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CVE-2026-39821

9.6 · CRITICAL

The ToASCII and ToUnicode functions incorrectly accept Punycode-encoded labels that decode to an ASCII-only label.

Published May 22, 2026CWE-1289EPSS 0.69%

Overview

CVE-2026-39821 is a critical-severity vulnerability. It was published on May 22, 2026 and has a CVSS 3.1 base score of 9.6 (CRITICAL).

This vulnerability has a CVSS 3.1 base score of 9.6, rated CRITICAL. It can be exploited remotely over the network. Some level of privileges is required for exploitation.

Technical description

The ToASCII and ToUnicode functions incorrectly accept Punycode-encoded labels that decode to an ASCII-only label. For example, ToUnicode("xn--example-.com") incorrectly returns the name "example.com" rather than an error. This behavior can lead to privilege escalation in programs using the idna package. For example, a program which performs privilege checks on the ASCII hostname may reject "example.com" but permit "xn--example-.com". If that program subsequently converts the ASCII hostname to Unicode, it will inadvertently permits access to the Unicode name "example.com".

Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

References

Refgo.dev/cl/767220Refgo.dev/issue/78760Refgroups.google.com/g/golang-announce/c/94pEornpRlIRefgroups.google.com/g/golang-announce/c/iI-mYSI0lu8Refpkg.go.dev/vuln/GO-2026-5026Refaccess.redhat.com/errata/RHSA-2026:23262Refaccess.redhat.com/errata/RHSA-2026:23264Refaccess.redhat.com/errata/RHSA-2026:26546Refaccess.redhat.com/errata/RHSA-2026:26547Refaccess.redhat.com/errata/RHSA-2026:30650

How CyberStrike helps

Frequently asked questions

What is CVE-2026-39821?

CVE-2026-39821 is a critical-severity vulnerability. It was published on May 22, 2026 and has a CVSS 3.1 base score of 9.6 (CRITICAL).

How severe is CVE-2026-39821?

This vulnerability has a CVSS 3.1 base score of 9.6, rated CRITICAL. It can be exploited remotely over the network. Some level of privileges is required for exploitation.

How do I fix or remediate CVE-2026-39821?

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

How can CyberStrike help with CVE-2026-39821?

CyberStrike is an open-source, AI-augmented pentesting engine. Rather than flag a version number, it tests whether an issue like CVE-2026-39821 is actually reachable in your own applications and APIs, then verifies each finding before reporting it. That tells you if the weakness is a real, exploitable risk in your environment or a false positive you can safely deprioritize.

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