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Vulnerability record · CVE-2026-63374 · published 22 September 2026

CVE-2026-63374: Improper certificate validation vulnerability

AnyIO is a high level asynchronous concurrency and networking framework that works on top of either Trio or asyncio. Prior to 4.14.2, connect_tcp() and TLSStream.wrap() can validate internationalized host names after the standard library converts them with IDNA 2003 instead of IDNA 2008. When a connection to a non-ASCII domain is hijacked or redirected, an attacker can obtain a legitimate certificate for the different ASCII hostname produced by IDNA 2003 and present it to the client, causing the malicious endpoint's certificate to validate. This issue is fixed in version 4.14.2.

9.3 CVSS 4.0 Critical EPSS 0.29% · top 80.7% CWE-295 · Improper certificate validationCWE-297 · CWE-297 Awaiting Analysis
9.3CVSS 4.0 base score
0.29%EPSS exploitation probability, 30 days
NoNot in CISA KEV
0Affected product versions listed by NVD
4References
24 Sep 2026Last modified by NVD

Description

AnyIO is a high level asynchronous concurrency and networking framework that works on top of either Trio or asyncio. Prior to 4.14.2, connect_tcp() and TLSStream.wrap() can validate internationalized host names after the standard library converts them with IDNA 2003 instead of IDNA 2008. When a connection to a non-ASCII domain is hijacked or redirected, an attacker can obtain a legitimate certificate for the different ASCII hostname produced by IDNA 2003 and present it to the client, causing the malicious endpoint's certificate to validate. This issue is fixed in version 4.14.2.

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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:

References

LinkTags
https://github.com/agronholm/anyio/commit/68f58915f82d9be8109ebbbd8f5d70577d43f2ce
https://github.com/agronholm/anyio/pull/1208
https://github.com/agronholm/anyio/releases/tag/4.14.2
https://github.com/agronholm/anyio/security/advisories/GHSA-82r6-8w77-94w6

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Source: NIST National Vulnerability Database (record CVE-2026-63374), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.