The rapid complexity growth of electronic systems nowadays increases their vulnerability to hacking, such as fault injection, including insertion of glitches into the system clock to corrupt internal state through timing errors. As a countermeasure, a frequency locked loop (FLL) based clock glitch detector is proposed in this paper. Regulated from an external supply voltage, this FLL locks at 16-36X of the system clock, creating four phases to measure the system clock by oversampling at 64-144X. The samples are then used to sense the frequency and close the frequency locked loop, as well as to detect glitches through pattern matching. Implemented in a 5nm FINFET process, it can detect the glitches or pulse width variations down to 3.125% of the input 40MHz clock cycle with the supply varying from 0.5 to 1.0V.