Microsoft has issued a significant warning about the growing trend of cybercriminals exploiting legitimate file hosting services, such as SharePoint, OneDrive, and Dropbox, in Business Email Compromise (BEC) attacks. These platforms, widely used in enterprise environments, are being weaponized by threat actors to bypass traditional security defenses in a technique known as living-off-trusted-sites (LOTS). By abusing these trusted services, attackers blend their malicious activity into regular network traffic, making it harder for security systems to detect and prevent such threats. This tactic complicates attribution efforts, as the traffic appears legitimate to unsuspecting users and traditional security tools.
These BEC attacks typically begin with phishing emails sent to unsuspecting targets, containing links to files hosted on these trusted services. The files are often configured with restricted access and set to “view-only” mode, making it difficult for recipients to detect embedded malicious URLs or download the content. The phishing emails prompt the recipient to authenticate by entering their email address and a one-time password (OTP) sent through a notification service. Once the user is verified, they are redirected to a phishing page that acts as an adversary-in-the-middle (AitM), capturing their login credentials and two-factor authentication (2FA) tokens. This stolen information allows the attackers to compromise the user’s account and use it to carry out further attacks.
The impact of these attacks is broad and damaging. With stolen credentials and 2FA tokens, cybercriminals can gain access to enterprise networks, carry out financial fraud, and exfiltrate sensitive data. Additionally, the compromised accounts are often used to move laterally within the organization, expanding the attack’s reach. These campaigns typically target trusted vendors or partners first, gaining access to their accounts and using them to stage attacks on other organizations. This not only erodes trust between businesses but also opens up avenues for more sophisticated BEC attacks. Microsoft’s Threat Intelligence team emphasized that, while these campaigns are opportunistic, they involve complex social engineering techniques and advanced methods to evade detection and exploit multiple accounts across different organizations.
Compounding the threat, cybersecurity firm Sekoia recently highlighted the rise of the Mamba 2FA phishing kit, a phishing-as-a-service (PhaaS) tool that has been actively used by threat actors since November 2023. Mamba 2FA allows attackers to bypass non-phishing-resistant multi-factor authentication (MFA) methods by impersonating Microsoft 365 login pages. Sold on a subscription basis for $250 per month, the kit automates phishing campaigns that use HTML attachments designed to appear as legitimate Microsoft 365 login requests. It handles OTPs and app-based notifications, with stolen credentials and session cookies sent directly to the attackers via Telegram. This tool, along with other phishing kits, lowers the barrier to entry for cybercriminals, enabling even less technically skilled actors to launch sophisticated phishing attacks.
