AnyIO process-pool workers can block indefinitely on undrained stderr
CVE-2026-64847 (GitHub advisory GHSA-5p39-cfhj-2xmp) is a medium-severity vulnerability with a CVSS 4.0 base score of 6.8, classed as Allocation of Resources Without Limits or Throttling (CWE-770), in anyio (PyPI). Affected versions: < 4.14.2. It is fixed in 4.14.2.
Published 18 Sept 2026Updated 18 Sept 2026
- CVE ID
- CVE-2026-64847
- GHSA ID
- GHSA-5p39-cfhj-2xmp
- EPSS
- 0.12%higher than 2% of CVEs
- Weakness (CWE)
- CWE-770Allocation of Resources Without Limits or Throttling
- KEV status
- Not in KEV
- Published
- 18 Sept 2026
- Updated
- 18 Sept 2026
- Ecosystem
- PyPI
- Assigned by (CNA)
- GitHub_M
Summary
Source: OSV (GitHub advisory)
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.
Technical details
Source: GitHub Advisory Database
CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:NMedium6.8Base score from GitHub Advisory Database
- Attack Vector AV
- Local
- Attack Complexity AC
- Low
- Attack Requirements AT
- None
- Privileges Required PR
- Low
- User Interaction UI
- None
- Confidentiality (Vulnerable System) VC
- None
- Integrity (Vulnerable System) VI
- None
- Availability (Vulnerable System) VA
- High
- Confidentiality (Subsequent Systems) SC
- None
- Integrity (Subsequent Systems) SI
- None
- Availability (Subsequent Systems) SA
- None
CISA SSVC decision points
- Exploitation
- none
- No public exploitation evidence
- Automatable
- no
- Needs manual steps per target
- Technical impact
- partial
- Limited control of the affected system
CISA Coordinator assessment (SSVC 2.0.3) via CVE.org.
Affected packages
Source: OSV (GitHub advisory)
- anyioPyPI
pkg:pypi/anyio- Affected versions
- < 4.14.2
- Fixed in
- 4.14.2
Show 69 listed versions
- 1.0.0
- 1.0.0a1
- 1.0.0a2
- 1.0.0b1
- 1.0.0b2
- 1.0.0rc1
- 1.0.0rc2
- 1.1.0
- 1.2.0
- 1.2.1
- 1.2.2
- 1.2.3
| Package | Affected versions | Fixed in |
|---|---|---|
anyio PyPI pkg:pypi/anyio |
Show 69 listed versions
|
|
Fix
Source: OSV (GitHub advisory)
Fix available
Upgrade to 4.14.2 or later.
Patch commits
Fixed stderr writes in a worker subprocess causing a deadlock (#1207)
+31 −3 across 3 files
- docs/versionhistory.rst+6 −3
- src/anyio/to_process.py+1 −0
- tests/test_to_process.py+24 −0
Release commits
The commits each fixed release tag points at. They usually bump the version rather than contain the fix itself.
Vulnerability intelligence
Root cause analysis
Vulnerable functions
References
Fixes 2
Advisories 4
Package 1
Other links 1
Questions about CVE-2026-64847
- What is the GHSA ID for CVE-2026-64847?
- GitHub tracks CVE-2026-64847 as GHSA-5p39-cfhj-2xmp.
- What is the CVSS score of CVE-2026-64847?
- CVE-2026-64847 has a CVSS 4.0 base score of 6.8 (Medium), 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.
- Which versions are affected by CVE-2026-64847?
- anyio (PyPI): < 4.14.2.
- How do I fix CVE-2026-64847?
- Upgrade to 4.14.2 or later. The fix is in commit f1b7301 and pull request #1207.
- What is the Package URL (PURL) for CVE-2026-64847?
- The affected package's PURL is pkg:pypi/anyio.
- Is CVE-2026-64847 exploited in the wild?
- CISA's Known Exploited Vulnerabilities catalog doesn't list CVE-2026-64847. That means no confirmed exploitation has been catalogued, not that it is safe to leave unpatched. CISA's SSVC assessment rates exploitation as "none". EPSS estimates a 0.12% chance of exploitation in the next 30 days, higher than 2% of CVEs.
Data sources
Upstream records
- OSV (GitHub advisory)LoadedRaw JSON
- GitHub Advisory DatabaseLoadedRaw JSON
- OSV (CVE record)LoadedRaw JSON
- CVE.orgLoaded
- CISA KEV catalogLoaded
Fetched 4 days ago (9 Oct 2026, 18:09 UTC). Records refresh automatically after 24 hours.
Where each part of this page came from
- Title and description
- OSV (GitHub advisory)
- Severity score
- GitHub Advisory Database
- Weakness (CWE)
- GitHub Advisory Database
- Affected packages
- OSV (GitHub advisory)
- Fix
- OSV (GitHub advisory)
- References
- Computed by CVEminer
- SSVC
- CVE.org
- KEV status
- CISA KEV catalog
- EPSS
- GitHub Advisory Database
OSV and GitHub Advisory Database content is licensed CC BY 4.0. Values marked as computed or derived were calculated by CVEminer from the upstream data.