CVE-2026-63374

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

Published Sep 22, 2026 CVSS 4.0 · 9.3 Critical EPSS < 1% KEV · Not listed
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.

Remediation

No solution or workaround provided in the CVE record

Tracking

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Advisories

Source ID Title
Github GHSA Github GHSA GHSA-82r6-8w77-94w6 AnyIO: TLSStream IDNA 2003 host name encoding enables potential TLS certificate spoofing
History
Type Values Removed Values Added
References
Metrics threat_severity

None

cvssV3_1

{'score': 7.4, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N'}

threat_severity

Moderate

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'yes', 'Exploitation': 'none', 'Technical Impact': 'total'}, 'version': '2.0.3'}

Type Values Removed Values Added
First Time appeared Agronholm
Agronholm anyio
Vendors & Products Agronholm
Agronholm anyio
Type Values Removed Values Added
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.
Title AnyIO: TLSStream IDNA 2003 host name encoding enables potential TLS certificate spoofing
Weaknesses CWE-295
CWE-297
References
Metrics cvssV4_0

{'score': 9.3, '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

Sources

Status
PUBLISHED
Assigner
GitHub_M
Published
Updated
Sep 28, 2026, 4:32 PM
Reserved
Jul 16, 2026, 5:39 PM
Status
Awaiting Analysis
Published
Sep 22, 2026, 4:17 PM
Modified
Sep 28, 2026, 5:17 PM
Severity
Moderate
Public date
Sep 22, 2026, 3:23 PM