Unfortunately method of generating new file names is very simple and weak. So, if we are capable of predicting next file name we can cause effective DoS for one or even all sessions, no matter we have access to repository, or not. On heavily loaded server it is hard to predict next PID value and in this way we can stop only one cvs session from normal working. There is a cvs server option which allow us to use another temporary directory. Unfortunately it's not really correct solution, cause cvs users still can stop server from working.
Simple exploit is attached.