CVE-2026-63374: AnyIO TLSStream IDNA 2003 host name encoding certificate spoofing

CVE-2026-63374 · Severity: critical · CVSS 4 · Published 2026-09-22

AnyIO is a Python library for asynchronous network I/O. When applications connect to internationalized domain names (non-ASCII characters), AnyIO improperly validates TLS certificates using outdated IDNA 2003 encoding instead of modern standards. An attacker who redirects network traffic can present a valid certificate for the IDNA-encoded domain name, causing the client to accept a spoofed connection. This allows attackers to intercept encrypted communication and steal sensitive data like credentials or session tokens.

Technologies: anyio (PyPI). Vendors: PyPI.

Quick answers

Is CVE-2026-63374 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-63374 being exploited in the wild.
How severe is CVE-2026-63374?
Critical severity, with a CVSS score of 4, published 22 September 2026.
Which products and versions are affected?
anyio (PyPI) < 4.14.2, ECOSYSTEM:introduced=0,fixed=4.14.2.
Is there a fix for CVE-2026-63374?
Yes. anyio (PyPI): upgrade to 4.14.2 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-63374 attacked?
It is attacked over the network.
Am I affected by CVE-2026-63374?
Check whether you run anyio (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • anyio (PyPI): upgrade to 4.14.2 or later
    pip install "anyio>=4.14.2"

Technical details

The vulnerability exists in AnyIO's `connect_tcp()` and `TLSStream.wrap()` methods, which use IDNA 2003 encoding for internationalized domain names during TLS certificate validation. This creates a mismatch between the hostname the user intends to connect to and the hostname validated against the certificate. An attacker who achieves network-level hijacking (e.g., via DNS spoofing, ARP poisoning, or BGP hijacking) can obtain a legitimate TLS certificate for the IDNA 2003-encoded version of the domain and present it during the handshake. The client will validate the certificate as correct due to the encoding mismatch, allowing the attacker to perform man-in-the-middle attacks. The attack requires network-level access but no privileges or user interaction. The vulnerability affects versions prior to 4.14.2, which implements the fix.

Affected products

  • agronholm AnyIO < 4.14.2

Identifiers

  • CVE-2026-63374
  • GHSA-82r6-8w77-94w6
  • PYSEC-2026-4025

Timeline

  • 2026-07-07: disclosed
  • 2026-09-18: advisory
  • 2026-09-18: patched: patched in v4.14.2

References

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