T1137.005 - Outlook Rules

Adversaries may abuse Microsoft Outlook rules to obtain persistence on a compromised system. Outlook rules allow a user to define automated behavior to manage email messages. A benign rule might, for example, automatically move an email to a particular folder in Outlook if it contains specific words from a specific sender. Malicious Outlook rules can be created that can trigger code execution when an adversary sends a specifically crafted email to that user.(Citation: SilentBreak Outlook Rules)

Once malicious rules have been added to the user’s mailbox, they will be loaded when Outlook is started. Malicious rules will execute when an adversary sends a specifically crafted email to the user.(Citation: SilentBreak Outlook Rules)

Atomic Tests:

Currently, no tests are available for this technique.

Detection

Microsoft has released a PowerShell script to safely gather mail forwarding rules and custom forms in your mail environment as well as steps to interpret the output.(Citation: Microsoft Detect Outlook Forms) This PowerShell script is ineffective in gathering rules with modified PRPR_RULE_MSG_NAME and PR_RULE_MSG_PROVIDER properties caused by adversaries using a Microsoft Exchange Server Messaging API Editor (MAPI Editor), so only examination with the Exchange Administration tool MFCMapi can reveal these mail forwarding rules.(Citation: Pfammatter - Hidden Inbox Rules) SensePost, whose tool Ruler can be used to carry out malicious rules, forms, and Home Page attacks, has released a tool to detect Ruler usage.(Citation: SensePost NotRuler)

Collect process execution information including process IDs (PID) and parent process IDs (PPID) and look for abnormal chains of activity resulting from Office processes. Non-standard process execution trees may also indicate suspicious or malicious behavior.