CVE-2026-63374: AnyIO: TLSStream IDNA 2003 host name encoding enables potential TLS certificate spoofing
Impact Services using internationalized (non-ASCII) domain names are potentially vulnerable to TLS connections made from AnyIO's connecttcp() or directly via TLSStream.wrap() where the connection has (through other means) been hijacked and redirected to a malicious server.
Published Sep 18, 2026
Affected packages
anyio
- Ecosystem
- PyPI
- Affected
- < 4.14.2
- Fixed in
- 4.14.2
- Check
pip show anyio
Advisory details
From the GitHub Advisory Database (CC BY 4.0).
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.
References
- https://github.com/agronholm/anyio/security/advisories/GHSA-82r6-8w77-94w6
- https://github.com/agronholm/anyio/pull/1208
- https://github.com/agronholm/anyio/commit/68f58915f82d9be8109ebbbd8f5d70577d43f2ce
- https://github.com/agronholm/anyio/releases/tag/4.14.2
- https://github.com/advisories/GHSA-82r6-8w77-94w6
How this page is made: the data panel comes from the GitHub Advisory Database, CISA’s KEV catalog, the NVD and FIRST EPSS, and updates twice a day. No analysis has been written for this entry yet. Methodology