CVE-2016-9910 is a medium-severity (CVSS 6.1) Cross-site Scripting (XSS) vulnerability in html5lib. A fix is available for html5lib — see the affected versions and patch details below.
Cross-site Scripting in html5lib
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
html5libReal-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
The serializer in html5lib before 0.99999999 might allow remote attackers to conduct cross-site scripting (XSS) attacks by leveraging mishandling of special characters in attribute values, a different vulnerability than CVE-2016-9909.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | html5lib | all versions | 0.99999999pip install --upgrade 'html5lib==0.99999999' |
Affected Products
html5libhtml5libDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for html5lib, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update html5lib to 0.99999999 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2016-9910 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.
Frequently Asked Questions
Is CVE-2016-9910 in your dependencies?
Find it across PyPI, including transitive dependencies.