Described embodiments provide a network processor that includes a security protocol processor to prevent replay attacks on the network processor. A memory stores security associations for anti-replay operations. A pre-fetch module retrieves an anti-replay window corresponding to a data stream of the network processor. The anti-replay window has a range of sequence numbers. When the network processor receives a data packet, the security hardware accelerator 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. Depending on the value, the data packet is either received or accepted. The anti-replay window might be updated to reflect the receipt of the most recent data packet.