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

> CVE-2026-63374 (GitHub advisory GHSA-82r6-8w77-94w6) is a critical-severity vulnerability with a CVSS 4.0 base score of 9.3, classed as Improper Certificate Validation (CWE-295), in anyio (PyPI). Affected versions: < 4.14.2. It is fixed in 4.14.2.

Canonical page: https://cveminer.com/vulnerabilities/CVE-2026-63374

| Field | Value |
| --- | --- |
| CVE | CVE-2026-63374 |
| GHSA | GHSA-82r6-8w77-94w6 |
| Severity | Critical |
| 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 Advisory Database) |
| CWE | CWE-295 Improper Certificate Validation; CWE-297 Improper Validation of Certificate with Host Mismatch |
| CISA KEV | Not listed |
| Published | 18 Sept 2026 |
| Modified | 18 Sept 2026 |

## Description

##### Impact
Services using internationalized (non-ASCII) domain names are potentially vulnerable to TLS connections made from AnyIO's `connect_tcp()` or directly via `TLSStream.wrap()` where the connection has (through other means) been hijacked and redirected to a malicious server. The attacker would obtain a legitimate certificate using the IDNA 2003 encoded version of the domain name and offer it to the connecting client, making the certificate validate properly on the client's end.

##### Patches
The vulnerability will be patched in v4.14.2.

##### Workarounds
Encode host names via the `idna` package prior to connecting.

## Affected packages

| Package | Ecosystem | PURL | Vulnerable versions | Fixed in |
| --- | --- | --- | --- | --- |
| anyio | PyPI | `pkg:pypi/anyio` | < 4.14.2 | 4.14.2 |

## Fix commits

- Patch: https://github.com/agronholm/anyio/commit/68f58915f82d9be8109ebbbd8f5d70577d43f2ce
- Pull request: https://github.com/agronholm/anyio/pull/1208

## Questions

**What is the GHSA ID for CVE-2026-63374?**

GitHub tracks CVE-2026-63374 as GHSA-82r6-8w77-94w6.

**What is the CVSS score of CVE-2026-63374?**

CVE-2026-63374 has a CVSS 4.0 base score of 9.3 (Critical), 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.

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

anyio (PyPI): < 4.14.2.

**How do I fix CVE-2026-63374?**

Upgrade to 4.14.2 or later. The fix is in commit 68f5891 and pull request #1208.

**What is the Package URL (PURL) for CVE-2026-63374?**

The affected package's PURL is pkg:pypi/anyio.

**Is CVE-2026-63374 exploited in the wild?**

CISA's Known Exploited Vulnerabilities catalog doesn't list CVE-2026-63374. That means no confirmed exploitation has been catalogued, not that it is safe to leave unpatched.

## References

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

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Source: CVEminer (https://cveminer.com/vulnerabilities/CVE-2026-63374), compiled from OSV (GitHub advisory), GitHub Advisory Database, CISA KEV catalog. OSV and GitHub Advisory Database content is licensed CC BY 4.0.