md5 39ca2651740c2cef91eb82161575348b

Hunt Hypothesis

The detection rule identifies potential malicious activity associated with the MD5 hash 39ca2651740c2cef91eb82161575348b, which may indicate the presence of known or emerging threats. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify and mitigate potential compromise early, especially given the low severity threshold which may mask more sophisticated attacks.

YARA Rule

rule md5_39ca2651740c2cef91eb82161575348b {
    strings: $ = /if\(md5\(@\$_COOKIE\[..\]\)=='.{32}'\) \(\$_=@\$_REQUEST\[.\]\).@\$_\(\$_REQUEST\[.\]\);/
    condition: any of them
}

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 1 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Scenario: A system update or patching tool (e.g., Microsoft Update, WSUS) downloads a file with the MD5 hash 39ca2651740c2cef91eb82161575348b as part of a legitimate update process.
  Filter/Exclusion: Check the file path against known update directories (e.g., C:\Windows\SoftwareDistribution, C:\Program Files\WindowsUpdate) or use a filter based on the process name (e.g., svchost.exe, wuauserv.exe).

• Scenario: A backup or synchronization tool (e.g., Veeam, rsync, or Azure Backup) transfers a file with the same MD5 hash during a routine data sync operation.
  Filter/Exclusion: Exclude files that match known backup directories (e.g., C:\Backup, D:\Sync) or filter by the process name of the backup tool (e.g., veeam.exe, rsync.exe).

• Scenario: A scheduled administrative task (e.g., via Task Scheduler or PowerShell) runs a script or executable that temporarily uses the hash 39ca2651740c2cef91eb82161575348b for a legitimate purpose (e.g., log file processing).
  Filter/Exclusion: Exclude files based on the task name or scheduled time, or filter by the user context (e.g., SYSTEM, Administrators).

• Scenario: A legitimate software installation or configuration tool (e.g., Docker, Ansible, or Puppet) includes a file with this hash as part of its installation package or configuration files.
  Filter/Exclusion: Exclude files located in known installation directories (e.g., C:\Program Files\Docker, `C