# CVE-2026-99203: pipe: preserve existing fasync readers on writer setup failure


- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 28 Oct 2026
- **Updated:** 28 Oct 2026
- **Also known as:** AZL-111365, BELL-CVE-2026-99203, DEBIAN-CVE-2026-99203, GHSA-j875-7x9h-g92w, UBUNTU-CVE-2026-99203

## What is CVE-2026-99203?

CVE-2026-99203 is a vulnerability in Linux, published on 28 Oct 2026. 

### Official description

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

pipe: preserve existing fasync readers on writer setup failure

F_SETFL and FIOASYNC decide whether to invoke ->fasync() before
pipe_fasync() takes pipe->mutex. Two enables of one O_RDWR FIFO can
therefore both enter the callback.

fasync_helper() allocates before checking for an existing entry. If the
first enable installs both entries, a second enable can update the
reader entry and then fail the allocation for the writer entry. The
error path removes the pre-existing reader entry, clearing FASYNC while
the writer entry remains registered. __fput() then skips ->fasync(), so a
later pipe notification can dereference the freed file or its f_owner.
An unprivileged memory-pressure reproducer triggers a KASAN
slab-use-after-free in send_sigio() through this path.

Keep the reader helper's result and undo the registration only if this
call added it. The pipe mutex serializes changes to both lists, so a
failed new registration still removes its reader, while a failed
duplicate preserves the existing registrations and FASYNC. Successful
duplicate enables continue to update fa_fd as before.

## Which versions are affected by CVE-2026-99203?


## How do I fix CVE-2026-99203?

Upgrade to a fixed version.

## Is CVE-2026-99203 being exploited?

There is no known exploitation of CVE-2026-99203 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-99203 and fix it if it is affected.

What it is: CVE-2026-99203 is a vulnerability in Linux, published on 28 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-99203 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-2026-99203/ (Markdown: https://insights.sbomify.com/CVE-2026-99203/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-99203
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-99203
- OSV: https://osv.dev/vulnerability/CVE-2026-99203

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-99203/
Summaries are generated and may contain mistakes; the official description above is the record.