A new technique to analyze FormBook malware infections

Avira Protection Labs, 3 months ago 4 min read

FormBook is a well known malware family of data-stealers and form-grabbers. Sold on hacking forums as ‘malware-as-a-service’ it  has previously been used to target the aerospace, defense, and financial sectors, and thoroughly researched. However, in this article we will use a new technique, developed by Avira researchers, to analyse the infection process, breaking it down individual steps.

Overview

By: Malina Rosu, specialist virus analyst, Avira Protection Labs

FormBook malware is typically spread through web browsers, mail, FTP clients and instant messaging applications via malicious attachments such as Office documents and PDF files. The malware can inject itself into processes and install function hooks through manual execution of phishing emails and  opening malicious attachments. Its capabilities include capturing screenshots, stealing credentials, keystroke logging, and clearing browser cookies. Data thefts are most effective when the victims use a virtual keyboard, auto-fill, or if they copy and paste information to fill a form.

The FormBook infection takes place in multiple stages. In our example the infection begins when a user receives an email with a malicious Office .docx file attachment presented as a purchase order. Other files included are a Rich Text Format file with seven other embedded Excel files, two script files (one VBS file, one image file which is actually a obfuscated script file) and some Powershell commands.

There are six different steps in the process, not all of which are immediately obvious, but can be understood when the entire process is completed. For example, the malware deletes the notepad.exe from System32 because the malware will  ultimately execute under this name.

the infection process

Infection Analysis

The infection vector in our example is an email. It apparently contains a valid purchase order (PO20200217.docx) that appears to have been sent to the wrong email address.

Phishing email

Stage 1: Analyzing the attachment – the docx documents

We execute the document in a virtual environment allowing us to check its malware capabilities. The document connects to a URL, and attempts to download the payload  via a connected URL – a Rich Text Format file.

Downloading paylod

It is a docx file with Open Office XML format. In settings.xml.rels resides an xml file with an external relationship of type attachedTemplate embedded in the document that redirects to an external resource.

After the payload is successfully downloaded, a PowerShell command is used to delete the notepad.exe from the System32 folder

Stage 2: Analyzing the first payload – the rtf document

Analyzing the .rtf file document with RTFScan, reveals eight OLE embedded objects. Seven out of the eight objects were similar Excel files, all being executed as seen in the Process Monitor capture:

7 file objects

Stage 3: Analyzing the second payload – the xls documents

We show the analysis flow for one of the Excel documents below, because they are all similar. The file name ‘payload_1.exe’ together with a low number of  detections from VirusTotal make them suspicious.

detection of malicious files

Execution of the Excel documents results in warning pop-ups for Enable Content, requesting the user to enable macros. Enabling macros have no effect on the document, it appears damaged.

blank document

At this point, many users would close the file and move on. However, before executing the file, we analyzed the file behaviour on Process Monitor. The files were not damaged at all, as shown below:

downloaded excel files

 

The attackers have hidden the document file on purpose, making it appear blank when opened. Now the obvious question is; how does the file manage to execute the PowerShell script? Well, remember the Enable Content warning? This points to the fact that there may be macro code:

macro code in the malicious file

The macro code is executed before the document is closed, and it appears that it executes the text it finds in the TextBox 1. But it’s hidden as it contains a Powershell command.

The command, hidden in the textbox, downloads and executes a Visual Basic script. As shown in the below image:

The script file also had only a few detections from AV scanners in VirusTotal.

Stage 4: Analyzing the first payload – the Visual Basic Script

To avoid detection the Visual Basic script is obfuscated using string manipulation functions such as reverse, split, replace, concatenation and hex encoding.

analyzing the payload

We hex-decoded the Fly subroutine. The malicious script that executed in the global namespace overwrites the initial Fly subroutine. This is shown by the two Fly subroutine calls at the very beginning of the malicious script.

The new Fly subroutine makes use of the Windows Management Instrumentation to execute the PowerShell script received as parameter. This is done with hidden property enabled to hide its presence from the user.

The PowerShell makes a GET request to the website. It manipulates the response to download the fifth stage payload.

The script achieves persistence by copying itself to “\Appdata\Roaming\” folder and adding itself to the current user registry Run key.

Stage 5: Analyzing the second payload – the att.jpg image

To establish if the jpg file was indeed an image, we use the Hex editor. The .jpg file had the first four bytes FF D8 FF EO, so it is unlikely to be what it claims. The character “-” linking the bytes also indicates that the file is not legitimate:

To examine the malicious file, and figure out what the file hides, we need to understand how the script manipulates the file after downloading it. It performs a split action by “-“, and then a hex to ascii decoding.  The below image shows two very long hex encoded strings – Cli1 and Cli2, and a function – UNpack.

 

The UNpack function performs a Gzip decompression of the Cli string. Since the magic number for a GZIP archive file is 1F 8B 08  (as seen in the first part of the string) it seems likely that it is a GZIP. We get the payload (a dynamic library file) after decompressing the Gzip byte array, and replacing “$%” in the Cli string. The dynamic library file – Pineapple.dll – is loaded in the following line of  script.

Following similar steps, we conclude that the second string is an executable file – attack2_cli2.exe_, part of the FormBook malware family.  Avira detects it as TR/Crypt.ZPACK.Gen. The file is executed under the name notepad.exe.

This is the main reason why the first document deleted the legitimate notepad executable file from System32 folder.

Conclusion

FormBook malware is very agile in its behavior and effective in hiding malicious intent. It can easily trick users into executing it. Despite not being broadly used, FormBook represents a real threat: It is stealthier and more powerful than other malware.

You can learn more about new and novel malware in the research section of the Avira Insights blog, or find out how you can improve your own detection rates by using Avira’s Anti-malware SDKs or cloud sandbox API.

IOCs

SHA256 File type
a8c6456fb40ee49af00ac856c7ec58f314bb593f5aca991d0c8adc18ca5e3868 eml
9227ad6740207c4150a96b38c42792e57aa6258ab56763c1e9c4df7d97239559 eml
468313d0dea135d610d08cda83d34523f48f0e7a9e829eb88ec24dcc2e7b8bfb eml
85218477e69ef0889cc78566c972c53332046f1c5d1201bae9bf288fafa27c53 docx
1e031429d31c0d3e456669636d6ab370d1ca8fe02df702c7b875bc50ae43245b rtf
f0b21a76949e1403e948acc020d6c3f03ffdb338a15bd3006e7eb5c40da69cf0 xls
811dfd3270bd9203c65d3e1f737cbabf2404b83507f90f416a2dcae28cc223d2 xls
f12de42e9bd05e89256f1aa70ba90cae8178826e3b751d4f98fd40d4119196d6 xls
74902beea45467517e9f29131b2633110c090dc07f4b158089fc74baaf84ab98 xls
ce7bb44456aed206be1bc2cda0e6e33a209b1939745dce5060b3342d135205bc xls
2dd3cec76010e43b0e594a2d3d693be9d6e14e946e1ddea1bec02ccbcae9b854 xls
f38b76f7792bb6d5d372c79fb943a777d05f32c5e041e7505ce623aff9b8de0e xls
96be0e8c566a0568f67efda85d0f522a9627973db4098d80c3a91ccd424540f7 xls
187ac08a1c4f8b8111824083a200824113474eca9e7b9807ed1d81e2d653b558 xls
3024bd7d2f89ec07350a13f7fa74a4e83e9f434266c37cbecaa56d68233ef416 xls
1d67c5459b79801bca4192c1b4e80344cd7706272e6e6b7cd6e8cbd7c10caad4 vbs
f9f7c1312d20d68fc09d4fea2b58a52b6374f5b2ec88be162f655146379e1231 exe
18a86d81ef592cb588e609a66c03793f93dd3466a7e9403280c81bd0034bbdad dll
47405c182c4f99dd120fb0b7fded1720a9e44bad379a6304cd067027fadfcd32 att.jpg – script file

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A new technique to analyze FormBook malware infections

Avira Protection Labs

Protection Lab is the heart of Avira’s threat detection and protection unit. The researchers at work in the Labs are some of the most qualified and skilled anti-malware researchers in the security industry. They conduct highly advance research to provide the best detection and protection to nearly a billion people world-wide.

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