Description

Microsoft 365 "Direct Send" is being abused by attackers to deliver sophisticated phishing that appearsto originate from within attack organizations. The attackers are abusing the feature, which is designed for unauthenticated message relays from internal devices (e.g., printers orapplications), to deliver legitimate-surfacing phishing emails and spoof internal email addresses. Proofpoint researchers found that attackers are bypassing Microsoft 365's built-in security controls by utilizing insecure third-party email appliances, which are being hosted most often on virtual private servers (VPS), as SMTP relays. These fake emails usually even when authentication checks like SPF, DKIM, or DMARC fail end up in the user inbox or spam folder. By using such topics as work reminders, payment requests, or voicemail notification, the phishing emails are designed to resemble official company message to confuse recipients. Spoof "From" addresses are used by attackers in the process of creating SMTP sessions when they have obtained an RDP connection with Windows Server 2022 virtual hosts. They bypass ill-configured relays that support encrypted transport but can utilize self-signed or outdated certificates, thus allowing attackers to evade detection. Although Microsoft's internal network can detect spoofing (e.g., "compauth=fail" in headers), the messages reach end users nonetheless, adding to the chances of social engineering and hijacking of credentials succeeding. Organisations need to use PowerShell scripts to switch off and disable extraneous Direct Send settings, impose strict email authentication protocols (SPF, DKIM, DMARC), and monitor for unauthorized relaying to combat the threat. To execute security measures for Microsoft 365security hardening, security operations teams need to utilize advancedemail security technologies and search for spoofing indicators in email headers. Such phishespresent how cloud technology abuse canerode business trust and lead tosignificant security risks.