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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. 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.
Multi-factor authentication—also known as two-factor authentication, MFA, or 2FA—is the gold standard for account security. 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). As the growing use of MFA has stymied account-takeover campaigns, attackers have found ways to strike back.
Microsoft observed a campaign that inserted an attacker-controlled proxy site between the account users and the work server they attempted to log into. 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. 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. The campaign began with a phishing email with an HTML attachment leading to the proxy server.
