Abstract. We present GHUMVEE, a multi-variant execution engine for software intrusion detection. GHUMVEE transparently executes and monitors diversified replicae of processes to thwart attacks relying on a predictable, single data layout. Unlike existing tools, GHUMVEE's interventions in the process' execution are not limited to system call invocations. Because of that design decision, GHUMVEE can handle complex, multi-threaded real-life programs that display non-deterministic behavior as a result of non-deterministic thread scheduling and as a result of pointer-value dependent behavior. This capability is demonstrated on GUI programs from the Gnome and KDE desktop environments.