University networks already stressed by file-sharing programs, viruses and hackers now face a new threat: students who sublet their network access to spammers for as little as $20 per month.

Tufts University, a 151-year-old school in Medford, Mass., last month discovered spammers were paying students to offer up their PCs as relay points that helped mask the true source of the spam. While university network executives interviewed were not aware of other cases on U.S. campuses, the phenomenon has cropped up in Israel.