CVE-2026-49825: lxml-html-clean Vulnerability in URL Attribute Sanitization

Background

lxml-html-clean is a Python package that provides a cleaner for HTML content, allowing developers to sanitize untrusted HTML by removing potentially harmful elements and attributes. It is commonly used in web applications to prevent XSS (Cross-Site Scripting) attacks. The package is designed to filter out unsafe attributes and ensure that the rendered HTML is safe for users. However, a vulnerability exists in the way it handles namespaced URL attributes, specifically xlink:href, which can lead to security risks if not addressed.

Vulnerability Detail

The vulnerability arises from the lxml_html_clean.Cleaner class, which fails to strip javascript: URLs from namespaced URL attributes like xlink:href. This oversight allows malicious payloads to pass through the sanitizer when the safe_attrs_only option is set to False. As a result, an attacker can inject JavaScript code that executes when a user clicks on the rendered link, leading to stored XSS vulnerabilities. The root cause is linked to an incomplete allow-list of attributes that does not include xlink:href, which is necessary for proper sanitization.

pip install --upgrade "lxml-html-clean==0.4.5"

Workaround

If you cannot update the package immediately, you can implement a temporary workaround by manually checking and removing any xlink:href attributes containing javascript: URLs in your HTML content. Here's a code snippet to illustrate this workaround:

from lxml import html

def sanitize_xlink_href(tree):
    for el in tree.iter():
        if 'xlink:href' in el.attrib and el.attrib['xlink:href'].startswith('javascript:'):
            del el.attrib['xlink:href']

# Example usage:
html_content = '<svg><a xlink:href="javascript:alert(1)">x</a></svg>'
tree = html.fromstring(html_content)
sanitize_xlink_href(tree)
print(html.tostring(tree).decode())

Conclusion

The CVE-2026-49825 vulnerability in the lxml-html-clean package poses a significant risk due to its failure to properly sanitize namespaced URL attributes. It is crucial for developers to update to the patched version or implement the provided workaround to mitigate the risk of XSS attacks.

See every vulnerability recorded for lxml-html-clean →

FAQ