![]() The custom form and the message format are tailor-made for each other. The custom form is triggered when the mailbox receives a specific message from the attacker that requires the mailbox to load the custom form. The attacker inserts a custom mail form template into the user's mailbox. The malware allows the attacker to steal (or steal again) the user's username and password or other credentials from local machine and perform other malicious activities. Typically, the application installs malware on the user's machine (for example, PowerShell Empire). Typically, the rule action is to launch an application on a remote (WebDAV) server. When the mailbox receives a message that matches the conditions of rule, the action of the rule is applied. The attacker sends the trigger email to the compromised mailbox, which is still being used as normal by the unsuspecting user. The rule conditions and message format are tailor-made for each other. The forwarding rule is triggered when the mailbox receives a specific message from the attacker that matches the conditions of the rule. The attacker creates a forwarding Inbox rule in the mailbox. The attacker signs in to that user's Exchange mailbox (Exchange Online or on-premises Exchange). The attacker steals a user's credentials. The attacks typically follow these patterns: The good news is: if you keep your Outlook clients patched to the latest version, you aren't vulnerable to the threat as current Outlook client defaults block both mechanisms. The malware steals credentials or performs other illicit activity. The rules or forms are typically designed to run remote code and install malware on the local machine. When the fresh installation of Outlook connects to the mailbox, all rules and forms are synchronized from the cloud. Reinstalling Outlook, or even giving the affected person a new computer won't help. By injecting custom forms into Outlook.There are two ways that an attacker can use Outlook to establish a persistence mechanism: This activity is called establishing a persistence mechanism. What is the Outlook Rules and Custom Forms injection attack?Īfter an attacker gains access to your organization, they'll try to establish a foothold to stay in or get back in after they've been discovered. ![]() Summary Learn how to recognize and remediate the Outlook rules and custom Forms injections attacks in Office 365. Learn about who can sign up and trial terms here. Did you know you can try the features in Microsoft 365 Defender for Office 365 Plan 2 for free? Use the 90-day Defender for Office 365 trial at the Microsoft 365 Defender portal trials hub. ![]()
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