CVE-2026-64847: Anyio AnyIO Process-pool Workers Can Block Vulnerability

MODERATE
Advisory Identifier: GHSA-5p39-cfhj-2xmp
Share
CWE-770
Published
Advisory Summary

CVE-2026-64847 is a MODERATE anyio process-pool workers can block vulnerability (CWE-770: Allocation of Resources Without Limits or Throttling) affecting Anyio libraries in the PyPI ecosystem. The issue involves anyio process-pool workers can block indefinitely on undrained stderr. Remediation is available by upgrading affected packages to version 4.14.2.

CVE-2026-64847 at a Glance

CVE Identifier CVE-2026-64847
GHSA Identifier GHSA-5p39-cfhj-2xmp
Severity MODERATE
CVSS v3.1 Base Score N/A
CWE Classification CWE-770: Allocation of Resources Without Limits or Throttling
Affected Product Anyio
Package Ecosystem PyPI
Published Date
Last Updated

How to Fix CVE-2026-64847

To remediate this vulnerability, upgrade all affected Anyio dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

anyio fixed: 4.14.2 
pip install anyio>=4.14.2
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show anyio

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-64847, inspect your installed packages using pip:

pip show anyio

Analyze the terminal output to locate any version of Anyio in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 4.14.2), your application is affected.

Note: The pip show anyio command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-770: Allocation of Resources Without Limits or Throttling, where the product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated. In Anyio, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### 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.

CVE-2026-64847 Affected Anyio Versions

1 component(s)
Fixed Versions:
4.14.2
Product / Package Ecosystem Affected Versions Fixed Version
anyio
pypi/anyio
PyPI
< 4.14.2
4.14.2

References

GitHub Advisory / Wiki ADVISORY
https://github.com/agronholm/anyio/security/advisories/GHSA-5p39-cfhj-2xmp
Anyio Source Repository REPOSITORY
https://github.com/agronholm/anyio/pull/1207
Security Fix Commit FIX COMMIT
https://github.com/agronholm/anyio/commit/f1b7301c8264b0d2e8d24a5788fd29e93dea4040
Anyio Source Repository REPOSITORY
https://github.com/agronholm/anyio
Anyio Release RELEASE
https://github.com/agronholm/anyio/releases/tag/4.14.2

Frequently Asked Questions

What is CVE-2026-64847?
CVE-2026-64847 is a security vulnerability in Anyio involving AnyIO process-pool workers can block indefinitely on undrained stderr under CWE-770: Allocation of Resources Without Limits or Throttling.
Which Anyio versions are affected?
Affected versions include releases prior to 4.14.2 across affected packages: anyio.
What version fixes CVE-2026-64847?
Upgrading to 4.14.2 (or the latest patched release for your respective branch) resolves the vulnerability.
Is anyio affected?
Yes, anyio is affected in releases prior to 4.14.2. Upgrade your dependency to 4.14.2 or later.
What CWE is associated with CVE-2026-64847?
CVE-2026-64847 is classified under CWE-770: Allocation of Resources Without Limits or Throttling (The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.).
How can I check PyPI dependencies for CVE-2026-64847?
Run the dependency tree command: 'pip show anyio'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-5p39-cfhj-2xmp",
  "modified" : "2026-09-18T17:17:10Z",
  "published" : "2026-09-18T17:17:10Z",
  "aliases" : [ "CVE-2026-64847" ],
  "summary" : "AnyIO process-pool workers can block indefinitely on undrained stderr",
  "details" : "### Impact\nAnyIO 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.\n\nAnyone who runs untrusted or faulty code that writes too much to `stderr` is at risk.\n\n### Patches\nThis vulnerability will be patched in v4.14.2.\n\n### Workarounds\nClosing `sys.stderr` in the target function should at least prevent the deadlock.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "anyio"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "4.14.2"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/agronholm/anyio/security/advisories/GHSA-5p39-cfhj-2xmp"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/agronholm/anyio/pull/1207"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/agronholm/anyio/commit/f1b7301c8264b0d2e8d24a5788fd29e93dea4040"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/agronholm/anyio"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/agronholm/anyio/releases/tag/4.14.2"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-770" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-18T17:17:10Z",
    "nvd_published_at" : null
  }
}