_P_IPV6
- If ETH_P_IPV6, p->mac_header must be at least
  sizeof(struct hop_jumbo_hdr)

Returning -E2BIG from skb_gro_receive() ensures that packets which cannot
become BIG TCP are cleanly flushed at <= 64KB and delivered intact without
dropping.

This issue does not exist in mainline (7.0+) because the subsystem was
rewritten in commit 81be30c1f5f2 ("net/ipv6: Drop HBH for BIG TCP on RX
side"), making this fix relevant only for older stable branches like
6.18.y.

## Which versions are affected by CVE-2026-80725?


## How do I fix CVE-2026-80725?

Apply the vendor's patch.

## Is CVE-2026-80725 being exploited?

There is no known exploitation of CVE-2026-80725 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-80725 and fix it if it is affected.

What it is: CVE-2026-80725 is a critical memory corruption vulnerability in Linux, published on 29 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-2026-80725 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-2026-80725/ (Markdown: https://insights.sbomify.com/CVE-2026-80725/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-80725
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-80725
- OSV: https://osv.dev/vulnerability/CVE-2026-80725

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-80725/
Summaries are generated and may contain mistakes; the official description above is the record.