CVE-2019-10010

CVE-2019-10010

Credits

• Mohit Fawaz for identifying the issue
• Sebastiaan Knijnenburg and Ross Tuck for responsibly disclosing/relaying the issue
• John MacFarlane for investigating it and implementing the upstream fix we mirrored here

References

• https://nvd.nist.gov/vuln/detail/CVE-2019-10010
• https://github.com/thephpleague/commonmark/releases/tag/0.18.3
• https://github.com/thephpleague/commonmark/issues/353
• https://github.com/FriendsOfPHP/security-advisories/blob/master/league/commonmark/CVE-2019-10010.yaml

In league/commonmark 0.18.2 and below, malicious users can insert double-encoded HTML entities into their Markdown like this:

[XSS](javascript:alert%28'XSS'%29)

This library would (correctly) unescape the & entity to & during the parsing step. However, the renderer step would fail to properly re-escape the resulting : string, thus producing the following malicious HTML output:

<p><a href="javascript&colon;alert('XSS')">XSS</a></p>

Browsers would interpret : as a : character and allow the JS to be executed when the link is clicked.

This vulnerability was present in the upstream library this project was forked from and therefore exists in all prior versions of league/commonmark.

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.

CVE-2019-10010 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 (0.18.3); upgrading removes the vulnerable code path.

Kodem intelligence

The new 0.18.3 release mirrors the fix made upstream - we no longer attempt to preserve entities when rendering HTML attributes like href, src, title, etc.

The $preserveEntities parameter of Xml::escape() is therefore no longer used internally, so it has been deprecated and marked for removal in the next major release (0.19.0).

1. What is CVE-2019-10010? CVE-2019-10010 is a medium-severity cross-site scripting (XSS) vulnerability in league/commonmark (composer), affecting versions < 0.18.3. It is fixed in 0.18.3. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2019-10010? CVE-2019-10010 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 league/commonmark are affected by CVE-2019-10010? league/commonmark (composer) versions < 0.18.3 is affected.
4. Is there a fix for CVE-2019-10010? Yes. CVE-2019-10010 is fixed in 0.18.3. Upgrade to this version or later.
5. Is CVE-2019-10010 exploitable, and should I be worried? Whether CVE-2019-10010 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-2019-10010 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-2019-10010? Upgrade league/commonmark to 0.18.3 or later.