NEW YORK – Hackers have struck elite US universities in recent weeks, pilfering sensitive data from donors and making off with students’ information in a string of digital heists that only add stress to schools already besieged with political pressure. Harvard University, Princeton and the University of Pennsylvania have each disclosed breaches within the past three weeks, months after a politically motivated attacker burrowed deep into networks at New York City’s Columbia University. The identity of the intruders in the most recent incidents remains a mystery. However, the hacker in each case duped school employees in order to access databases...