Description
By issuing instant messages to a client which contain a very large number of emoticons it is possible to cause the Microsoft Communicator to become nonresponsive for a certain period of time. During this period of time the phone does not respond to incoming invite messages and can even be forced to go into an offline state, eventually requiring the phone to reregister.
Recommendations
Microsoft has acknowledged the issue and is working on correcting it. I n the short term it is recommended that a VoIP aware IPS product, such as VoIPguard, with signatures to detect attempts to exploit this issue, be implemented to prevent it from being exploited. Implementing best practices can limit the exposure of this issue. Microsoft customers with a valid support agreement may wish to speak with their support contact in order to obtain further vendor details.