---
title: "CVE-2026-63374: AnyIO: TLSStream IDNA 2003 host name encoding enables potential TLS certificate spoofing"
url: https://erroneo.us/security/cve/CVE-2026-63374/
description: "CVE-2026-63374 (anyio (PyPI)): AnyIO is a high level asynchronous concurrency and networking framework that works on top of either Trio or asyncio. Prior to…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-09-22
date_modified: 2026-09-28
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-63374"
kev: false
cvss: 9.3
epss: 0.00289
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-63374: AnyIO: TLSStream IDNA 2003 host name encoding enables potential TLS certificate spoofing

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
- EPSS 0.3% , percentile 20: FIRST’s model estimates a 0.3% chance of exploitation activity in the next 30 days, higher than 20% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes anyio (PyPI) < 4.14.2, fixed in 4.14.2 . Check your lockfile, including transitive dependencies.

## What it is

AnyIO is a high level asynchronous concurrency and networking framework that works on top of either Trio or asyncio. Prior to 4.14.2, connecttcp() 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.

## Affected packages

- PyPI `anyio` < 4.14.2 → fixed: 4.14.2

## Affected products (CVE record)

- agronholm anyio: < 4.14.2

## Severity

- GitHub_M CVSS 4.0: 9.3 `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`
- GitHub CVSS 4.0: 9.3 `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`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation none , automatable yes , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.29% probability of exploitation activity in the next 30 days; percentile 20.

## Sources

- [GitHub Advisory GHSA-82r6-8w77-94w6](https://github.com/advisories/GHSA-82r6-8w77-94w6)
- [CVE.org record CVE-2026-63374](https://www.cve.org/CVERecord?id=CVE-2026-63374)
- [NVD entry CVE-2026-63374](https://nvd.nist.gov/vuln/detail/CVE-2026-63374)
- [github.com](https://github.com/agronholm/anyio/security/advisories/GHSA-82r6-8w77-94w6)
- [github.com](https://github.com/agronholm/anyio/pull/1208)
- [github.com](https://github.com/agronholm/anyio/commit/68f58915f82d9be8109ebbbd8f5d70577d43f2ce)
- [github.com](https://github.com/agronholm/anyio/releases/tag/4.14.2)

---
Source page: https://erroneo.us/security/cve/CVE-2026-63374/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt