Maldoc uses Windows API to perform process hollowing

A favorite technique by malware authors is to use macros in their office documents to utilize a normal system executable and replace the code inside, a technique known as “process hollowing”. The primary goal of this post is to identify this technique and understand how it is employed. I’ve also posted a video that walks through shellcode analysis using Ghidra on YouTube Starting with the Macros To get started, inspect the macros and see where the code begins execution. For this document, this begins with the Document_Open function – which can be found in the ThisDocument stream. As is often…

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Anti-Analysis in JavaScript Executed by Windows Script Host (WSH)

It’s common to see malicious office documents drop a JavaScript (JS) file to be executed by the Windows Script Host (WSH). The JS can then be used to create the necessary objects to create HTTP requests to retrieve and execute the next stage payload. For example, here is a document that drops the JS and executes it via CMD -> WSCRIPT (you can also see the use of CSCRIPT): What caught my eye with this sample was that there was no associated network traffic. While that doesn’t guarantee that the document didn’t achieve it’s objectives, I felt it was worth…

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Anti-Analysis in an Office Document

Please note: This was a blog post I originally authored for Bromium. Due to changes in how they host their blog content, it has fallen into the archives and become somewhat difficult to find. I’m posting this content here mainly as an archive. Office documents have been a favorite method of distribution for malware authors for several years. While most malware authors go to great lengths to hide the intention of their macros through obfuscation, it is seldom that I’ve encountered macros that also exhibit anti-analysis techniques. I recently examined an office document that contained such capabilities. You can find…

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Identifying a User Form in an Office Document

In this post, we will be looking into ways to identify and analyze the presence of a user form in an office document. As I discussed in a previous post, user forms are often used to store resources needed by the malware author such as scripts (PowerShell, VBS), shellcode and strings. We will be using OLEDUMP to assist in our analysis and by the end of this post, you will be able to identify and trace the usage of user forms and their objects throughout macro code. For this analysis, we will be looking at the following malicious office document….

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Analyzing Malicious Office Documents with OLEDUMP

Microsoft office documents are a common vehicle used by malware authors to deliver malware. These documents, used for malicious purposes, are commonly referred to as maldocs. While there has been a variety of ways in which they have been used, one of the more prevalent is through the use of macros. Macros are written in Visual Basic for Applications (VBA), which is well documented on the Microsoft Developer Network (MSDN). This API allows malware authors to hook into life-cycle events of a document, such as AutoOpen, AutoClose and AutoExit (MSDN) in order to achieve code execution with minimal interaction from…

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