VULNERABILITY INTELLIGENCE

CVE-2026-39821

CVSS score9.6 CRITICAL
EPSS probability1.83%
CISA KEVNot currently listed
Published2026-05-22
Last modified2026-09-17
CVSS vectorCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N

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".

Weakness classification

  • 134c704f-9b21-4f2e-91b3-4a467353bcc0: CWE-1289
  • 0b0ca135-0b70-47e7-9f44-1890c2a1c46c: CWE-1289

Primary references