GitHub Advisoryadvisory · PyPI

GHSA-82r6-8w77-94w6 AnyIO: TLSStream IDNA 2003 host name encoding enables potential TLS certificate spoofing

CRITICAL Published 2026-09-18 Modified 2026-09-18 Official page ↗
Aliases
CVE-2026-63374
Ecosystem
PyPI
Packages
anyio
Severity
Critical
CVSS vector
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
Weaknesses
CWE-295, CWE-297
Reviewed
2026-09-18

Affected packages

• PyPI anyio: < 4.14.2 → fixed in 4.14.2

Description

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.

Related

Alias · CVE (1)

  • CVE-2026-63374 AnyIO is a high level asynchronous concurrency and networking framework that works on top of either Trio or asyncio. vulnerability

Weakness · CWE (2)

  • CWE-295 Improper Certificate Validation weakness
  • CWE-297 Improper Validation of Certificate with Host Mismatch weakness

References