CVE-2026-39821

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".

Affected packages (2)

• Debian/golang-golang-x-netfrom 0
• Go/golang.org/x/netfrom 0, < 0.55.0

CVSS scores

References (4)

• ADVISORYhttps://security-tracker.debian.org/tracker/CVE-2026-39821
• PATCHhttps://go.dev/cl/767220
• REPORThttps://go.dev/issue/78760
• WEBhttps://groups.google.com/g/golang-announce/c/iI-mYSI0lu8