lxml-html-clean has <base> tag injection through default Cleaner configuration

QFinch assessment

The <base> tag passes through the default Cleaner configuration. While pagestructure=True removes html, head, and title tags, there is no specific handling for <base>, allowing an attacker to inject it and hijack relative links on the page.

Potential impact

The injection of a <base> tag allows an attacker to hijack the resolution of all relative URLs on the page. This results in three critical attack vectors: 1. Phishing & Redirection: Attackers can redirect user navigation (e.g., <a href="/login">) and form submissions (e.g., <form action="/auth">) to an attacker-controlled domain, effectively stealing credentials or sensitive data without the user realizing they have left the legitimate site. 2. Cross-Site Scripting (XSS): If the victim application loads JavaScript files using relative paths (e.g., <script src="assets/app.js">), the browser will attempt to fetch the script from the attacker's domain. This upgrades the vulnerability from HTML injection to full Stored XSS. 3. Defacement: Relative references to images (<img>) and stylesheets (<link>) will be loaded from the attacker's server, allowing for UI redressing or defacement.

Technical context

The <base> tag is not currently in the pagestructure kill set. Even though the specification says <base> must be inside <head>, browsers accept <base> tags outside of the head. If an attacker injects a <base> tag, it changes the base URL for all relative URLs on the page (links, images, scripts) to a domain controlled by the attacker.