>Can access rights leak to someone else? Can somebody in the future gain access to a resource because their cell in the ACM has access rights to this resource? ### Safety property Given initial state and HRU commands, does there exist a sequence of commands that results in access right $r$ appearing in a cell of the ACM in which it does not appear in the initial state? Safety property basically asks if someone can gain access to a resource that it should not have access to, because of a sequence of ACM operations. ### HRU undecidability result In general, it is undecidable whether a given configuration is safe for a given generic right. This proof was done by reduction to the [[Turing machine#Halting problem|Halting Problem of a Turing machine]]