Due to a security flaw, the vendor has warned that all versions from Ubuntu 6.06 onwards to 8.04 are vulnerable to a local security exploit that could result in system compromise once the attacker gains access to root.
The flaw is not a remote vulnerability. The attacker would have to have an account on the system in question in order to attempt the exploit, the result of which could range from crashing the system to compromising its data. The issue affects multiple editions of Ubuntu, including Kubuntu, Edubuntu and Xubuntu.
Canonical sent the warning email earlier this week. If you are using an older version of Ubuntu, now is a good time to update.