Abstract: A Polymorphic Anti-virus Module (PAM) (200) comprises a CPU emulator (210) for emulating the target program, a virus signature scanning module (250) for scanning decrypted virus code, and an emulation control module (220), including a static exclusion module (230), a dynamic exclusion module (240), instruction/interrupt usage profiles (224) for the mutation engines (162) of the known polymorphic viruses (150), size and target file types (226) for these viruses, and a table (228) having an entry for each known polymorphic virus (150). During emulation, the emulation control module (220) may observe use of a register-indirect memory write instruction using a register that has not been initialized. Such a random write can be used as an indication that the file is probably a data file and so is unlikely to harbor a virus.

Abstract: An emulation module (110) includes a pre-fetch queue (116) having an adjustable size (126) to eliminate any dependence of virus decryption routines on the size of the pre-fetch queue (116) when emulating executable files to test for the presence of virus infections. An executable file is tested by setting (210, 258) the size of the emulator's pre-fetch queue (116) and emulating (220) the file under the guidance of an emulation control module (130). Emulated instructions are monitored and a flag is set (230) when any instructions are modified (224) after being copied to the pre-fetch queue and subsequently executed (228). Emulation continues until the emulation control module (130) indicates (230) that the file should be scanned for virus signatures. If no virus signatures are detected (234) and the flag is set (244), the size of the pre-fetch queue is reduced (258) and the process is repeated.