The computer system at the center was "improperly accessed from outside of the Stanford network" on May 11, according to university General Counsel Debra Zumwalt. The system contained client and recruiter records dating back to 1996.
Client records generally include names, resumes, letters of recommendation and Social Security numbers, but no financial data, university officials said. Some recruiter records contained credit card numbers.
There is no evidence that any records were actually stolen in the break-in, officials said, but they are notifying affecting clients and recruiters. Altogether, 9,900 are being notified.
"Protection of confidential information is a high priority at Stanford," Zumwalt said in a statement. "And since this incident, we have been working to understand this breach of our system and ways to prevent a reoccurence."