Vulnerability-Lookup

GHSA-W2Q5-6Q6X-X959

Vulnerability from github – Published: 2026-05-26 13:30 – Updated: 2026-05-26 13:30

Details

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".

Severity

10.0 (Critical)


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

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JSON

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{
  "affected": [],
  "aliases": [
    "CVE-2026-39821"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-1289"
    ],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-05-22T16:16:20Z",
    "severity": "CRITICAL"
  },
  "details": "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\".",
  "id": "GHSA-w2q5-6q6x-x959",
  "modified": "2026-05-26T13:30:18Z",
  "published": "2026-05-26T13:30:18Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-39821"
    },
    {
      "type": "WEB",
      "url": "https://go.dev/cl/767220"
    },
    {
      "type": "WEB",
      "url": "https://go.dev/issue/78760"
    },
    {
      "type": "WEB",
      "url": "https://groups.google.com/g/golang-announce/c/iI-mYSI0lu8"
    },
    {
      "type": "WEB",
      "url": "https://pkg.go.dev/vuln/GO-2026-5026"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N",
      "type": "CVSS_V3"
    }
  ]
}

CVE-2026-39821 (GCVE-0-2026-39821)

Vulnerability from cvelistv5 – Published: 2026-05-22 15:01 – Updated: 2026-05-27 13:13

Title

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

Summary

Severity

9.6 (Critical)


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

SSVC

Exploitation: none Automatable: no Technical Impact: total

CISA Coordinator (v2.0.3)

CWE

CWE-1289 - Improper Validation of Unsafe Equivalence in Input

Assigner

References

4 references

URL	Tags
https://go.dev/cl/767220
https://go.dev/issue/78760
https://groups.google.com/g/golang-announce/c/iI-…
https://pkg.go.dev/vuln/GO-2026-5026

Impacted products

1 product

Vendor	Product	Version
golang.org/x/net	golang.org/x/net/idna	Affected: 0 , < 0.55.0 (semver)

Credits

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

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JSON

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{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "cvssV3_1": {
              "attackComplexity": "LOW",
              "attackVector": "NETWORK",
              "availabilityImpact": "NONE",
              "baseScore": 9.6,
              "baseSeverity": "CRITICAL",
              "confidentialityImpact": "HIGH",
              "integrityImpact": "HIGH",
              "privilegesRequired": "LOW",
              "scope": "CHANGED",
              "userInteraction": "NONE",
              "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N",
              "version": "3.1"
            }
          },
          {
            "other": {
              "content": {
                "id": "CVE-2026-39821",
                "options": [
                  {
                    "Exploitation": "none"
                  },
                  {
                    "Automatable": "no"
                  },
                  {
                    "Technical Impact": "total"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-05-23T03:55:58.522682Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
          }
        ],
        "problemTypes": [
          {
            "descriptions": [
              {
                "cweId": "CWE-1289",
                "description": "CWE-1289 Improper Validation of Unsafe Equivalence in Input",
                "lang": "en",
                "type": "CWE"
              }
            ]
          }
        ],
        "providerMetadata": {
          "dateUpdated": "2026-05-27T13:13:15.606Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
        {
          "collectionURL": "https://pkg.go.dev",
          "defaultStatus": "unaffected",
          "packageName": "golang.org/x/net/idna",
          "product": "golang.org/x/net/idna",
          "programRoutines": [
            {
              "name": "Profile.process"
            },
            {
              "name": "Profile.ToASCII"
            },
            {
              "name": "Profile.ToUnicode"
            },
            {
              "name": "ToASCII"
            },
            {
              "name": "ToUnicode"
            }
          ],
          "vendor": "golang.org/x/net",
          "versions": [
            {
              "lessThan": "0.55.0",
              "status": "affected",
              "version": "0",
              "versionType": "semver"
            }
          ]
        }
      ],
      "credits": [
        {
          "lang": "en",
          "value": "KC1zs4 (https://github.com/KC1zs4)"
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "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\"."
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "description": "CWE-1289: Improper Validation of Unsafe Equivalence in Input",
              "lang": "en"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-05-22T15:01:21.462Z",
        "orgId": "1bb62c36-49e3-4200-9d77-64a1400537cc",
        "shortName": "Go"
      },
      "references": [
        {
          "url": "https://go.dev/cl/767220"
        },
        {
          "url": "https://go.dev/issue/78760"
        },
        {
          "url": "https://groups.google.com/g/golang-announce/c/iI-mYSI0lu8"
        },
        {
          "url": "https://pkg.go.dev/vuln/GO-2026-5026"
        }
      ],
      "title": "Invoking failure to reject ASCII-only Punycode-encoded labels in golang.org/x/net/idna"
    }
  },
  "cveMetadata": {
    "assignerOrgId": "1bb62c36-49e3-4200-9d77-64a1400537cc",
    "assignerShortName": "Go",
    "cveId": "CVE-2026-39821",
    "datePublished": "2026-05-22T15:01:21.462Z",
    "dateReserved": "2026-04-07T18:13:03.526Z",
    "dateUpdated": "2026-05-27T13:13:15.606Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

Action not permitted

GHSA-W2Q5-6Q6X-X959

CVE-2026-39821 (GCVE-0-2026-39821)

Tags

Sightings

Nomenclature