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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

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 · score source

EPSS

• CVE-2026-63374: 0.29% estimated exploitation activity in the next 30 days; percentile 21.53 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact Services using internationalized (non-ASCII) domain names are potentially vulnerable to TLS connections made from AnyIO's connecttcp() 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.

Published by the source

18 Sep 2026, 17:17 UTC

Last modified by the source

1 Oct 2026, 17:56 UTC

Source last checked

26 Oct 2026, 13:52 UTC

1. Revision 4 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-82r6-8w77-94w6

2. Revision 3 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-82r6-8w77-94w6

4. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-82r6-8w77-94w6

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# Patch Next advisory brief: CVE-2026-63374

Advisory: AnyIO: TLSStream IDNA 2003 host name encoding enables potential TLS certificate spoofing
IDs: CVE-2026-63374, GHSA-82r6-8w77-94w6
Record: https://patchnext.com/advisories/cve-2026-63374 (revision 4, source checked 26 Oct 2026, 13:52 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: anyio 4.14.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI anyio: affected < 4.14.2; fixed in 4.14.2

## Suggested inspection (read-only)
- `python -m pip show anyio`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on anyio.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-82r6-8w77-94w6 (GitHub-reviewed advisory GHSA-82r6-8w77-94w6 lists affected ranges: PyPI anyio >=0 <4.14.2)
- Verified: https://osv.dev/vulnerability/GHSA-82r6-8w77-94w6 (OSV record GHSA-82r6-8w77-94w6 lists affected ranges: PyPI anyio >=0 <4.14.2 (fixed))

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