CVE WatchJul 17, 2026

CVE-2026-39821

The ToASCII and ToUnicode functions incorrectly accept Punycode-encoded labels that decode to an ASCII-only label. For example, ToUnicode("x

CVSS

9.6

Critical

EPSS

0.5%

p38

KEV

—

Exploit Today

0-100

Published: May 22, 2026 · Last modified: Jul 17, 2026 · CWE-1289

EPSS · 30d

0.5%EPSS · 30 days0.5%

2026-06-302026-07-17

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".

Official references

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