The Month of Apple Bugs (MOAB) project has identified a vulnerability within iChat.
The vulnerability can lead to a denial of service attack using Apple's Bonjour 'zero configuration' networking technology.
The bug researchers believe that if a malicious application repeatedly advertisers a user's presence using Bonjour, iChat will keep adding that user to the contacts list.
This blocks iChat users from finding additional people on the network and makes communication over the network unstable.
The researchers also describe a second associated bug. Apparently, it's possible to build a malicious application that will cause the iChat Agent to crash.
"Apple iChat Bonjour functionality is affected by several remotely exploitable denial of service flaws which can be triggered via advertising presence services over multicast DNS," the MOAB researchers claim.
Security experts at Secunia rate the risk as "not critical".
"The vulnerability is caused due to an error in iChat agent using Bonjour within the processing of TXT key hashes. This can be exploited to cause the application to crash by broadcasting a specially crafted TXT key hash," Secunia explains.
Both vulnerabilities can be protected against by refraining to use iChat with Bonjour on anything other than a trusted network.