The U.S. Joint Chiefs of Staff's unclassified email system was brought back online Monday and the Department of Defense (DOD) confirmed that the system had been swept of malicious software.
The email system had been down since the attack was discovered on July 25. DOD officials said the system was compromised through a phishing campaign, and they maintain that no sensitive data was stolen. Restoration of the email system allowed about 4,000 Defense Department workers to regain access to their email, which was shut down as a precaution.
Initial reports blamed the attack on a Russian hacking group called APT29.
The system was isolated by the department's cyber security team and cleaned of any nefarious activity, spokeswoman Lt. Col. Valerie Henderson said in a statement, adding that the team also put new measures in place to harden the email system from future attacks.