VBA (Visual Basic for Applications) project source code investigation examines macro code in Microsoft Office documents. These include .doc, .xls, .xlsm, .dotm, and similar files. The goal is to determine whether they contain malware or ransomware droppers. When an analyst receives a suspicious Office file, examining the VBA source code is a critical first step.
Why Do Hackers Target VBA Source Code?
Indeed, essentially, VBA macros are embedded in nearly all Microsoft Office applications and can execute code with the same privileges as the logged-in user. Cybercriminals exploit this capability because:
- VBA code is easy to write and can be rapidly modified to evade antivirus signatures.
- Social engineering convinces users to click “Enable Macros”, executing the payload automatically.
- Malicious macros can download and run ransomware, exfiltrate data, or establish persistence — all from within a trusted Office document.
- VBA projects can be password-locked by attackers to prevent inspection, making forensic analysis harder.
Attackers have used malicious Office macros to deliver ransomware and enable data exfiltration at scale.
How Do Attackers Deploy Malware Through VBA?
Stage 1: Lure Document Delivery
The attacker delivers a malicious Office document via phishing email, fake invoice, or HR communication. The document displays a convincing message such as “Content is protected — please enable macros to view” to manipulate the recipient.
Stage 2: Macro Execution
Subsequently, once macros are enabled, the VBA code executes. Common techniques include using Shell, WScript.Shell, or PowerShell calls to download a remote payload, decode base64-encoded strings that reconstruct a malware executable, or write malicious files directly to disk.
Stage 3: Payload Deployment
Finally, the dropped payload — often ransomware, a remote access trojan (RAT), or a backdoor — executes and establishes persistence on the victim’s system. The original Office document may delete itself after execution to remove evidence.
What Are the Challenges in VBA Source Code Investigation?
- First, password-locked VBA projects — Attackers deliberately password-protect the VBA project to prevent investigators from viewing the code. The Office application will not display the source code without the correct password.
- Second, obfuscated code — Attackers randomise variable and function names, encode strings in base64 or XOR, and disrupt code flow to deliberately confusing to slow analysis.
- Moreover, p-code vs. source discrepancy — Office stores both the VBA source text and a compiled p-code (pseudocode). Attackers sometimes manipulate one without updating the other, meaning the visible source code may differ from what actually executes.
- Multi-stage payloads — The first-stage macro may only download a second-stage payload from an external URL, making static analysis of the document alone insufficient.
How to Investigate VBA Project Source Code?
Step 1: Extract the VBA Project
Office documents (.docm, .xlsm) are ZIP archives. Rename the file extension to .zip and extract it. The VBA source code is stored in the xl/vbaProject.bin (Excel) or word/vbaProject.bin (Word) file — a Compound Binary File that can be parsed with tools like olevba (part of the oletools suite).
Step 2: Unlock Password-Protected VBA Projects
Additionally, sometimes the VBA project is password-locked. In that case, specialised recovery tools can bypass or reset the password. This grants access to the protected source code without losing the file.
Step 3: Analyse with oletools (olevba)
Furthermore, run olevba suspicious_document.xlsm to extract and display all VBA source code, flagging suspicious patterns such as Shell calls, AutoOpen/AutoClose macros, encoded strings, and external URL references. The tool automatically highlights the most suspicious indicators of compromise (IoCs).
Step 4: Detonate in a Sandbox
Moreover, for a complete picture, run the document in an isolated sandbox environment such as Any.run, Cuckoo Sandbox, or a dedicated analysis VM with network monitoring. This reveals the malware’s runtime behaviour, including what URLs it contacts, what files it drops, and what registry changes it makes.
Step 5: Extract and Document IoCs
Finally, from your static and dynamic analysis, extract all indicators of compromise: URLs, IP addresses, domain names, file hashes, registry keys, and mutexes. Document these in your forensic report for use in threat intelligence and court evidence.
How to Protect Against VBA Macro Malware?
- First, disable macros by default in Microsoft Office Group Policy settings and only allow digitally signed macros from trusted publishers.
- Additionally, train employees to never enable macros in documents received via email, especially from unknown senders.
- Furthermore, deploy an email security gateway that scans Office attachments for macro content before delivery.
- Finally, keep Office and Windows fully patched — many macro-based attacks exploit unpatched vulnerabilities in addition to social engineering.
How to Report a Malware Incident in India?
If a malicious Office document infected your organisation, report the incident promptly. You can report to CERT-In at incident@cert-in.org.in. Also file a complaint on cybercrime.gov.in.1930. For professional forensic investigation of malicious VBA code and malware removal, contact cyber expert Anuraag Singh.
