Phishing attacks utilizing encrypted RPMSG attachments sent through compromised Microsoft 365 accounts are targeting users to steal their Microsoft credentials while evading detection by email security gateways.
RPMSG files, created using Microsoft’s Rights Management Services, provide an extra layer of protection by restricting access to authorized recipients. Attackers exploit the authentication requirements of RPMSG files by tricking targets into providing their credentials through fake login forms.
These attacks involve emails originating from compromised accounts, such as that of Talus Pay, and prompt recipients to click a button to decrypt the message, redirecting them to a fake Office 365 webpage where they are asked to sign in.
Once authenticated, recipients are presented with a phishing email and directed to a fake SharePoint document hosted on Adobe’s InDesign service. Clicking further leads to a decoy page that collects various system information through a malicious script, including visitor ID, connect token and hash, system language, browser details, and more. The collected data is then used to create a cloned Microsoft 365 login form that sends entered usernames and passwords to attacker-controlled servers.
Trustwave researchers note that detecting and countering these targeted phishing attacks can be challenging due to their low volume and the use of trusted cloud services like Microsoft and Adobe.
To mitigate the risks of this type of phishing attack, Trustwave advises companies to educate their users about the threat and caution against decrypting or unlocking unexpected messages from external sources. Enabling Multi-Factor Authentication (MFA) for Microsoft 365 accounts is also recommended to prevent compromise.
The utilization of encrypted RPMSG attachments conceals the phishing messages from email scanning gateways, making it crucial to raise awareness and implement security measures to protect against this evolving phishing tactic.
