Power and clock transients can also be used in some processors to affect the decoding and execution of individual instructions. Every transistor and its connection paths act like an RC element with a characteristic time delay; the maximum usable clock frequency of a processor is determined by the maximum delay among its elements. Similarly, every flip-flop has a characteristic time window (of a few picoseconds) during which it samples its input voltage and changes its output accordingly. This window can be anywhere inside the specified setup cycle of the flip-flop, but is quite fixed for an individual device at a given voltage and temperature.
So if we apply a clock glitch (a clock pulse much shorter than normal) or a power glitch (a rapid transient in supply voltage), this will affect only some transistors in the chip. By varying the parameters, the CPU can be made to execute a number of completely different wrong instructions, sometimes including instructions that are not even supported by the microcode. Although we do not know in advance which glitch will cause which wrong instruction in which chip, it can be fairly simple to conduct a systematic search.Glitch attacks undermine what is perhaps the most common and fundamental assumption (law #1) of secure computing - that software will always run as it was coded. The breaking of this paradigm created a situation in the late 90s in which pretty much every smart card in the world could be hacked at a low cost. It took the smart card industry several years to design and deploy new hardware that was resistant to this kind of attack (solutions include various forms of input regulators, filters and detectors). Attempts to prevent this form of attack using special secure coding techniques proved ineffective - you can't really solve a security hole using software if you can't rely on the software running as coded.
Yesterday a hacker named gligli announced that he performed such a glitch attack on the Xbox 360, the details of which he described in a Free60.org wiki:
We found that by sending a tiny reset pulse to the processor while it is slowed down does not reset it but instead changes the way the code runs, it seems it's very efficient at making bootloaders memcmp functions always return "no differences". memcmp is often used to check the next bootloader SHA hash against a stored one, allowing it to run if they are the same. So we can put a bootloader that would fail hash check in NAND, glitch the previous one and that bootloader will run, allowing almost any code to run.As far as I know this is the first successful glitch attack performed on a consumer electronics device; first but not last. Consumer electronics (CE) device security engineers have never made the paradigm shift to a world with glitch attacks. It is quite likely that a similar glitch attack can be done on other CE devices be they other game consoles, cell phones or tablets. This is really big news and it's surprising that it hasn't made more waves in the security blog-sphere.
One of the reasons CE devices have been considered less susceptible to glitch attacks is that they run at a much higher clock frequency (many smart cards run at under 5 MHz while modern CE devices run at over 200 MHz). The Xbox 360 helpfully has a signal that allows hackers to reduce the clock frequency to 520 KHz - making glitch attacks much easier. If the Xbox security team had more smart card design experience they probably wouldn't have made such a mistake.