CVE-2026-63374 AnyIO: TLSStream IDNA 2003 host name encoding enables potential TLS certificate spoofing
Attackers can spoof TLS certificates for internationalized domains by exploiting IDNA 2003 encoding differences in AnyIO's connection validation.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-63374?
CVE-2026-63374 is a critical vulnerability in agronholm anyio, published on 22 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-63374?
3 open source packages ship an affected version, including anyio.
| Package | Affected | Fixed in |
|---|---|---|
anyio PyPI | All versions | 4.14.2 |
anyio PyPI | All versions | 4.14.2 |
anyio PyPI | All versions | 4.14.2 |
Linux distribution packages (1)
graphifyy openSUSE:Tumbleweed | All versions | 0.9.97-1.1 |
How do I fix CVE-2026-63374?
Upgrade anyio to 4.14.2 or later.
Is CVE-2026-63374 being exploited?
There is no known exploitation of CVE-2026-63374 in the wild and no public exploit code.
Sources
Also known as CLEANSTART-2026-YH72790, CLEANSTART-2026-YN00002, DEBIAN-CVE-2026-63374, ECHO-d875-04b9-4f82, GHSA-82r6-8w77-94w6, MINI-35v9-r4fw-2jmc and 26 more.