Detects mailsearcher module from Trickbot Trojan

Hunt Hypothesis

This hunt targets adversaries executing the Trickbot MailSearcher module to harvest email metadata and identify potential targets for phishing campaigns within the organization. Proactively hunting for this behavior in Azure Sentinel is critical because early detection of this reconnaissance activity allows security teams to isolate compromised endpoints before Trickbot escalates to credential theft or lateral movement phases.

YARA Rule

rule MALW_mailsercher_trickbot_module :
Trojan
{
meta:
author = "Marc Salinas @Bondey_m"
description = " Detects mailsearcher module from Trickbot Trojan"
strings:
$str_mails_01 = "mailsearcher"
$str_mails_02 = "handler"
$str_mails_03 = "conf"
$str_mails_04 = "ctl"
$str_mails_05 = "SetConf"
$str_mails_06 = "file"
$str_mails_07 = "needinfo"
$str_mails_08 = "mailconf"
condition:
all of ($str_mails_*)
}

Deployment Notes

This YARA rule can be deployed in the following contexts:

• Microsoft Defender for Endpoint — Custom indicators / advanced hunting
• Email Gateway — Attachment scanning
• File Share Monitoring — Periodic scanning of shared drives
• YARA CLI — Manual threat hunting on endpoints

This rule contains 8 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 4 specific false positive scenarios for the “Detects mailsearcher module from Trickbot Trojan” rule in an enterprise environment, along with suggested filters or exclusions:

• Scenario: Microsoft Outlook Add-in Deployment

  • Context: The organization deploys a third-party email security add-in (e.g., Proofpoint Email Security or Barracuda Sentinel) via Group Policy. During the installation or update phase, the setup executable extracts a module named mailsearcher.dll (or similar) into the Outlook program directory (C:\Program Files\Microsoft Office\root\Office16) to scan incoming messages for phishing links before they reach the user’s inbox.
  • Suggested Filter: Exclude execution of processes containing “mailsearcher” when the parent process is msiexec.exe or setup.exe and the file path resides within standard Office installation directories (C:\Program Files\Microsoft Office*).

• Scenario: Scheduled Archive Indexing Job

  • Context: An IT administrator has configured a nightly scheduled task using Windows Task Scheduler to run a legacy internal utility named MailSearcher.exe. This tool scans the local OST/PST files of shared mailboxes to index metadata for compliance reporting. The job runs under the context of a dedicated service account (e.g., svc-mail-index) rather than an interactive user session.
  • Suggested Filter: Create an exclusion rule that ignores alerts where the process name is MailSearcher.exe and the User Account matches known service accounts (e.g., names starting with svc-, domain\service_) or the command line arguments include specific flags like /scheduled or /compliance.

• Scenario: Endpoint Protection Heuristic Scanning

  • Context: The enterprise uses a next-generation antivirus solution such as **CrowdStrike Falcon