"The suspected Russian hackers behind the worst U.S. cyber attack in years leveraged reseller access to Microsoft Corp services to penetrate targets that had no compromised network software from SolarWinds," Joseph Menn and Raphael Satter at Reuters report:
While updates to SolarWinds' Orion software was previously the only known point of entry, security company CrowdStrike Holdings Inc said Thursday hackers had won access to the vendor that sold it Office licenses and used that to try to read CrowdStrike's email. It did not specifically identify the hackers as being the ones that compromised SolarWinds, but two people familiar with CrowdStrike's investigation said they were.
CrowdStrike uses Office programs for word processing but not email. The failed attempt, made months ago, was pointed out to CrowdStrike by Microsoft on Dec. 15.
CrowdStrike, which does not use SolarWinds, said it had found no impact from the intrusion attempt and declined to name the reseller.
"They got in through the reseller's access and tried to enable mail 'read' privileges," one of the people familiar with the investigation told Reuters. "If it had been using Office 365 for email, it would have been game over."
Many Microsoft software licenses are sold through third parties, and those companies can have near-constant access to clients' systems as the customers add products or employees.
More at Reuters: Suspected Russian hackers used Microsoft vendors to breach customers