A computer device has a kernel driver in a kernel mode of the operating system which records an access token as initially associated with a user process. Later, the user process presents its access token when requesting certain operations through the operating system. The kernel driver detects that the user process has been subject to an escalation of privilege attack by evaluating the access token in its presented form as against the initially recorded access token and, in response, performs a mitigation action such as suspending the user process.