Risk: High
An attacker who sends a stream of specifically-crafted network traffic to a Skype client network can cause the client to overwrite part of the heap, including the heap integrity control data. Since the attacker cannot control the address where the data is written, the most likely effect will be that the Skype will abort execution due to an internal error, although other unpredictable behaviour is possible.
Such a crash will lead to a loss of availability of the Skype application until it is restarted by the user. Skype has been able to induce Skype clients to crash, but has not been able to cause the client to execute specific instructions.