CVE-2026-39821

Medium

Elevated severity or exploit probability.

CVSS base

9.6 CRITICAL

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N

EPSS — probability of exploitation (30 days)

0.7%

51.4th percentile

CISA KEV

Not listed

Weakness / dates

CWE-1289

Published 2026-05-22 · modified 2026-09-17

CVSS breakdown

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N

Attack Vector	N	Network
Attack Complexity	L	Low
Privileges Required	L	Low
User Interaction	N	None
Scope	C	Changed
Confidentiality	H	High
Integrity	H	High
Availability	N	None

Timeline

2026-05-22 — Published (NVD)
2026-09-17 — Last modified (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".

Affected

golang

References

Official: NVD · CVE.org