How Advanced Attackers Take Aim at Office 365

Microsoft Office 365 is quickly becoming the most common business email platform and a hot target for cybercriminals following their enterprise victims to the cloud. The size, complexity, and data stores in Office 365 make it an appealing and often easy target for advanced actors.

Since he started working at Mandiant more than four years ago, manager Josh Madeley has seen attackers use Office 365 in ways he never previously considered while handling large intrusion investigations. As more companies adopted the platform, advanced persistent threat (APT) groups started to use tactics nobody had discussed let alone developed investigative methodology for, he says. 

Office 365 encompasses not only Microsoft Exchange, but also Teams, SharePoint, OneDrive, and other popular enterprise tools. The sheer amount of data stored in the full platform means attackers often don't need to compromise the on-premises network to get what they're after.

The past year alone has seen multiple cases in which APT actors have realized Office 365 is a "gold mine of information," adds Doug Bienstock, principal consultant at Mandiant. His work in red teaming and incident response has led Bienstock down the path of investigating the Office 365 platform from an attacker's perspective. They're becoming experts in it, he says. 

Madeley and Bienstock's research into APT attacks targeting Office 365 has given them insight into how and why advanced adversaries use it. Most APTs target an organization to steal data or gain additional access; sometimes they target one business with the goal of breaching another. Advanced attackers seek long-term, persistent access and will use techniques that enable them to keep access to a mailbox for a period of months or years.

Email isn't the only target, they say. Many seek access to Teams, where developers chat about API keys and vulnerabilities, or SharePoint and OneDrive, where sensitive files are usually stored. "It's really a huge treasure trove of information," Bienstock says. "Especially in the last six months with COVID, it's what a lot of organizations are dependent on to work together."

Unskilled attackers typically target Office 365 with business email compromise (BEC). In the APT model, intruders acquire administrator credentials for the infrastructure so they can access the configuration of Office 365 and learn how it integrates with the on-premises network. Active Directory, Madeley and Bienstock say, is an example of a main integration point; APT actors have taken advantage of this to ensure persistent access to Office 365 content. Sometimes they exploit bugs in the way Office 365 validates configuration changes, and sometimes they act as an admin to access data.

Legacy software is a consistent issue across organizations and software vendors, says Bienstock. In one 2020 investigation he and Madeley and managed, an attacker learned if they accessed Office 365 using a piece of software from 2016, they could bypass the business' multifactor authentication (MFA) requirement. This wasn't an issue Microsoft was aware of at the time, but it was a legacy loophole that had never been closed.

To pull this off, an attacker would have to download an old version of the target software and go through trial and error to see what worked – not something an unskilled actor would do.

On the data theft side, Madeley says some attackers abuse Office 365's built-in security tools designed for admins to detect malicious activity. Some actors are using these tools to sneak in, noticing the logging mechanisms are insufficient in detecting and analyzing which data was stolen. These tools have the greatest access but don't raise any red flags, he points out.

Madeley and Bienstock will discuss more of these attack methods in their upcoming Black Hat USA talk, "My Cloud is APT's Cloud: Investigating and Defending Office 365," on August 6, 2020.

Improving Defenses: What Businesses Can Do
"As we work with organizations, we realize they don't necessarily have the same experience in investigating these things," Madeley says of cloud intrusions. While he's an advocate of moving to the cloud and believes cloud platforms are more secure in most instances, he also sees a learning curve for businesses that have managed servers in a more traditional way versus the growing platform-as-a-service (PaaS) models.

There is a misconception that Office 365 is just an email platform, Bienstock adds, "but it's really much more than that." He and Madeley are trying to shine a spotlight on this and help people realize they need to invest time and effort in improving their security posture and educating analysis and defenders. Microsoft updates its platforms often, and they need to be in the loop.

Madeley advises organizations to take a closer look at their shared security model and identify where Microsoft assumes responsibility for certain parts of the infrastructure, and where they assume the remaining portion. "Where is that gap? What's between that gap? Where does Microsoft start and stop, and where does user knowledge of tools start and stop?" he asks.

Businesses should also enable MFA if they haven't already, Bienstock adds. "You'd be surprised how many organizations of all shapes and sizes are not enforcing multifactor for Office 365," he says.

Related Content:

Check out The Edge, Dark Reading's new section for features, threat data, and in-depth perspectives. Today's featured story: "The Entertainment Biz Is Changing, but the Cybersecurity Script Is One We've Read Before."