Russian hackers accessed the unclassified email accounts of 4,000 military and civilians at the Pentagon, including the chairman of the Joint Chiefs of Staff. 

Washington Times:

The system has been shut down since roughly July 24, according to U.S. officials, who told NBC News on Thursday that a “sophisticated cyberattack” by the hackers affected roughly 4,000 military and civilian personnel who work for the Joint Chiefs of Staff.

While it was not clear whether the attack was sanctioned by the Russian government or was the work of individuals, NBC cited the unnamed officials as saying the suspected hackers coordinated the cyber attack via social media accounts.

The officials also said no classified information was seized or compromised and that only unclassified accounts and emails were hacked — and that the system is expected to be back online before the end of this week.

The development comes months after U.S. officials revealed a separate intrusion by Russian hackers.

In April, Defense Secretary Ashton Carter said the earlier incident, which also involved an unclassified U.S. military network, exposed the growing cyber threat — as well as the improving U.S. ability to respond.

“Adversaries should know that our preference for deterrence and our defensive posture don’t diminish our willingness to use cyber options if necessary,” Mr. Carter said in prepared remarks during an April address at Stanford University, according to Reuters.

What kind of stuff is contained in your personal email account?  There are defintely some things I wouldn't want Russian hackers or anyone else finding out.  There are some key personnel at the Pentagon who have now had their personal lives hacked, perhaps giving the Russians an opportunity to apply pressure on them.

So while no classified data was compromised, the damage is still considerable.  While Pentagon cyber security is probably much better than that found in the private sector, this incident is worrisome because it reveals an ambitious agenda by Russia to compromise our most secret and valuable networks.