spam mailer

Hunt Hypothesis

The hypothesis is that the detected behavior indicates an adversary using a spam mailer to distribute malicious content, potentially as part of a broader phishing or credential harvesting campaign. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify and mitigate early-stage threat activity that could lead to larger compromises.

YARA Rule

rule spam_mailer {
    strings:
        $ = "<strong>WwW.Zone-Org</strong>"
        $ = "echo eval(urldecode("
    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
• Source Rule

False Positive Guidance

• Scenario: System administrator uses a legitimate email tool to send out a bulk notification to all users
  Filter/Exclusion: Exclude emails sent from the domain of the email tool (e.g., @mail.example.com) or from specific user accounts designated for administrative tasks.

• Scenario: A scheduled job runs a script that generates and sends test emails to a local SMTP server for validation
  Filter/Exclusion: Exclude emails sent to a local SMTP server (@localhost or 127.0.0.1) or from scripts running under a specific service account.

• Scenario: A security analyst uses a sandboxed environment to test a phishing email attachment
  Filter/Exclusion: Exclude emails sent from or to sandboxed environments (e.g., @sandbox.example.com) or those containing known test payloads.

• Scenario: A DevOps team deploys a CI/CD pipeline that sends automated alerts via email to a monitoring team
  Filter/Exclusion: Exclude emails sent from the CI/CD tool’s email relay (e.g., @ci.example.com) or those with specific subject lines or headers indicating automated alerts.

• Scenario: A user receives a legitimate marketing email from a company’s official email domain
  Filter/Exclusion: Exclude emails from known marketing domains (e.g., @marketing.example.com) or those matching a whitelist of approved email domains.