A common hack is the (Distributed) denial-of-service ((D)DoS) attack. Here, the malicious attacker attempts to consume, indeed overload, resources on the target system to such an extent that the system either crashes, or at the very least, becomes completely unresponsive (hung).
Interestingly, on an untuned system, performing this type of attack is quite easy; as an example, let's imagine we have shell access (not root, of course, but as a regular user) on a server. We could attempt to have it run out of disk space (or at least run short) quite easily by manipulating the ubiquitous dd(1) (disk dump) command. One use of dd is to create files of arbitrary lengths.
For example, to create a 1 GB file filled with random content, we could do the following:
$ dd if=/dev/urandom of=tst count=1024 bs=1M
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB, 1...