# 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.

- **Severity:** Critical 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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 22 Sep 2026
- **Updated:** 28 Sep 2026
- **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, MINI-3mx4-w5qp-h38g, MINI-4279-xmgr-pccq, MINI-4m54-p583-5g35, MINI-685f-9w8v-6j23, MINI-8wr4-72w4-5v7j, MINI-c9jc-c5qq-97m4, MINI-f2f3-qj9x-26mq, MINI-fr4g-x4fc-c768, MINI-gxfj-6xff-g4gp, MINI-hgmg-7v9c-hjxh, MINI-j6x2-cwg7-8p5v, MINI-jhrv-99pw-xf4q, MINI-jjrg-hgvr-p9jc, MINI-mfc4-7rh5-5vfw, MINI-pggj-m5h2-5pcv, MINI-q4g7-v7pq-58mw, MINI-qfxw-p8c6-c27m, MINI-r494-6f7q-r4c7, MINI-rf9q-8h25-xmh9, MINI-v37w-mwvp-fgv7, MINI-vjmr-m746-m35g, MINI-x732-g48r-whpv, PYSEC-2026-4025, ROOT-APP-PYPI-CVE-2026-63374, UBUNTU-CVE-2026-63374, openSUSE-SU-2026:12324-1

## 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.

Weakness: CWE-297 (Improper Validation of Certificate with Host Mismatch), CWE-295 (Improper Certificate Validation)

## Which versions are affected by CVE-2026-63374?

3 open source packages ship an affected version, including anyio.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `anyio` | PyPI | All versions | 4.14.2 |
| `anyio` | PyPI | All versions | 4.14.2 |
| `anyio` | PyPI | All versions | 4.14.2 |
| `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.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63374 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-63374 in the wild and no public exploit code.

Affected packages and fixed versions:
- anyio (PyPI): affected all versions; fixed in 4.14.2
- anyio (PyPI): affected all versions; fixed in 4.14.2
- anyio (PyPI): affected all versions; fixed in 4.14.2

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-63374/ (Markdown: https://insights.sbomify.com/CVE-2026-63374/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63374
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63374
- OSV: https://osv.dev/vulnerability/CVE-2026-63374

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63374/
Summaries are generated and may contain mistakes; the official description above is the record.