Summary
JustHTML has a Sanitizer Bypass (in Markdown)
to_markdown() does not sufficiently escape text content that looks like HTML. As a result, untrusted input that is safe in to_html() can become raw HTML in Markdown output.
This is not specific to tokenizer raw-text states like <title>, <noscript>, or <plaintext>, although those states can trigger the behavior. The root cause is broader: Markdown text serialization leaves angle brackets unescaped in text nodes.
Details
When converting a parsed document to Markdown, text nodes are escaped for a small set of Markdown metacharacters, but HTML-significant characters such as < and > are preserved. That means content parsed as text, including entity-decoded text or text produced by RCDATA/RAWTEXT-style parsing, can be emitted into Markdown as raw HTML.
Examples of affected input include:
- Text produced from entity-decoded input such as
<script>...</script> - Text inside elements like
<title>,<textarea>,<noscript>(when parsed as raw text), and<plaintext>
This is distinct from actual <script> or <style> elements in the DOM. Those are already dropped by default in to_markdown() unless html_passthrough=True.
Proof of Concept
General case
from justhtml import JustHTML
doc = JustHTML("<p><img src=x onerror=alert(1)></p>", fragment=True)
print(doc.to_html())
print()
print(doc.to_markdown())
Impact
Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is GHSA-3RCM-VJRC-P45J? GHSA-3RCM-VJRC-P45J is a medium-severity cross-site scripting (XSS) vulnerability in justhtml (pip), affecting versions <= 1.11.0. It is fixed in 1.12.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
- Which versions of justhtml are affected by GHSA-3RCM-VJRC-P45J? justhtml (pip) versions <= 1.11.0 is affected.
- Is there a fix for GHSA-3RCM-VJRC-P45J? Yes. GHSA-3RCM-VJRC-P45J is fixed in 1.12.0. Upgrade to this version or later.
- Is GHSA-3RCM-VJRC-P45J exploitable, and should I be worried? Whether GHSA-3RCM-VJRC-P45J 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
- What actually determines whether GHSA-3RCM-VJRC-P45J 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.
- How do I fix GHSA-3RCM-VJRC-P45J? Upgrade
justhtmlto 1.12.0 or later.