# CVE-2026-81009: io_uring/query: cap user size passed to copy_struct_to_user

> A local attacker can cause a denial of service by requesting excessive memory zeroing in the Linux kernel io_uring query interface.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 11 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-81009, DEBIAN-CVE-2026-81009, GHSA-w44c-cv6c-x94c, UBUNTU-CVE-2026-81009

## What is CVE-2026-81009?

CVE-2026-81009 is a denial of service vulnerability in Linux, published on 11 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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

io_uring/query: cap user size passed to copy_struct_to_user

io_handle_query_entry() clamps hdr.size for the inbound copy_from_user()
but keeps the original user value as usize. copy_struct_to_user() uses
that usize and, when it is larger than the kernel result, clear_user()s
the trailing bytes.

As hdr.size is a __u32, a query can request nearly 4 GiB of zeroing,
including on the error path where res_size stays 0. The interface is
reachable without a ring via IORING_REGISTER_QUERY.

Reject sizes larger than PAGE_SIZE, as recommended for copy_struct_*
interfaces.

## Which versions are affected by CVE-2026-81009?


## How do I fix CVE-2026-81009?

Apply the vendor's patch.

## Is CVE-2026-81009 being exploited?

There is no known exploitation of CVE-2026-81009 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-81009 and fix it if it is affected.

What it is: CVE-2026-81009 is a denial of service vulnerability in Linux, published on 11 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-81009 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-2026-81009/ (Markdown: https://insights.sbomify.com/CVE-2026-81009/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-81009
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-81009
- OSV: https://osv.dev/vulnerability/CVE-2026-81009

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-81009/
Summaries are generated and may contain mistakes; the official description above is the record.