ICQ security
ICQ is literally filled with security holes, mainly because you do not connect through a server but connect directly to the client your talking to. Aside from that problem, the real risk lies in the client-operations. In most IM-protocols requests are sent to the server and validated/executed there, ICQ doesn't do this, all requests/actions are executed and validated at the client. This is why there are so many ICQ 'hacking-tools' out there.
Sofar a number of ICQ versions are available for download, with a lot of 'nice' features, but (still full of security holes and in their beta testversion/buggy). Sofar no bugs have been reported in ICQ 5 but keep watching below is a report about the severity of bugs found (note, none were patched, at this time no bugs were reported for ICQ 5). To see a list of vulnerabilities in all versions of ICQ go here