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

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Summary

CVECVE-2026-39821
StatePUBLISHED
AssignerGo
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-05-22 16:16:20 UTC
Updated2026-05-29 15:26:37 UTC
DescriptionThe 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".

Risk And Classification

Primary CVSS: v3.1 9.6 CRITICAL from ADP

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

EPSS: 0.000500000 probability, percentile 0.159000000 (date 2026-05-28)

Problem Types: CWE-1289 | CWE-1289: Improper Validation of Unsafe Equivalence in Input | CWE-1289 CWE-1289 Improper Validation of Unsafe Equivalence in Input

VersionSourceTypeScoreSeverityVector
3.1ADPDECLARED9.6CRITICALCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N
3.1134c704f-9b21-4f2e-91b3-4a467353bcc0Secondary9.6CRITICALCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N

CVSS v3.1 Breakdown

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

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

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Application Golang Net All All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Golang.orgxnet Golang.org/x/net/idna affected 0.55.0 semver Not specified

References

ReferenceSourceLinkTags
pkg.go.dev/vuln/GO-2026-5026 [email protected] pkg.go.dev Vendor Advisory
groups.google.com/g/golang-announce/c/iI-mYSI0lu8 [email protected] groups.google.com Mailing List
go.dev/issue/78760 [email protected] go.dev Issue Tracking
go.dev/cl/767220 [email protected] go.dev Issue Tracking
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: KC1zs4 (https://github.com/KC1zs4) (en)

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