CVE-2026-39821

Summary

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 Software

VendorProductVersion RangeStatus
Go standard librarynet/http0 < 1.25.13affected
Go standard librarynet/http1.26.0-0 < 1.26.6affected
Go standard librarynet/http1.27.0-0 < 1.27.0-rc.3affected
Go standard librarynet/http/internal/http20 < 1.25.13affected
Go standard librarynet/http/internal/http21.26.0-0 < 1.26.6affected
Go standard librarynet/http/internal/http21.27.0-0 < 1.27.0-rc.3affected
golang.org/x/netgolang.org/x/net/idna0 < 0.55.0affected

Weaknesses

  • CWE-1289: Improper Validation of Unsafe Equivalence in Input

ADP Enrichment

CISA ADP Vulnrichment

  • SSVC:
  • Exploitation: none
    • Automatable: no
    • Technical Impact: total

golang.org/x/net/idna: golang: net/http: golang.org/x/net/idna: Privilege escalation via incorrect Punycode label processing

Additional References

References