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

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

Critical PyPI

Severity: 9.3 / 10; Critical · CVSS 4.0
Exploit chance (EPSS): 0.29%; Chance of exploitation in the next 30 days. Higher than 21.50% of scored vulnerabilities.
Known exploited: No; Not in CISA's catalog
Weakness: CWE-295 Improper Certificate Validation; CWE-297 Improper Validation of Certificate with Host Mismatch
Fix: 4.14.2; Upgrade anyio to this version or later

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 versions

PyPI

Package	Vulnerable	Fixed in
anyio	`< 4.14.2`	`4.14.2`
anyio	`< 4.14.2`	`4.14.2`

How to fix

Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.

anyio PyPI → `4.14.2`

pip

pip install --upgrade "anyio>=4.14.2"

poetry

poetry add "anyio>=4.14.2"

uv

uv add "anyio>=4.14.2"

Severity breakdown

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

Attack vector: Network
Attack complexity: Low
Attack requirements: None
Privileges required: None
User interaction: None
Confidentiality (vulnerable system): High
Integrity (vulnerable system): High
Availability (vulnerable system): None
Confidentiality (subsequent systems): None
Integrity (subsequent systems): None
Availability (subsequent systems): None

Weakness

CWE-295: Improper Certificate Validation

The product does not validate, or incorrectly validates, a certificate.

How to prevent it

Architecture and Design, Implementation: Certificates should be carefully managed and checked to assure that data are encrypted with the intended owner's public key.
Implementation: If certificate pinning is being used, ensure that all relevant properties of the certificate are fully validated before the certificate is pinned, including the hostname.

CWE-297: Improper Validation of Certificate with Host Mismatch

The product communicates with a host that provides a certificate, but the product does not properly ensure that the certificate is actually associated with that host.

How to prevent it

Architecture and Design: Fully check the hostname of the certificate and provide the user with adequate information about the nature of the problem and how to proceed.
Implementation: If certificate pinning is being used, ensure that all relevant properties of the certificate are fully validated before the certificate is pinned, including the hostname.

References

Advisories

Fixes and patches

Packages

Timeline

Sep 18, 2026 Published
Oct 1, 2026 Last updated by the source
Nov 1, 2026 Latest EPSS score

Advisory	Severity	Why it's related
GHSA-5p39-cfhj-2xmp AnyIO process-pool workers can block indefinitely on undrained stderr	Medium	Also affects anyio
GHSA-3w57-8xmc-8v26 AnyIO run_process/open_process ignores extra_groups and can retain parent supplementary groups	High	Also affects anyio
GHSA-4487-j8r4-4g2f Forge: verifyCertificateChain() removes trusted CA certificates with the same subject from the CA store	Low	Same weakness, CWE-295
GHSA-5x38-2vff-x55p Forge: X.509 basicConstraints bypass in verifyCertificateChain() via deprecated extension OIDs 2.5.29.10 and 2.5.29.13	High	Same weakness, CWE-295
EEF-CVE-2026-115693 Custom cacerts or cacertfile do not restrict trust to the pinned CA in hackney	Not rated yet	Same weakness, CWE-295
GHSA-6q48-6c7w-9gw2 certifi bundles emSign root CAs with compromised intermediate CA keys	Critical	Same weakness, CWE-295
GHSA-2m5v-pch6-74hx undici vulnerable to TLS certificate validation bypass via dropped tls options in Agent	Critical	Same weakness, CWE-295
GHSA-vrv5-2jg9-4mmr Microsoft Security Advisory CVE-2026-96222 - .NET Spoofing Vulnerability	High	Same weakness, CWE-295

Is this in your code?

MergeHarbor checks every repository's dependencies against this database, ranks what matters by exploitability and reachability, routes it to the owning team and opens the fix pull request.

Check your repos with MergeHarbor