From Joomla! Documentation
A vulnerable extension is one that has been found to contain (or contribute to) a security vulnerability.
Vulnerable extensions are not necessarily poorly-coded. As the Web evolves, technical requirements and commonly accepted coding practices change. Active projects release new versions of their extensions as requirements change. For this reason, it is important to:
- Know the version numbers of all installed extensions.
- Use only the latest stable version of all extensions.
- Completely remove all files of insecure or unused extensions.
- Check the Vulnerable Extension List on a regular basis and remove or update any extension version found to be vulnerable.
A RSS feed is also available http://feeds.joomla.org/JoomlaSecurityVulnerableExtensions