CVE-2026-39821: Golang Net

Critical severity, CVSS 9.6. EPSS: 0.7% chance of exploitation in the next 30 days.

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 products

• Golang Net: before 0.55.0 (fixed in 0.55.0)

Published 2026-05-22. Last modified 2026-09-17.