Described embodiments provide a network processor that includes a security sub-processor to prevent replay attacks on the network processor. A memory stores an anti-replay window corresponding to a data stream of the network processor. The anti-replay window has N bits initialized to correspond to data packet sequence numbers in the range 1 to N. The anti-replay memory is stored in a plurality of data words. A plurality of flip-flops store word valid bits corresponding to each of the data words. A multiplexer selects the word valid bit corresponding to a data word requested by the security processor, and an AND gate performs a bitwise AND operation between the selected data word and word valid bit. When the network processor receives a data packet, the security sub-processor determines a value of the received sequence number with respect to minimum and maximum values of a sequence number range of the anti-replay window.