# CVE-2026-98621: net: gue: reject invalid REMCSUM offsets

> An integer underflow in the Linux kernel GUE protocol allows remote attackers to cause memory corruption and potential code execution via crafted packets.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 18 Oct 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-109034, BELL-CVE-2026-98621, DEBIAN-CVE-2026-98621, GHSA-2cjw-phmm-3745, UBUNTU-CVE-2026-98621

## What is CVE-2026-98621?

CVE-2026-98621 is a vulnerability in Linux, published on 18 Oct 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: gue: reject invalid REMCSUM offsets

The REMCSUM option carries an absolute checksum start and checksum field
offset. gue_remcsum() passes them to skb_remcsum_process(), whose
partial path stores offset - start in the u16 skb->csum_offset variable.
If offset is less than start, this underflows.

A forwarded packet can retain CHECKSUM_PARTIAL and reach a NETIF_F_HW_CSUM
driver which trusts the metadata, leading skb_copy_and_csum_dev() to write
two bytes about 64 KiB beyond the destination buffer.

Reject reversed tuples in validate_gue_flags(), after the existing length
validation, so all GUE parsers enforce the ordering in one place.

## Which versions are affected by CVE-2026-98621?


## How do I fix CVE-2026-98621?

Apply the vendor's patch.

## Is CVE-2026-98621 being exploited?

There is no known exploitation of CVE-2026-98621 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98621 and fix it if it is affected.

What it is: CVE-2026-98621 is a vulnerability in Linux, published on 18 Oct 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-2026-98621 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-2026-98621/ (Markdown: https://insights.sbomify.com/CVE-2026-98621/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98621
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98621
- OSV: https://osv.dev/vulnerability/CVE-2026-98621

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98621/
Summaries are generated and may contain mistakes; the official description above is the record.