CVE-2026-53606: Sanitize html URIs Through Action, Formaction, Data, Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-vccv-cmxp-4j9h",
  "modified" : "2026-07-31T21:43:52Z",
  "published" : "2026-07-31T21:43:51Z",
  "aliases" : [ "CVE-2026-53606" ],
  "summary" : "sanitize-html has incomplete URI scheme validation in that allows javascript: URIs through action, formaction, data, poster, and background attributes",
  "details" : "## Summary\n\nsanitize-html uses `allowedSchemesAppliedToAttributes` (default: `['href', 'src', 'cite']`) to gate the `naughtyHref()` function that blocks dangerous URI schemes like `javascript:` and `vbscript:`. The HTML specification defines 10+ attributes that accept URIs (`action`, `formaction`, `data`, `poster`, `background`, `ping`, `xlink:href`, `dynsrc`, `lowsrc`), but none of these are included in the default gate list. When a developer allows any of these attributes in their configuration, `javascript:` URIs pass through completely unmodified, enabling XSS.\n\nThe library has zero awareness of these URI-bearing attributes — none appear anywhere in the 854-line source file (verified by grep). No warning mechanism exists, and the README provides no security guidance about expanding `allowedSchemesAppliedToAttributes` when allowing form or media attributes.\n\n## Severity\n\nExploitation requires non-default configuration: the developer must explicitly allow a non-default tag (e.g., `form`) AND a non-default attribute (e.g., `action`). Default configuration is NOT vulnerable. However, this is a common configuration pattern for CMS platforms, form builders, and rich content editors.\n\n## Affected Versions\n\nAll versions of sanitize-html from v1.18.0 (which introduced `allowedSchemesAppliedToAttributes`) through at least v2.17.2. The default list has been `['href', 'src', 'cite']` since introduction and has never been expanded.\n\n## Root Cause\n\n**File**: `index.js:329` (sanitize-html 2.10.0, confirmed same in 2.17.x)\n\n```javascript\n// Line 329 — The gate that controls scheme validation\nif (options.allowedSchemesAppliedToAttributes.indexOf(a) >= 0) {\n    if (naughtyHref(name, value)) {\n        delete frame.attribs[a];\n        return;\n    }\n}\n```\n\n**Default list at line 829**:\n```javascript\nallowedSchemesAppliedToAttributes: ['href', 'src', 'cite'],\n```\n\nThe `naughtyHref()` function (lines 627-667) correctly blocks `javascript:`, `vbscript:`, and other dangerous schemes. However, it has exactly 2 call sites in the entire codebase (lines 330 and 395), both inside the `indexOf` gate. There is no ungated path.\n\nWhen attribute name is `action`, `formaction`, `data`, `poster`, `background`, etc.:\n- `indexOf('action')` returns `-1`\n- The `if` block is skipped entirely\n- `naughtyHref()` is never called\n- `javascript:alert(1)` passes through unmodified\n\nThe `escapeHtml()` function at line 464 provides no defense — it only encodes `& < > \"` characters, which are not present in `javascript:alert(1)`.\n\n**Data Flow**:\n```\nAttacker input: <form action=\"javascript:alert(document.cookie)\">\n1. htmlparser2 parses → tag='form', attribs={action:'javascript:alert(document.cookie)'}\n2. index.js:298 → allowedAttributes check: 'action' in developer config → PASS\n3. index.js:329 → ['href','src','cite'].indexOf('action') → -1 → SKIP naughtyHref()\n4. index.js:464 → escapeHtml('javascript:alert(document.cookie)') → unchanged\n5. OUTPUT: <form action=\"javascript:alert(document.cookie)\">\n```\n\n## Steps to Reproduce\n\n```javascript\nconst sanitize = require('sanitize-html');\n\n// ===== VECTOR 1: form action (100% reliable, all modern browsers) =====\nconst v1 = sanitize(\n    '<form action=\"javascript:alert(document.cookie)\"><button>Submit</button></form>',\n    {\n        allowedTags: ['form', 'button'],\n        allowedAttributes: { form: ['action'] }\n    }\n);\nconsole.log('V1 (action):', v1);\n// OUTPUT: <form action=\"javascript:alert(document.cookie)\"><button>Submit</button></form>\n// XSS triggers when user submits the form\n\n// ===== VECTOR 2: button