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CVE-2026-64847 — anyio

Fix: agronholm/anyio@f1b7301

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

Also known asGHSA-5p39-cfhj-2xmpPYSEC-2026-4024
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Nov 1, 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.

EPSS Exploitation Probability

via FIRST.org ↗
0.2%probability of exploitation in next 30 days
Lower Risk+0.00%
Lower risk than most CVEs5th percentile — riskier than 5% of all scored CVEsHighest risk
0.00%0.22%0.44%0.66%0.2%0.2%Oct 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

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

    Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

This vulnerability is rated as Moderate severity by Red Hat because triggering the denial of service requires executing worker tasks that generate substantial diagnostic output, impacting application availability without compromising system confidentiality or integrity. Within Red Hat supported products, AnyIO is…

Workaround published by Red Hat
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.
Source: Red Hat security advisory for CVE-2026-64847 (CC BY 4.0)
ProductFixed inAdvisory
Red Hat AI Inference Server 3.2rhaiis/vllm-cuda-rhel9:1790621718RHSA-2026:73929
Red Hat AI Inference Server 3.2rhaiis/vllm-rocm-rhel9:1790621713RHSA-2026:73930
Red Hat AI Inference Server 3.4rhaii/vllm-cpu-rhel9:1790075793RHSA-2026:70965
Red Hat AI Inference Server 3.4rhaii/vllm-spyre-rhel9:1790076141RHSA-2026:70969
Red Hat AI Inference Server 3.4rhaii/vllm-cuda-rhel9:1790090131RHSA-2026:70979
Red Hat AI Inference Server 3.4rhaii/vllm-rocm-rhel9:1790109620RHSA-2026:70995
Red Hat AI Inference Server 3.4rhaii/model-opt-cuda-rhel9:1790076498RHSA-2026:70996

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

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CVE-2026-64847: anyio — Fixed in 4.14.2