Parallels have released a security update to address the security vulnerability (details here). An authenticated user in Plesk for linux is able to act as root and is able to run arbitrary codes. This may result in complete server computerization.
Last day Parallels issued a security advisory to address these vulnerabilities.
These security updates will automatically visible in parallels Plesk panel, It takes only a few clicks to apply these. The security hot-fix for Plesk 9.x is also available for download in Plesk knowledge base here.