The US Justice Department and the FBI have received information from researchers at Cyberreason, of a long term infiltration of dozens of unnamed North American companies, for purposes of stealing company secrets and IP. This effort that ran for a number of years is alleged to be sponsored by the Chinese government.
The amount of data that is believed stolen in this attack is many hundreds of gigabytes, which included information that could/has aided further attacks.
If the hackers are stealthy in their attacks, and carry out no actions that are destructive or easily detected, they can linger in a system for years, harvesting whatever information they want. If no ransom demands follow the exfiltration of the system and little has remained in the logs and files, then who knows they have been there?
