A computer implemented method of detecting malicious code in a software application executing with a runtime environment in a computer system, the method including: receiving a definition of one or more runtime exception trigger conditions, each trigger condition defining criteria for the runtime environment to enter a software exception state and having associated a definition of a resulting state of the runtime environment having entered the exception state; monitoring the runtime environment to detect the satisfaction of a detected trigger condition by the runtime environment and, in response to a determination that the runtime environment fails to enter a resulting state associated with the detected trigger condition, identifying the software application as including malicious code.