6.1

CVSS Score

3.1

-

CVSS Score

Basic Information

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CVE-2026-28348

CVE-2026-28348: lxml_html_clean: CSS @import Filter Bypass via Unicode Escapes

lxml_html_clean is a project for HTML cleaning functionalities copied from lxml.html.clean. Prior to version 0.4.4, 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. This issue has been patched in version 0.4.4.

(GitHub Advisory)

Miggo Vulnerability Database

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CVE-2026-28348

CVE-2026-28348:

6.1

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
lxml-html-clean	pip	<= 0.4.3	0.4.4

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability exists in the lxml-html-clean library, specifically within the Cleaner._has_sneaky_javascript method in lxml_html_clean/clean.py. The root cause of the vulnerability is the incorrect sanitization of CSS content. The function used style.replace('\\', '') to strip backslashes from the input style string. This was intended as a security measure, but it created a bypass for CSS filters. Attackers could use CSS Unicode escape sequences, such as \69 for the character 'i', to construct malicious payloads like @\69mport. After the vulnerable line of code processed this payload, it would become @69mport, which would not be caught by the blacklist looking for @import. However, a browser's CSS parser would correctly decode the original escape sequence and execute the @import directive. The patch fixes this by replacing the naive backslash removal with a new method, _decode_css_unicode_escapes, which correctly interprets and decodes these escape sequences before the security checks are performed. This ensures that the sanitized content is evaluated in the same way a browser would, thus preventing the bypass.

Vulnerable functions

Cleaner._has_sneaky_javascript

lxml_html_clean/clean.py

The function `_has_sneaky_javascript` was vulnerable because it improperly sanitized CSS style content by removing backslashes. This allowed attackers to bypass security filters for keywords like `@import` and `expression()` by using CSS Unicode escape sequences (e.g., `@\69mport`). The line `style = style.replace('\\', '')` would transform the payload in a way that it no longer matched the blacklist, but would still be correctly interpreted by a web browser, leading to potential XSS or data exfiltration.

Vulnerability Intelligence
Miggo AI

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WAF Protection Rules

WAF Rule

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Reasoning

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