Parsedown before 1.7.2, when safe mode is used and HTML markup is disabled, might allow attackers to execute arbitrary JavaScript code if a script (already running on the affected page) executes the contents of any element with a specific class. This occurs because spaces are permitted in code block infostrings, which interferes with the intended behavior of a single class name beginning with the language- substring.

References

• erusev/parsedown#699
• https://github.com/FriendsOfPHP/security-advisories/blob/master/erusev/parsedown/CVE-2019-10905.yaml
• https://nvd.nist.gov/vuln/detail/CVE-2019-10905
• https://github.com/erusev/parsedown/releases/tag/1.7.2