External CSS loading via Unicode escape bypass
Published Mar 5, 2026 · Updated Mar 5, 2026
Improper CSS escaping in fedora-python lxml_html_clean before 0.4.4 allows remote attackers to load external CSS through cleaned HTML. The _has_sneaky_javascript() method deletes backslashes instead of decoding CSS Unicode escapes, so escaped @import and expression() tokens evade its keyword checks. Exploitation requires an application to clean attacker-controlled CSS and a user to render the result; external CSS can exfiltrate data or alter the UI, while older browsers execute expression().
Summary
What happened
Improper CSS escaping in fedora-python lxml_html_clean before 0.4.4 allows remote attackers to load external CSS through cleaned HTML. The _has_sneaky_javascript() method deletes backslashes instead of decoding CSS Unicode escapes, so escaped @import and expression() tokens evade its keyword checks. Exploitation requires an application to clean attacker-controlled CSS and a user to render the result; external CSS can exfiltrate data or alter the UI, while older browsers execute expression().
The record
- CVE
- CVE-2026-28348
- Published
- Mar 5, 2026
- Updated
- Mar 5, 2026
- Vendor
- fedora-python
- Product
- lxml_html_clean
- Classifications
- CWE-116, T1203
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Mar 5, 2026CVE publishedPublication date reported by the CVE source.
- Mar 5, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=< 0.4.4
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- Upstream Unicode-escape CSS bypass proof of conceptproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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