CVE-2026-39821

Aliases:GO-2026-5026
Advisory lineage Upstream: 0 Downstream: 128
Analyzed
Published: 22 May 2026, 15:01
Last modified:22 Jul 2026, 12:08

Vulnerability Summary

Overall Risk (default)
high
70/100
CVSS Score
9.6 CRITICAL
v3.1 (cve.org)
EPSS Score
0.48% LOW
0% probability 0.00%
KEV
Not listed
Ransomware
No reports
Public exploits
None found
Dark Web
Not detected

Timeline

22 May 2026, 15:01
Published
Vulnerability first disclosed
22 Jul 2026, 12:08
Last Modified
Vulnerability information updated

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 Metrics

  • v3.1•CRITICAL•Score: 9.6CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N

EPSS Trends

Current EPSS score: 0.48%• Percentile: 38%

Techniques & Countermeasures

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

    The product receives an input value that is used as a resource identifier or other type of reference, but it does not validate or incorrectly validates that the input is equivalent to a potentially-unsafe value.

Affected Systems

  • golang.org/x•net

    < 0.55.0

  • golang•net

    < 0.55.0

  • golang.org/x/net•golang.org/x/net/idna

    < 0.55.0

References (85)