The story was first reported in the Yale Daily News, the school's student newspaper.
Princeton admissions workers allegedly used the birth dates and Social Security numbers of 11 students who had applied to both schools to tap into the Yale site, said Yale spokesman Tom Conroy.
The site, launched by the admissions office this year, was programmed to show students a fireworks display if they had been accepted and a rejection notice if they hadn't, the newspaper said.
Princeton officials did not immediately have a comment. According to reports, the school has temporarily suspended a dean of admissions pending an investigation into the incident.
Yale has also complained to the FBI. An FBI representative in Connecticut said that the bureau is assessing the situation to determine if there were violations of federal laws.
"We are deeply concerned about the privacy of our students. We have therefore notified appropriate law enforcement authorities, as well as the applicants whose web locations were accessed," Yale General Counsel Dorothy K. Robinson said in a statement. "We have also notified Princeton, and expect that they will follow up appropriately."
Conroy said that Yale plans to strengthen the security at its admissions site.