The hackers who recently broke into Microsoft’s network and monitored top executives’ email for two months did so by gaining access to an aging test account with administrative privileges, a major gaffe on the company’s part, a researcher said.

The new detail was provided in vaguely worded language included in a post Microsoft published on Thursday. It expanded on a disclosure Microsoft published late last Friday. Russia-state hackers, Microsoft said, used a technique known as password spraying to exploit a weak credential for logging into a “legacy non-production test tenant account” that wasn’t protected by multifactor authentication. From there, they somehow acquired the ability to access email accounts that belonged to senior executives and employees working in security and legal teams.

A “pretty big config error”

In Thursday’s post updating customers on findings from its ongoing investigation, Microsoft provided more details on how the hackers achieved this monumental escalation of access. The hackers, part of a group Microsoft tracks as Midnight Blizzard, gained persistent access to the privileged email accounts by abusing the OAuth authorization protcol, which is used industry-wide to allow an array of apps to access resources on a network. After compromising the test tenant, Midnight Blizzard used it to create a malicious app and assign it rights to access every email address on Microsoft’s Office 365 email service.

In Thursday’s update, Microsoft officials said as much, although in language that largely obscured the extent of the major blunder. They wrote:

Threat actors like Midnight Blizzard compromise user accounts to create, modify, and grant high permissions to OAuth applications that they can misuse to hide malicious activity. The misuse of OAuth also enables threat actors to maintain access to applications, even if they lose access to the initially compromised account. Midnight Blizzard leveraged their initial access to identify and compromise a legacy test OAuth application that had elevated access to the Microsoft corporate environment. The actor created additional malicious OAuth applications. They created a new user account to grant consent in the Microsoft corporate environment to the actor controlled malicious OAuth applications. The threat actor then used the legacy test OAuth application to grant them the Office 365 Exchange Online full_access_as_app role, which allows access to mailboxes. [Emphasis added.]

Kevin Beaumont—a researcher and security professional with decades of experience, including a stint working for Microsoft—pointed out on Mastodon that the only way for an account to assign the all-powerful full_access_as_app role to an OAuth app is for the account to have administrator privileges. “Somebody,” he said, “made a pretty big config error in production.”

https://arstechnica.com/?p=1999478