Decrypt IC firmware is one of the most widely used non-invasive methods include playing around with the supply voltage and clock signal. Under-voltage and over-voltage IC decryption could be used to disable protection circuit or force a processor to do the wrong operation.
For these reasons, some secured processors have a voltage detection circuit which has been used to against the MCU code reading, but this circuit cannot react to fast transients. Power and clock transients can also be used in some processors to affect the mcu program decoding and execution of individual instructions.
Another problem that affects hardware security is the fact that usually a whole family of chips from one manufacturer has the same implementation of the security protection. It means that once an attacker finds a way to overcome the security in one device, very likely he would be able to break another. Manufacturers do change the security protection from time to time, but again that affects a wide range of products simultaneously.
Nowadays attackers are very clever. They do not believe in what the manufacturers claim about the security of their products. They are constantly looking for new and low-cost attack methods, and they never give up. As a result there is a permanent battle between the manufacturers who are trying to improve the security of their products and the attackers who are constantly breaking these products.
There is no real change in this process within the last decade – only temporary shifts of the front line from time to time. For sure modern smartcards are extremely secure, but attackers are not idle and sometimes are very successful. That forces the developers to update their products quite often.