Cybersecurity & Information Security

Business Email Compromise and Inbox Rules Investigation

6 min readPublished: August 5, 2026

Professional visual illustration on BEC investigation in Incident Response and DFIR

Quick answer

BEC investigation is a controlled process that balances damage containment with evidence preservation. Document the source, time, and tools, save the Hash, build a Timeline, and separate fact, interpretation, and decision.

Incident Response and DFIR require a balance between speed, evidence preservation, business continuity, and documentation. A correct action is an action that can be explained, reproduced, and reviewed after the incident. This article focuses on BEC investigation and is intended for SOC analysts and Microsoft 365 investigators. The goal is to provide a working methodology that can be applied in practice, in a professional interview, and in a work environment, without settling for a dictionary definition.

The main challenge is that data is almost always partial. Received chain, Return-Path, and SPF can indicate a direction, but their meaning depends on the time, asset, user, and expected activity. Therefore, we will build the investigation around an investigative question, required evidence, and clear criteria for completion.

The practical scenario in the article is: A compromised account scenario with a hidden forwarding rule. All examples are lab data or process descriptions. When dealing with Penetration Testing, Web, or Cloud, one should only work with explicit authorization, a defined Scope, and the ability to stop the test.

BEC vs. Spoofing

The topic 'BEC vs. Spoofing' is a central part of working on a BEC investigation. It is recommended to break it down into three questions: What is the input, what decision do you want to make, and what evidence is sufficient to justify it? These questions prevent the automatic use of a tool without understanding the purpose.

In practice, record the Received chain, Return-Path, SPF, DKIM, DMARC, compare them to expected behavior, and define at least one Pivot. The result should be verifiable by another analyst, including limitations and next steps.

Sign-in and Identity evidence

The topic 'Sign-in and Identity evidence' is a central part of working on a BEC investigation. It is recommended to break it down into three questions: What is the input, what decision do you want to make, and what evidence is sufficient to justify it? These questions prevent the automatic use of a tool without understanding the purpose.

Inbox/Forwarding rules

The topic 'Inbox/Forwarding rules' is a central part of working on a BEC investigation. It is recommended to break it down into three questions: What is the input, what decision do you want to make, and what evidence is sufficient to justify it? These questions prevent the automatic use of a tool without understanding the purpose.

Mailbox audit and OAuth

The topic 'Mailbox audit and OAuth' is a central part of working on a BEC investigation. It is recommended to break it down into three questions: What is the input, what decision do you want to make, and what evidence is sufficient to justify it? These questions prevent the automatic use of a tool without understanding the purpose.

Scoping, Containment and Notification

The response to a BEC investigation should reduce risk without erasing evidence that is still needed. Start with a reversible and focused action, confirm ownership and authority, and document time, perpetrator, and outcome.

Long-term remediation addresses the root cause: permissions, configuration, Validation, Telemetry, process, or training. After implementation, perform a Retest and monitor for signs of recurrence, instead of settling for closing a Ticket.

Unique investigation focus areas

In this topic, it is recommended to build a focused evidence map in advance. The main investigation focus areas are: Received chain, Return-Path, SPF, DKIM, DMARC, Message-ID, mailbox rules. The list is not an automatic Checklist; each item is chosen because it can link an entity, action, and time or explain legitimate behavior.

Received chain: Define the expected value, what would be considered abnormal, and what additional source would corroborate the finding.
Return-Path: Define the expected value, what would be considered abnormal, and what additional source would corroborate the finding.
SPF: Define the expected value, what would be considered abnormal, and what additional source would corroborate the finding.
DKIM: Define the expected value, what would be considered abnormal, and what additional source would corroborate the finding.
DMARC: Define the expected value, what would be considered abnormal, and what additional source would corroborate the finding.
Message-ID: Define the expected value, what would be considered abnormal, and what additional source would corroborate the finding.

When one of the focus areas is unavailable, the gap should be documented and an alternative chosen. For example, if a Process identifier is unstable, time, Host, User, and Parent can be used; if Payload is encrypted, Metadata, volume, frequency, and TLS/DNS context are used.

Recommended workflow

Define a Scope and a single working question regarding BEC investigation.
List the data sources and required evidence: Received chain, Return-Path, SPF, DKIM.
Create a short Baseline of normal behavior or expected outcome.
Perform the minimum test in a lab environment and record time, input, and output.
Build a Timeline or comparison table and separate fact from interpretation.
Pivot to an additional source to confirm or refute the initial explanation.
Summarize the decision, limitations, recommended action, and Retest criteria.

Practical scenario

The chosen scenario is a compromised account with a hidden forwarding rule. The purpose of the exercise is not to prove attack capability, but to practice collection, comparison, and documentation safely. Before starting, define dummy data, a time window, and an expected outcome.

At the end of the exercise, submit a product that another analyst or reviewer can critique: a screenshot or Export of the evidence, a short Timeline, initial assumption, corroborating evidence, limitation, and recommendation. When there is insufficient evidence, the correct conclusion is that the scenario was not proven.

Stage	What is performed	Output
Preparation	Define Scope, time, and objective. Note which fields or evidence from Received chain, Return-Path, SPF are expected to appear.	Short test plan
Data generation	Perform a safe and simulated action related to BEC investigation, without real information or impact on a production system.	Controlled event/Request/Flow
Collection	Collect the raw evidence and context from an additional source. Verify Time zone, identifiers, and integrity.	Two linked pieces of evidence
Analysis	Write what each piece of evidence proves, what it does not prove, and what is the possible legitimate explanation.	Interim conclusion
Completion	Choose closure, escalation, Finding, or Tuning; add recommendation and Retest.	Documented output

Practical Checklist

Check and document: Source of evidence.
Check and document: Collection time and time zone.
Check and document: Hash and Chain of Custody.
Check and document: Tool and version.
Check and document: Response actions performed.
Check and document: Timeline and working assumptions.
Note Time zone, tool version, and collection time.
Save the raw data before filtering or modification.
Write what the finding proves and what is still unknown.
Define owner and next action with a deadline.

Common mistakes

Modifying the system before preserving evidence.
Not documenting time zone.
Not calculating Hash.
Mixing fact and conjecture.
Not documenting who held the evidence.
Prioritizing theoretical completeness over immediate damage containment.

Summary and CTA

Business Email Compromise and Inbox Rules investigation is a topic that connects technical knowledge with work discipline. Start with a question, collect only relevant evidence, maintain context and time, and choose an action that can be justified and re-tested.

In HPI's Cybersecurity & AI track, these principles are practiced using systems, logs, and labs. A natural continuation is to move on to the linked articles, perform the lab exercise, and save the output as part of a professional portfolio.

FAQ

Does BEC investigation alone prove an attack or vulnerability?

No. It provides a signal or finding that requires context, verification, and an additional source. A professional conclusion relies on a chain of evidence and consistency with expected behavior.

What to do when some data is missing?

Document the missing data, check for an alternative source, and reduce the level of certainty. Do not complete fields with assumptions or present Unknown as valid.

How long should evidence be kept?

The time depends on policy, regulation, cost, and the type of incident. It is important to define Retention, Legal hold, and the ability to export evidence in a verifiable format in advance.

How to practice without risking a real system?

Use virtual machines, dummy data, CTF, or a dedicated lab. In authorized tests, define Scope, Stop conditions, and backup before starting work.