formaction (100% reliable) =====\nconst v2 = sanitize(\n    '<button formaction=\"javascript:alert(1)\">Click</button>',\n    {\n        allowedTags: ['button'],\n        allowedAttributes: { button: ['formaction'] }\n    }\n);\nconsole.log('V2 (formaction):', v2);\n// OUTPUT: <button formaction=\"javascript:alert(1)\">Click</button>\n\n// ===== VECTOR 3: object data =====\nconst v3 = sanitize(\n    '<object data=\"javascript:alert(1)\"></object>',\n    {\n        allowedTags: ['object'],\n        allowedAttributes: { object: ['data'] }\n    }\n);\nconsole.log('V3 (data):', v3);\n// OUTPUT: <object data=\"javascript:alert(1)\"></object>\n\n// ===== CONTROL: href IS scheme-checked (expected behavior) =====\nconst ctrl = sanitize(\n    '<a href=\"javascript:alert(1)\">click</a>',\n    {\n        allowedTags: ['a'],\n        allowedAttributes: { a: ['href'] }\n    }\n);\nconsole.log('Control (href):', ctrl);\n// OUTPUT: <a>click</a>   ← href correctly stripped by naughtyHref()\n```\n\n**Observed behavior**: `javascript:` preserved on `action`/`formaction`/`data` but correctly stripped on `href`.\n\n**Expected behavior**: `javascript:` should be stripped on ALL URI-bearing attributes, or at minimum, the library should warn developers when they allow URI-bearing attributes not covered by scheme validation.\n\n## Impact\n\nAn attacker can achieve XSS in applications that use sanitize-html with non-default configurations allowing URI-bearing attributes:\n\n- `<form action=\"javascript:...\">` — XSS on form submission (all modern browsers)\n- `<button formaction=\"javascript:...\">` — per-button XSS override (all modern browsers)\n- `<object data=\"javascript:...\">` — object load XSS (Chrome, Firefox)\n- `<video poster=\"javascript:...\">` — limited browser support but spec-valid\n\n**Common vulnerable configurations**:\n- CMS platforms allowing form elements for user-generated content\n- Form builder applications\n- Rich text editors with extended tag allowlists\n- Email template editors allowing media/embed tags\n\n**Mitigating factors**:\n- Default configuration is NOT vulnerable\n- Requires double opt-in: non-default tag + non-default attribute\n- CSP `form-action` directive mitigates form-based vectors\n- Developers CAN manually add attributes to `allowedSchemesAppliedToAttributes`\n\n## Remediation\n\n**Option 1 (Recommended)**: Expand the default `allowedSchemesAppliedToAttributes` list:\n\n```javascript\n// index.js line 829, change from:\nallowedSchemesAppliedToAttributes: ['href', 'src', 'cite'],\n\n// to:\nallowedSchemesAppliedToAttributes: [\n    'href', 'src', 'cite', 'action', 'formaction',\n    'data', 'poster', 'background', 'ping',\n    'xlink:href', 'dynsrc', 'lowsrc'\n],\n```\n\n**Option 2**: Apply `naughtyHref()` to ALL attributes by default (invert the gate logic).\n\n**Option 3**: Add a runtime warning when developers allow URI-bearing attributes not in `allowedSchemesAppliedToAttributes` (analogous to `vulnerableTags` warning for `script`/`style` at lines 124-129).\n\n## Reporter\n\nKevin Lee (Changseon Lee)\nOPCIA Corp. / PeanutAI Inc.\nSeoul, South Korea\nGitHub: crattack",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "sanitize-html"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.18.0"
      }, {
        "fixed" : "2.17.5"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.17.4"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/apostrophecms/apostrophe/security/advisories/GHSA-vccv-cmxp-4j9h"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-53606"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/apostrophecms/apostrophe/pull/5464"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/apostrophecms/apostrophe/commit/5a88e9630cbbdde33154ef8abe7557ddf7be418b"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/apostrophecms/apostrophe"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/apostrophecms/apostrophe/releases/tag/sanitize-html@2.17.5"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-31T21:43:51Z",
    "nvd_published_at" : "2026-06-12T21:16:24Z"
  }
}