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Security Advisories

GO-2026-5026

https://pkg.go.dev/vuln/GO-2026-5026

Severity

9.6

Critical

CVSS V3

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Summary

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

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".

References

  • https://nvd.nist.gov/vuln/detail/CVE-2026-39821
  • https://github.com/advisories/GHSA-w2q5-6q6x-x959
  • https://pkg.go.dev/vuln/GO-2026-5026
  • https://go.dev/cl/767220
  • https://go.dev/issue/78760
  • https://groups.google.com/g/golang-announce/c/iI-mYSI0lu8
  • https://groups.google.com/g/golang-announce/c/94pEornpRlI

Affected packages

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