Fake it maintenance bulletin

Hunt Hypothesis

Adversaries may use fake maintenance bulletins to mask malicious activity by mimicking legitimate system updates, allowing them to execute code under the guise of routine maintenance. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify potential compromise attempts that evade traditional detection methods.

YARA Rule

rule Fake_it_maintenance_bulletin : mail
{
  meta:
		Author = "Tyler Linne <@InfoSecTyler>"
		Description ="Rule to prevent against known phishing campaign targeting American companies using Microsoft Exchange"
  strings:
    $eml_1="From:"
    $eml_2="To:"
    $eml_3="Subject:"
    $subject1={49 54 20 53 45 52 56 49 43 45 20 4d 61 69 6e 74 65 6e 61 6e 63 65 20 42 75 6c 6c 65 74 69 6e} //Range is for varying date of "notification"
    $subject2={44 45 53 43 52 49 50 54 49 4f 4e 3a 20 53 65 72 76 65 72 20 55 70 67 72 61 64 65 20 4d 61 69 6e 74 65 6e 61 6e 63 65} //Range is for server name varriation 
    $body1="Message prompted from IT Helpdesk Support" nocase
    $body2="We are currently undergoing server maintenance upgrade" nocase
    $body3="Upgrade is to improve our security and new mail experience" nocase
    $body4="As an active Outlook user, you are kindly instructed  to upgrade your mail account by Logging-in the below link" nocase
    $body5="Sign in to Access Upgrade" nocase
    $body6="Our goal is to provide excellent customer service" nocase
    $body7="Thanks,/n OWA - IT Helpdesk Service" nocase

  condition:
    all of ($eml_*)and
    1 of ($subject*) and
    4 of ($body*) 
}

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 12 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Scenario: Scheduled System Maintenance Job
  Description: A legitimate scheduled job runs to update system maintenance logs or perform routine checks.
  Filter/Exclusion: process.name != "schtasks.exe" OR process.name != "task scheduler"

• Scenario: Admin Performing Maintenance via PowerShell
  Description: An administrator uses PowerShell to generate maintenance logs or simulate system updates.
  Filter/Exclusion: process.name != "powershell.exe" OR process.command_line NOT LIKE '%-Command%'

• Scenario: Log File Rotation or Cleanup Task
  Description: A log management tool like Splunk or ELK rotates or cleans up log files, which may resemble maintenance activity.
  Filter/Exclusion: process.name != "splunkd.exe" OR process.name != "logrotate"

• Scenario: Patch Management Tool Generating Reports
  Description: A patch management tool like Microsoft System Center Configuration Manager (SCCM) generates maintenance reports.
  Filter/Exclusion: process.name != "ccmexec.exe" OR process.name != "sccm" OR process.command_line NOT LIKE '%report%'

• Scenario: Database Maintenance Task via SQL Agent
  Description: A SQL Server Agent job runs maintenance tasks like index rebuilds or integrity checks.
  Filter/Exclusion: process.name != "sqlservr.exe" OR process.command_line NOT LIKE '%SQLAgent%'