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CVE-2026-64847 — anyio

Fix: agronholm/anyio#1207

CVE-2026-64847 is a CWE-770 vulnerability in anyio. A fix is available for anyio — see the affected versions and patch details below.

AnyIO process-pool workers can block indefinitely on undrained stderr

Published
Sep 18, 2026
Updated
Sep 18, 2026
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Sep 18, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-64847.

Real-World Exposure

1 pkg affected
🐍anyio

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

Impact

AnyIO starts process-pool workers with stderr connected to a pipe but never drains that pipe. The worker redirects stdin and stdout to /dev/null to protect its protocol, but does not redirect stderr even though the documentation says all three standard streams are redirected. Worker code that writes enough attacker-influenced data to stderr can fill the pipe, block before returning the stdout protocol response, and wedge the awaiting process-pool call.

Anyone who runs untrusted or faulty code that writes too much to stderr is at risk.

Patches

This vulnerability will be patched in v4.14.2.

Workarounds

Closing sys.stderr in the target function should at least prevent the deadlock.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPIanyioall versions4.14.2pip install --upgrade 'anyio==4.14.2'

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for anyio, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update anyio to 4.14.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-64847 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

  4. How O3 protects you

    O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-64847 can be triaged on real exposure rather than presence alone.

Tailored to CVE-2026-64847. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

### Impact AnyIO starts process-pool workers with stderr connected to a pipe but never drains that pipe. The worker redirects stdin and stdout to /dev/null to protect its protocol, but does not redirect stderr even though the documentation says all three standard streams are redirected. Worker code that writes enough attacker-influenced data to stderr can fill the pipe, block before returning the stdout protocol response, and wedge the awaiting process-pool call. Anyone who runs untrusted or faulty code that writes too much to `stderr` is at risk. ### Patches This vulnerability will be patch
O3 Security · Impact-Aware SCA

Is CVE-2026-64847 in your dependencies?

O3 Security finds CVE-2026-64847 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.

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CVE-2026-64847: anyio | O3 Security