Secunia has issued an advisory today for Ubuntu users - an upgrade has been released to correct a few vulnerabilities.

Ubuntu has issued an update for php. This fixes some vulnerabilities, which can be exploited by malicious users to disclose potentially sensitive information, bypass certain security restrictions, gain escalated privileges, cause a DoS (Denial of Service), compromise a vulnerable system, and by malicious people to disclose potentially sensitive information, bypass certain security restrictions, cause a DoS (Denial of Service), and potentially compromise a vulnerable system.

The Secunia post has links to all of the packages you'll need (for each kind of architecture) to get things up to date and corrected. This issue has been marked as "Moderately Critical".