ubuntu-cve-2026-39821
Vulnerability from osv_ubuntu
Published
2026-05-22 16:16
Modified
2026-06-30 16:20
Summary
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
9.6 (Critical)
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N
8.2 (High)
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:N
N/A (UNKNOWN)
medium
References
To clipboard 

{
  "affected": [
    {
      "ecosystem_specific": {
        "availability": "Available with Ubuntu Pro: https://ubuntu.com/pro",
        "binaries": [
          {
            "binary_name": "golang-go.net-dev",
            "binary_version": "1:0.0+git20170629.c81e7f2+dfsg-2ubuntu0.1~esm3"
          },
          {
            "binary_name": "golang-golang-x-net-dev",
            "binary_version": "1:0.0+git20170629.c81e7f2+dfsg-2ubuntu0.1~esm3"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:Pro:18.04:LTS",
        "name": "golang-golang-x-net-dev",
        "purl": "pkg:deb/ubuntu/golang-golang-x-net-dev@1:0.0+git20170629.c81e7f2+dfsg-2ubuntu0.1~esm3?arch=source\u0026distro=esm-apps/bionic"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1:0.0+git20170629.c81e7f2+dfsg-2ubuntu0.1~esm3"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "1:0.0+git20170629.c81e7f2+dfsg-1ubuntu1",
        "1:0.0+git20170629.c81e7f2+dfsg-1ubuntu2",
        "1:0.0+git20170629.c81e7f2+dfsg-2",
        "1:0.0+git20170629.c81e7f2+dfsg-2ubuntu0.1~esm1",
        "1:0.0+git20170629.c81e7f2+dfsg-2ubuntu0.1~esm2"
      ]
    },
    {
      "ecosystem_specific": {
        "availability": "Available with Ubuntu Pro: https://ubuntu.com/pro",
        "binaries": [
          {
            "binary_name": "golang-go.net-dev",
            "binary_version": "1:0.0+git20190811.74dc4d7+dfsg-1ubuntu0.1~esm3"
          },
          {
            "binary_name": "golang-golang-x-net-dev",
            "binary_version": "1:0.0+git20190811.74dc4d7+dfsg-1ubuntu0.1~esm3"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:Pro:20.04:LTS",
        "name": "golang-golang-x-net-dev",
        "purl": "pkg:deb/ubuntu/golang-golang-x-net-dev@1:0.0+git20190811.74dc4d7+dfsg-1ubuntu0.1~esm3?arch=source\u0026distro=esm-apps/focal"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1:0.0+git20190811.74dc4d7+dfsg-1ubuntu0.1~esm3"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "1:0.0+git20190811.74dc4d7+dfsg-1",
        "1:0.0+git20190811.74dc4d7+dfsg-1ubuntu0.1~esm1",
        "1:0.0+git20190811.74dc4d7+dfsg-1ubuntu0.1~esm2"
      ]
    }
  ],
  "aliases": [],
  "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": "UBUNTU-CVE-2026-39821",
  "modified": "2026-06-30T16:20:57Z",
  "published": "2026-05-22T16:16:00Z",
  "references": [
    {
      "type": "REPORT",
      "url": "https://ubuntu.com/security/CVE-2026-39821"
    },
    {
      "type": "REPORT",
      "url": "https://www.cve.org/CVERecord?id=CVE-2026-39821"
    },
    {
      "type": "REPORT",
      "url": "https://groups.google.com/g/golang-announce/c/iI-mYSI0lu8"
    },
    {
      "type": "REPORT",
      "url": "https://github.com/golang/go/issues/78760"
    },
    {
      "type": "REPORT",
      "url": "https://go.dev/cl/767220"
    },
    {
      "type": "REPORT",
      "url": "https://go.dev/issue/78760"
    },
    {
      "type": "REPORT",
      "url": "https://pkg.go.dev/vuln/GO-2026-5026"
    },
    {
      "type": "ADVISORY",
      "url": "https://ubuntu.com/security/notices/USN-8416-1"
    }
  ],
  "related": [
    "USN-8416-1"
  ],
  "schema_version": "1.7.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N",
      "type": "CVSS_V3"
    },
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:N",
      "type": "CVSS_V3"
    },
    {
      "score": "medium",
      "type": "Ubuntu"
    }
  ],
  "upstream": [
    "CVE-2026-39821"
  ]
}



Log in or create an account to share your comment.




Tags
Taxonomy of the tags.





Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

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.