Security firms are warning of a big increase in attacks that target instant messaging systems.

Security Center researchers uncovered 21 new malicious code attacks traveling over IM networks in April. Worms that topped the list include ArcServe, IMspread, QVOD, IRCBot and Tiotua. Most, if not all, of these worms target the public IM networks such as AIM, Yahoo, and GoogleTalk. For businesses that have deployed closed instant messaging systems, the risk is minimal. Most of these threats travel like e-mail worms and users have to click  a link to get infected.

Responza’s rule of thumb – don’t accept messages from screen names you’re not familiar with and never, never, never accept links from strangers.