lxml-html-clean has <base> tag injection through default Cleaner configuration
Date: 2026-03
Affected: pip/lxml-html-clean
Attack vector: xss
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2026-28350
CWE: CWE-116
CVSS: 6.1 (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N)

### Summary The `<base>` tag passes through the default `Cleaner` configuration. While `page_structure=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. ### Details The `<base>` tag is not currently in the `page_structure` 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. ### PoC ```python from lxml_html_clean import clean_html # The base tag is preserved in the output result = clean_html('<base href="http://evil.com/"><a href="/account">Account</a>') print(result) # Output: <div><base href="http://evil.com/">...<a href="/account">Account</a></div> ``` ### 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


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