1. A security method for a computational system, the security method comprising: marking as tainted, at least some storage locations to which information is transferred from an untrusted source; and selectively propagating, by a computing device, the taint to additional storage locations in correspondence with information flows of instruction sequences executed in the computational system, wherein the selectivity of the taint propagating is based at least in part on an aging attribute that is varied according to aging of the taint and the aging attribute is used in making a decision on the selective propagation of the taint.