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GHSA-xvp8-3mhv-424c
No affected components available
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
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:
- 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. - 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. - Defacement: Relative references to images (
<img>) and stylesheets (<link>) will be loaded from the attacker's server, allowing for UI redressing or defacement.
The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. The attacker needs the user to perform some action, like clicking a link. The vulnerability can affect other systems as well, not just the initial system. There is a low impact on the confidentiality of the information. There is a low impact on the integrity of the data.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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