There was a security issue found in Ubuntu versions 6.06 LTS, 8.04 LTS, 9.10, 10.04 LTS, 10.10 also the coresponding versions of Kubuntu, Edubuntu and Xubuntu were affected.
The dhcp client utility incorrectly filtered crafted responses, as a result an attacker could use this flaw with a malicious DHCP server to execute arbitrary code, that could end in root privilege escalation.
In my opinion this is a very bad issue, by gaining root access to a Linux machine practically give attackers the possibility to do whatever they want with it. This can end with very bad consequiences to your machine and data stored on it.
More information about the bug and and how it can be fixed can be found at the source link: http://seclists.org/bugtraq/2011/Apr/98