do u want to have ur critical data accessible to any person?a firewall is needed to protect!although i know by default ubuntu comes with ports closed.also do u believe every binary formats esp proprietory one in elf format?elf injection thing is there where a s/w in elf format can be used to attain root permissions through its vulnerabilities.that is why Debian Etch(stable) got security fixes!

.though all these comes into use in only in serve scenario.
with cross platform lang like java you are vulnerable with stealing data for eg,the passwd cheat demo done by heisse security shows that!
as frank as i can

though my knowledge reg network is little.