GEN:SID 1:2458
Message CHAT Yahoo IM successful chat join
Summary This event is generated when a host in your network that has Yahoo Instant Messenger running has joined a chat room or is examining chat rooms to join.
Impact Possible policy violation.  Instant Messenger programs may not be appropriate in certain network environments.
Detailed Information Yahoo IM provides a means of allowing users who share similar interests to join a chat room and exchange messages.  While there are no known exploits associated with exchanging messages, this type of activity may not be appropriate in certain network environments.  
Affected Systems Any host running Yahoo Instant Messenger.
Attack Scenarios No known attacks.
Ease of Attack No known attacks.
Corrective Action Disallow the use of IM clients on the protected network and enforce or implement an organization wide policy on the use of IM clients.
Additional References Yahoo Protocol
http://www.cse.iitb.ac.in/~varunk/YahooProtocol.htm