The common PNG library used by Beonex Communicator contains a buffer overflow which potentially lets an attacker use a simple image file embedded in a webpage or email to execute local code and thus take over the system (gain the same rights as the user). There is no known exploit, but it is theoretically exploitable.
The Windows version 0.8-stable-1 shiped with the vulnerable PNG library. It has been fixed in the 0.8-stable-2 build. There is also a security fix available for those who have the vulnerable version installed and want to update that instead of downloading the whole package again.
The Linux version uses the system library, so it is up to you or your system administrator to update that - Linux distributions are publishing updates now. Please check with the vendor of your Linux version.