Methods are provided herein for communications bus signal fingerprinting. A security module monitors a plurality of voltage lines of at least one electronic control unit (ECU) electrically coupled to a communications bus. A voltage differential across at least two of the plurality of voltage lines of the at least one ECU is measured. The voltage differential is compared to a plurality of predetermined signal fingerprints associated with the at least one ECU. A variance in the compared voltage differential is identified relative to one or more of the plurality of predetermined signal fingerprints. Data characterizing the identified variance is provided.