A method and system for preventing undesired behaviors by executable code modules in a peer-to-peer computer system are provided. When a code module is received, an assembly inspection module queries a blacklist for the received code module. When the received code module is found on the blacklist, the computer system prevents execution of the received code module. Each peer includes an assembly inspection module. When the received code module is not found on the blacklist, the assembly inspection module inspects the received executable code module, prior to execution, to determine whether the code module can perform any undesired behaviors. If so, the received code module is added to the blacklist and prevented from executing.