When a suspected email mailbox compromise is reported, initiating an investigation promptly is critical. However, to ensure the investigation is effective, certain minimum intelligence requirements must be met. This blog outlines the bare minimum data needed to start investigating a suspected email mailbox compromise, whether the intelligence comes from an internal team or a third-party source.

Why Focus on the Minimum?

In the early stages of a suspected compromise, time is of the essence. Gathering only the essential data allows investigators to quickly assess the situation without being overwhelmed by extraneous details. The following requirements are designed to provide just enough context to confirm the compromise, identify the scope, and determine initial response actions.

Minimum Intelligence Requirements

The following data points are the absolute minimum needed to kickstart an investigation into a suspected email mailbox compromise:

• Account Details: The email address or account identifier (e.g., username) of the suspected compromised mailbox. This is critical to pinpoint the affected account.
• Timeframe of Suspected Activity: A rough estimate of when the suspicious activity was first observed (e.g., date and time of unusual emails or logins). This helps narrow down the scope of logs to review.
• Description of Suspicious Activity: A brief summary of what triggered the suspicion, such as unauthorized emails sent, unfamiliar login alerts, or reports from users or third parties. This provides context for the investigation.
• Source of the Report: Whether the information comes from an internal user (e.g., the account owner or IT team) or a third party (e.g., a customer receiving a phishing email). This helps assess the reliability of the report and potential external impact.
• Access to Relevant Logs: Availability of basic audit logs, such as login history or email activity logs, from the email provider (e.g., Microsoft 365, Gmail). If logs are not immediately accessible, confirmation of who can provide them (e.g., internal IT or third-party vendor) is sufficient to start.

Note: While additional data like IP addresses, email headers, or full forensic logs can enhance the investigation, they are not strictly required to begin. The goal is to confirm the compromise and contain it before diving into deeper analysis.

Internal vs. Third-Party Intelligence

The source of the intelligence does not significantly alter the minimum requirements, but it may affect how the data is obtained:

• Internal Party: Internal reports often come from the account owner or IT team, providing direct access to account details and logs. For example, an employee might report receiving a multi-factor authentication (MFA) push notification they didn’t initiate.
• Third Party: Third-party reports may originate from external contacts (e.g., a customer receiving a phishing email) or security vendors. These reports might lack direct access to logs, but the email address, timeframe, and activity description are usually sufficient to start.

Next Steps After Gathering Minimum Data

Once these minimum data points are collected, investigators can take immediate actions, such as:

• Verifying the reported activity by checking available logs.
• Securing the account (e.g., resetting passwords, enabling MFA, or temporarily suspending access).
• Communicating with the account owner or third parties to gather additional details.

Conclusion

Investigating a suspected email mailbox compromise doesn’t require an exhaustive dataset to begin. By focusing on the bare minimum intelligence—account details, timeframe, activity description, report source, and log access—investigators can quickly confirm the issue and take action to mitigate damage. Whether the intelligence comes from an internal team or a third party, these core data points provide a solid foundation for a timely and effective response.