Sixty-one of the 102 people who got the test calls, including managers and a contractor, complied with a request that the employee provide his or her user name and temporarily change his or her password to one the caller suggested, according to the Treasury Inspector General for Tax Administration, an office that does oversight of Internal Revenue Service.
The caller asked for assistance to correct a computer problem.
The report said that by failing to question the identity of the caller the employees were putting the IRS at risk of providing unauthorized people access to taxpayer data that could be used for identity theft and other fraudulent schemes.
"This is especially disturbing because the IRS has taken many steps to raise employee awareness of the importance of protecting their computers and passwords," said Inspector General J. Russell George.
Only eight of the 102 employees contacted either the inspector general's office or IRS security offices to validate the legitimacy of the caller.
The report said the IRS took measures to improve security after two similar test telephone calls in 2001 and 2004. "However, the corrective actions have not been effective," it said.
The IRS agreed with recommendations from the inspector general that it should take steps to make employees more aware of hacker tactics such as posing as an internal employee and to remind people to report such incidents to security officials.
The IRS has nearly 100,000 employees and contractors with access to tax return information processed on about 240 computer systems and more than 1,500 databases.