Summary
AnyIO process-pool workers can block indefinitely on undrained stderr
Workarounds
Closing sys.stderr in the target function should at least prevent the deadlock.
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.
The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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This vulnerability will be patched in v4.14.2.
Frequently Asked Questions
- What is CVE-2026-64847? CVE-2026-64847 is a medium-severity allocation of resources without limits or throttling vulnerability in anyio (pip), affecting versions < 4.14.2. It is fixed in 4.14.2. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
- Which versions of anyio are affected by CVE-2026-64847? anyio (pip) versions < 4.14.2 is affected.
- Is there a fix for CVE-2026-64847? Yes. CVE-2026-64847 is fixed in 4.14.2. Upgrade to this version or later.
- Is CVE-2026-64847 exploitable, and should I be worried? Whether CVE-2026-64847 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-64847 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-64847? Upgrade
anyioto 4.14.2 or later.