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

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• 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.

From the GitHub advisory

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.

Weakness type: CWE-295, CWE-297.

Product: anyio (PyPI) · Assigned by: GitHub_M · Published: 2026-09-22

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: none
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• 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.

Timeline

• 2026-09-18 — GitHub-reviewed advisory GHSA-82r6-8w77-94w6 published (critical)
• 2026-09-22 — CVE record published by GitHub_M
• 2026-09-28 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-82r6-8w77-94w6 github.com
2. CVE.org record CVE-2026-63374 cve.org
3. NVD entry CVE-2026-63374 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. github.com github.com

Critical PyPI advisories · Today’s Security Digest