BEC Mailbox Containment

1. Prerequisites

• Confirmed mailbox compromise
• Completion of BEC investigation activities
• Access to email administration platform
• Access to Identity Provider
• Access to affected mailbox

2. Step-by-Step Instructions

1. Confirm Affected Mailbox

   • Verify affected account details.

2. Disable Attacker Access

   • Revoke:
     • Sessions
     • Tokens
     • Active authentication

3. Reset Credentials

   • Force password reset.
   • Require MFA re-registration where appropriate.

4. Remove Malicious Inbox Rules

   • Remove:
     • Forwarding rules
     • Auto-delete rules
     • Hidden rules

5. Remove Mail Forwarding

   • Disable:
     • External forwarding
     • Suspicious forwarding destinations

6. Remove Unauthorised Access

   • Remove:
     • Delegates
     • Shared access
     • OAuth grants

7. Review Mailbox Configuration

   • Validate:
     • Security settings
     • Mail flow settings
     • Access controls

8. Monitor for Continued Activity

   • Review:
     • Authentication attempts
     • Mailbox changes
     • New rules

9. Validate Containment

   • Confirm:
     • Attacker access removed
     • Mailbox secured
     • No malicious configuration remains

10. Escalate and Hand Off

    • Coordinate with recovery activities.
    • Update the incident record.

3. Post-Action

• Document all containment actions.
• Preserve evidence of malicious configuration.
• Record validation results.
• Participate in recovery activities.

Contributor

Vishal Thakur
GitHub: https://github.com/malienist

Contributed to the Arcana Incident Response Documentation Framework.