The attackers used those email threads and the hacked employee's forged identity to convince the other party to make a payment. In the days following the cookie theft, the threat actors accessed employee email accounts and looked for messages to use in business email compromise scams, which tricked targets into wiring large sums of money to accounts they believed belonged to co-workers or business partners. "In multiple cases, the cookies had an MFA claim, which means that even if the organization had an MFA policy, the attacker used the session cookie to gain access on behalf of the compromised account." "From our observation, after a compromised account signed into the phishing site for the first time, the attacker used the stolen session cookie to authenticate to Outlook online ()," members of the Microsoft 365 Defender Research Team and the Microsoft Threat Intelligence Center wrote in a blog post. The campaign began with a phishing email with an HTML attachment leading to the proxy server.Įnlarge / The phishing website intercepting the authentication process. Once the authentication was completed, the threat actor stole the session cookie the legitimate site sent, so the user doesn't need to be reauthenticated at every new page visited. When the user entered a password into the proxy site, the proxy site sent it to the real server and then relayed the real server's response back to the user. Microsoft observed a campaign that inserted an attacker-controlled proxy site between the account users and the work server they attempted to log into. As the growing use of MFA has stymied account-takeover campaigns, attackers have found ways to strike back. It requires the account user to prove their identity in the form of something they own or control (a physical security key, a fingerprint, or face or retina scan) in addition to something they know (their password). Multi-factor authentication-also known as two-factor authentication, MFA, or 2FA-is the gold standard for account security.
The threat actors behind the operation, who have targeted 10,000 organizations since September, have used their covert access to victim email accounts to trick employees into sending the hackers money. On Tuesday, Microsoft detailed an ongoing large-scale phishing campaign that can hijack user accounts when they're protected with multi-factor authentication measures designed to prevent such takeovers.
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