A novel phishing scam has been identified that gains access to information on Office 365 accounts without obtaining usernames and passwords. The campaign also manages to bypass multi-factor authentication controls that has been set up to prevent stolen credentials from being used to remotely access email accounts from unfamiliar locations or devices.

The campaign takes advantage of the OAuth2 framework and the OpenID Connect protocol that are used to authenticate Office 365 users. The phishing emails include a malicious SharePoint link that is used to fool email recipients into granting an application permissions that allow it to access user data without a username and password.

The phishing emails are typical of several other campaigns that abuse SharePoint. They advise the recipient that a file has been shared with them and they are required to click a link to view the file. In this case, the file being shared appears to be a pdf document. The document includes the text “q1.bonus” which suggests that the user is being offered additional money. This scam would be particularly effective if the sender name has been spoofed to appear as if the email has been sent internally by the HR department or a manager.

Clicking the link in the email directs the user to a genuine Microsoft Online URL where they will be presented with the familiar Microsoft login prompt. Since the domain starts with login.microsoftonline.com the user may believe that they are on a genuine Microsoft site (they are) and that it is safe to enter their login credentials (it is not). The reason why it is not safe can be seen in the rest of the URL, but for many users it will not be clear that this is a scam.

Entering in the username and password does not provide the credentials to the attacker. It will authenticate the user and also a rogue application.

By entering in a username and password, the user will be authenticating with Microsoft and will obtain an access token from the Microsoft Identity Platform. OAuth2 authenticates the user and OIDC delegates the authorization to the rogue application, which means that the application will be granted access to user data without ever being provided with credentials. In this case, the authentication data is sent to a domain hosted in Bulgaria.

The user is required to enter their login credentials again and the rogue app is given the same permissions as a legitimate app. The app could then be used to access files stored in the Office 365 account and would also be able to access the user’s contact list, which would allow the attacker to conduct further attacks on the organization and the user’s business contacts.

The phishing campaign was identified by researchers at Cofense who warn access only needs to be granted once. Access tokens have an expiration date, but this method of attack allows the attackers to refresh tokens, so that potentially gives the attackers access to documents and files in the Office 365 account indefinitely.

With multi-factor authentication enabled, businesses may feel that they are immune to phishing attacks. Multi-factor authentication is important and can prevent stolen credentials from being used to access Office 365 and other accounts, but MFA is not infallible as this campaign shows.

This campaign highlights how important it is to have an email security solution that uses predictive technology to identify new phishing scams that have not been seen before and do not include malicious attachments. Phishing attacks such as this are likely to bypass Office 365 antispam protections and be delivered to inboxes, and the unusual nature of this campaign may fool users into unwittingly allowing hackers to access their Office 365 accounts.

For further information on how you can secure your Office 365 accounts and block sophisticated phishing attacks, give us a call today to find out how SpamTitan can improve your email defenses.