Professor Mike Schmitt has written a three-part article on foreign cyber election interference for the European Journal of International Law Talk series. The three parts are available to read here:

Part 1: https://www.ejiltalk.org/foreign-cyber-interference-in-elections-an-international-law-primer-part-i/

Part 2: https://www.ejiltalk.org/foreign-cyber-interference-in-elections-an-international-law-primer-part-ii/

Part 3: https://www.ejiltalk.org/foreign-cyber-interference-in-elections-an-international-law-primer-part-iii/