Self-checking software tamper resistance mechanisms employing checksums, including advanced systems as recently proposed by Chang and Atallah (2002) and Horne et al. (2002), have been promoted as an alternative to other software integrity verification techniques. Appealing aspects include the promise of being able to verify the integrity of software independent of the external support environment, as well as the ability to automatically integrate checksumming code during program compilation or linking. In this paper, we show that the rich functionality of many modern processors, including UltraSparc and x86-compatible processors, facilitates automated attacks which defeat such checksumming by self-checking programs.