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CVE-2026-63374 — Vulnerability in anyio — CVE Details

CRITICAL

AnyIO: TLSStream IDNA 2003 host name encoding enables potential TLS certificate spoofing

Published September 18, 2026Updated October 1, 2026Source: osv

Summary

### Impact Services using internationalized (non-ASCII) domain names are potentially vulnerable to TLS connections made from AnyIO's `connect_tcp()` or directly via `TLSStream.wrap()` where the connection has (through other means) been hijacked and redirected to a malicious server. The attacker would obtain a legitimate certificate using the IDNA 2003 encoded version of the domain name and offer it to the connecting client, making the certificate validate properly on the client's end. ### Patches The vulnerability will be patched in v4.14.2. ### Workarounds Encode host names via the `idna` package prior to connecting.

Advisory Timeline

  • First published September 18, 2026 (24 days ago).
  • Last revised October 1, 2026 — 13 days after initial publication.

Remediation

Upgrade to the fixed version using your package manager.

pip
Update anyio to 4.14.2 or later
pip install "anyio>=4.14.2"

After upgrading, run your dependency scanner again to confirm the vulnerability is resolved.

Affected Packages (1)

Affects 1 ecosystem: pypi (1).

PackageEcosystemAffectedFixed In
anyio
pypi
1.0.0, 1.0.0a1, 1.0.0a2, 1.0.0b1 (+65 more)4.14.2

Vulnerability Classification

Common Weakness Enumeration (CWE) identifiers for this vulnerability type.

CVSS Score Breakdown

What the CVSS (Common Vulnerability Scoring System) 9.1 score means for each attack dimension.

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
—
Confidentiality
—
Integrity
—
Availability
—

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

References

Frequently Asked Questions

What is CVE-2026-63374?
AnyIO: TLSStream IDNA 2003 host name encoding enables potential TLS certificate spoofing This vulnerability has been assigned a severity rating of CRITICAL (CVSS score: 9.1/10).
How do I check if my project is affected by CVE-2026-63374?
CVE-2026-63374 affects anyio. Use GeekWala's free vulnerability scanner to check your dependencies against CVE-2026-63374 and 200,000+ other known vulnerabilities.

Severity & Exploitability

CVSS Score
9.1

Exploitation is straightforward and causes maximum impact. Patch immediately.

Also Known As

GHSA-82r6-8w77-94w6
PYSEC-2026-4025

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