Express XSS Sanitizer: allowedTags/allowedAttributes bypass leads to permissive sanitization (XSS risk)

Description

A vulnerability has been identified in express-xss-sanitizer (<= 2.0.1) where restrictive sanitization configurations are silently ignored.

When a developer explicitly sets:

allowedTags: [] allowedAttributes: {}

the library incorrectly treats these values as "not provided" due to length/emptiness checks, and falls back to sanitize-html's default configuration.

As a result, instead of stripping all HTML tags and attributes, the sanitizer allows a permissive set of tags (e.g., <a>, <p>, <div>, etc.) and attributes (e.g., href on <a>).

This behavior violates the expected API contract and may lead to security issues such as content injection or XSS, depending on how the sanitized output is used.

Impact

Developers intending to fully strip HTML content by providing empty allowedTags or allowedAttributes configurations may unknowingly allow a wide range of HTML elements and attributes.

This can result in:

• Injection of unintended HTML content (e.g., <div>, <table>, headings)
• Injection of links via <a href="...">
• Potential XSS vectors depending on downstream usage

The impact depends on how the sanitized output is rendered or consumed, but the root issue is a mismatch between developer intent and actual behavior.

Proof of Concept

const { sanitize } = require('express-xss-sanitizer');
const sanitizeHtml = require('sanitize-html');

const input = '<a href="http://evil.com">click</a><p>phish</p>';

// Using express-xss-sanitizer (v2.0.1)
sanitize(input, { allowedTags: [], allowedAttributes: {} });
// => '<a href="http://evil.com">click</a><p>phish</p>'

// Expected behavior (sanitize-html directly)
sanitizeHtml(input, { allowedTags: [], allowedAttributes: {} });
// => 'clickphish'

Root Cause

The issue was caused by validation logic that checked for non-empty arrays/objects:

• allowedTags required length > 0
• allowedAttributes required Object.keys(...).length > 0

This caused empty configurations ([]) and ({}) to be ignored, resulting in fallback to default permissive settings.

Fix

The validation logic has been updated to respect explicitly provided empty configurations.

Now, if allowedTags or allowedAttributes are provided (even if empty), they are passed directly to sanitize-html without being overridden.