A phishing email campaign detailed earlier this month is expanding with the use of additional email services to hide malicious intent, according to a warning from software giant Microsoft.
Dubbed ‘Compact’ Campaign, the operation has been ongoing since December 2020, targeting thousands of users. In early March, researchers with the WMC Global Threat Intelligence Team estimated that more than 400,000 Outlook Web Access and Office 365 credentials had been compromised in multiple, connected campaigns.
At the time, the researchers revealed that the adversary behind the campaign was leveraging trusted domains to ensure that phishing emails successfully bypass email protections.
Compromised accounts at the SendGrid email delivery service were used to send many of the emails. After the researchers and SendGrid started terminating the sending accounts, the threat actor switched to MailGun to send the phishing messages.
Now, Microsoft says that the phishing messages are relying on compromised accounts on email marketing services and leverage configuration settings to bypass phishing protections that organizations might have in place.
In addition to SendGrid, the tech giant reveals, the campaign’s operators abused Amazon SES last year, and started leveraging Mailgun for the same purposes since January.
“Microsoft Defender for Office 365 data shows that this phishing operation is still active today and continues to expand,” the company said on Twitter.
“The attackers abuse another legitimate service to further mask the malicious intent of their phishing emails. To evade domain reputation-based solutions, they use Appspot to create multiple unique phishing URLs per recipient,” the tech giant added.
Microsoft also notes that Appspot has been notified on the abuse, and that the company has already confirmed that the reported URLs are malicious. Appspot already took action against the offending projects and is working with Microsoft on tracking this operation.
Some of the phishing emails used in these attacks masquerade as notifications from video conferencing services, while recent attacks spoof security solutions and productivity tools, Microsoft reveals.
“Because this campaign uses compromised email marketing accounts, we strongly recommend orgs to review mail flow rules for broad exceptions that may be letting phishing emails through,” the company concludes.
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