White House spokesman Jay Carney told reporters the White House is equipped with mitigation measures that identified the attack, isolated it and prevented its spread.
He said there was no indication that any data was removed.
“There are distinctions between those networks that contain classified information and those that don’t, and the attack was against an unclassified network,” Carney said.
Carney described the attack as “spear-phishing” and said such efforts against government computer systems are “not infrequent.” Carney spoke in Henderson, Nev., where President Barack Obama is preparing for his first debate against rival Mitt Romney on Wednesday.
“Phishing” is a tactic that involves sending an email that falsely claims to be from a legitimate enterprise in an attempt to trick the user into turning over information.
The White House would not say whether the recent attack was linked to China.
US War Secretary Leon Panetta, during a visit to China last month, raised the subject of China-based cyber-attacks against American companies and the government.
News of the most recent attack came as the Obama administration is preparing an executive order with new rules to protect US computer systems.
After Congress failed this summer to pass a comprehensive cyber-security bill, the White House said it would use executive branch authorities to improve computer security, especially for networks tied to essential US industries, such as electric grids, water plants, and banks.