Ubuntu Linux forum was hacked last July 14th. Canonical the company developing Ubuntu disclosed that the forum's database was hacked and data was compromised including hashed passwords of 2 Million users. The hackers used a known vulnerability in an unpatched software the forum is using in its servers.
In the Linux world, the number one security isssue is updating the operating system and the software that we use with it. The first ones to violate this are the servers. Servers simply run old vulnerable software.