CVE-2026-39821

9.6 CRITICAL
Published: May 22, 2026 Modified: May 29, 2026
View on NVD

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".

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CVSS v3.x Details

0.0 Low Medium High Critical 10.0
Vector String
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N

References to Advisories, Solutions, and Tools

Patch Vendor Advisory Exploit Third Party Advisory
https://go.dev/cl/767220
Source: security@golang.org
Issue Tracking
https://go.dev/issue/78760
Source: security@golang.org
Issue Tracking
https://groups.google.com/g/golang-announce/c/iI-mYSI0lu8
Source: security@golang.org
Mailing List
https://pkg.go.dev/vuln/GO-2026-5026
Source: security@golang.org
Vendor Advisory

4 reference(s) from NVD

Quick Stats

CVSS v3 Score
9.6 / 10.0
EPSS (Exploit Probability)
0.1%
16th percentile
Exploitation Status
Not in CISA KEV

Weaknesses (CWE)

Affected Vendors

golang