On-chip sensor to detect power supply vulnerabilities. The on-chip sensor employs a sensitive delay chain and an insensitive delay chain to detect power supply undershoots and overshoots without requiring external off-chip components. Undershoots and overshoots outside a user-defined threshold are detected. The undershoots and overshoots are indicated by a relative difference in phase of the two delay chains. The two delay chains are programmable to detect various frequencies.