Microsoft: Malicious OAuth applications abuse cloud email services spread spam

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https://www.microsoft.com/security/blog/2022/09/22/malicious-oauth-applications-used-to-compromise-email-servers-and-spread-spam/

Microsoft researchers recently investigated an attack where malicious OAuth applications were deployed on compromised cloud tenants and then used to control Exchange Online settings and spread spam. The investigation revealed that the threat actor launched credential stuffing attacks against high-risk accounts that didn’t have multi-factor authentication (MFA) enabled and leveraged the unsecured administrator accounts to gain initial access. The unauthorized access to the cloud tenant enabled the actor to create a malicious OAuth application that added a malicious inbound connector in the email server. The actor then used the malicious inbound connector to send spam emails that looked like they originated from the targets’ domain. The spam emails were sent as part of a deceptive sweepstakes scheme meant to trick recipients into signing up for recurring paid subscriptions…

For the attack to succeed, the threat actor needed to compromise cloud tenant users with sufficient permissions that would allow the actor to create an application in the cloud environment and give it admin consent. The actor performed credential stuffing attacks against their targets, attempting to access users with the global admin role. The authentication attempts, which originated from a single IP address, were launched against the Azure Active Directory PowerShell application (app ID: 1b730954-1685-4b74-9bfd-dac224a7b894). The same application was later used to deploy the rest of the attack…

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… Leveraging its cross-signal capabilities, Microsoft 365 Defender alerts customers using Microsoft Defender for Office 365, Microsoft Defender for Cloud Apps, Application governance add-on, and Azure Active Directory Identity Protection to detect the techniques covered in the attack through the attack chain.

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