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.
- 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
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
Which versions are affected by CVE-2026-64847?
3 open source packages ship an affected version, including anyio.
| Package | 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.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-64847 being exploited?
There is no known exploitation of CVE-2026-64847 in the wild and no public exploit code.
Sources
Also known as CLEANSTART-2026-HN56879, CLEANSTART-2026-XS44337, CLEANSTART-2026-XX95180, DEBIAN-CVE-2026-64847, ECHO-b8cf-50b8-5b03, GHSA-5p39-cfhj-2xmp and 25 more.