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CVE-2026-39821

Invoking failure to reject ASCII-only Punycode-encoded labels in golang.org/x/net/idna

Published: May 22, 2026Updated: May 29, 2026 Sources: CVE List NVDCWE-1289
NVD MITRE
9.6CVSSCRITICAL
EPSS Score
0.4%
Top 72.5%
CISA KEV
Not in KEV
Exploits
No Known Exploits
Remediation
Patch Available

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".

CVSS Vector Breakdown

AV:NAC:LPR:LUI:NS:CC:HI:HA:N
Exploitability
AV:NAttack Vector
Network
AC:LAttack Complexity
Low
PR:LPrivileges Required
Low
UI:NUser Interaction
None
Scope
S:CScope
Changed
Impact
C:HConfidentiality
High
I:HIntegrity
High
A:NAvailability
None
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Weaknesses

CWE-1289

Affected Products

golang.org/x/net/idnagolang.org/x/net
Library
OSS Libraries / go
netgolangOSS Libraries
golang.org/x/netoss-projectOSS Librariesaka golang.org/x/net
golangoss-projectUSOSS Libraries

Exploitability

Official Patch Available

References

https://go.dev/cl/767220
go.dev
https://go.dev/issue/78760
go.dev
https://groups.google.com/g/golang-announce/c/iI-mYSI0lu8
groups.google.com
and 1 more references View all →

Timeline

Published
May 22, 2026
Last Updated
May 29, 2026

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