A phishing campaign is using a malicious Microsoft Office 365 app to access victims’ accounts without stealing their credentials.

In early December, PhishLabs analyzed a phishing email that impersonated an employee at a targeted organization in its sender address. The email complemented this spoofing tactic by incorporating the name of the targeted organization into the subject line and the title of an internal SharePoint or OneDrive file share.

Clicking on the “Open” button redirected recipients to a link that leverages login.microsoftonline.com, a legitimate hostname controlled by Microsoft, to prompt users to log in to their accounts via the tech giant’s official login page. At that point, the campaign then presented users with a permissions request page for an Office 365 app, which attackers might have created with the help of a compromised organization’s development credentials. Agreeing to those permissions enabled attackers to access a recipient’s inbox, contacts, OneDrive files and other data all without ever directly stealing their account credentials.

Similarities to a 2017 Google Phishing Operation

According to PhishLabs’ analysis, this campaign is similar to an operation that made it onto security researchers’ radar back in May 2017. ThreatPost reported at the time that the campaign spread to users via their contacts and informed them of someone wanting to share a Google Doc with them.

Once they clicked on the “Open in Docs” button, the campaign redirected victims to a legitimate Google OAuth consent screen. Upon authenticating themselves, the operation prompted victims to permit a malicious application named “Google Docs” to access their Gmail and contacts through Google’s OAuth2 service implementation.

How to Defend Against a Microsoft Phishing Campaign

Security professionals can defend against a Microsoft phishing campaign, such as the one described above, by using test phishing engagements to teach their employees about social engineering attacks. For instance, these exercises can help employees learn to look out for grammar and spelling errors that sometimes manifest in phishing emails.

Security teams should also limit users’ access based on what they need to execute their job duties effectively. Implementing this model of least privilege will prevent malicious actors from abusing compromised employees’ unnecessarily high privileges to access critical business resources.