# CVE-2026-64847: AnyIO process-pool workers can block indefinitely on undrained stderr

> Attackers can cause indefinite blocking in AnyIO process-pool workers by filling the undrained stderr pipe, leading to denial of service.

- **Severity:** Medium 6.8 (`CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`)
- **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:** 18 Sep 2026
- **Updated:** 18 Sep 2026
- **Also known as:** CLEANSTART-2026-HN56879, CLEANSTART-2026-XS44337, CLEANSTART-2026-XX95180, DEBIAN-CVE-2026-64847, ECHO-b8cf-50b8-5b03, GHSA-5p39-cfhj-2xmp, MINI-2grf-8x39-gfrj, MINI-49jh-j7h3-8x5f, MINI-4mf7-w85g-ww48, MINI-4qh2-qpwp-pjh5, MINI-6rxq-v998-f99v, MINI-7mmh-wchh-vrrc, MINI-92h2-8g2v-xp3c, MINI-f6cw-2wh3-m2g5, MINI-g95j-wgqv-hch9, MINI-gx2v-vq6x-ppv6, MINI-hqxp-mm4m-cqgv, MINI-j2hg-9hjq-j6r8, MINI-jcx6-hr7x-j59f, MINI-m54j-mr42-p596, MINI-mj4h-2c2m-mxm5, MINI-mj8x-jgjh-qq57, MINI-mpxx-p8j5-3f59, MINI-mwv2-q4cf-2rwh, MINI-pqq3-22qg-6r73, MINI-rgm8-3xw7-2wxm, MINI-rpr8-457v-wxr9, MINI-v4h3-mjr7-ww4m, MINI-x8qr-j6vw-cjmx, PYSEC-2026-4024, UBUNTU-CVE-2026-64847

## What is CVE-2026-64847?

CVE-2026-64847 is a medium-severity denial of service vulnerability in agronholm anyio, published on 18 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

AnyIO is a high level asynchronous concurrency and networking framework that works on top of either Trio or asyncio. Prior to 4.14.2, AnyIO starts process-pool workers with standard error connected to a pipe that the parent never drains, even though the documented behavior redirects all three standard streams. Worker code that writes enough attacker-influenced data to sys.stderr can fill the pipe and block before returning the standard-output protocol response, causing the awaiting process-pool call to remain blocked indefinitely. Applications that run untrusted or faulty worker code capable of producing substantial standard-error output are affected. This issue is fixed in version 4.14.2.

Weakness: CWE-770 (Allocation of Resources Without Limits or Throttling)

## Which versions are affected by CVE-2026-64847?

3 open source packages ship an affected version, including anyio.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `anyio` | PyPI | All versions | 4.14.2 |
| `anyio` | PyPI | All versions | 4.14.2 |
| `anyio` | PyPI | All versions | 4.14.2 |

## How do I fix CVE-2026-64847?

Upgrade anyio to 4.14.2 or later.

## Is CVE-2026-64847 being exploited?

There is no known exploitation of CVE-2026-64847 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-64847 and fix it if it is affected.

What it is: CVE-2026-64847 is a medium-severity denial of service vulnerability in agronholm anyio, published on 18 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-2026-64847 in the wild and no public exploit code.

Affected packages and fixed versions:
- anyio (PyPI): affected all versions; fixed in 4.14.2
- anyio (PyPI): affected all versions; fixed in 4.14.2
- anyio (PyPI): affected all versions; fixed in 4.14.2

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-2026-64847/ (Markdown: https://insights.sbomify.com/CVE-2026-64847/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-64847
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-64847
- OSV: https://osv.dev/vulnerability/CVE-2026-64847

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-64847/
Summaries are generated and may contain mistakes; the official description above is the record.