T1036.007 Double File Extension

T1036.007 Double File Extension

Sub-technique of: T1036

High-Level Description

Adversaries may abuse a double extension in the filename as a means of masquerading the true file type. A file name may include a secondary file type extension that may cause only the first extension to be displayed (ex: File.txt.exe may render in some views as just File.txt). However, the second extension is the true file type that determines how the file is opened and executed. The real file extension may be hidden by the operating system in the file browser (ex: explorer.exe), as well as in any software configured using or similar to the system’s policies.

Adversaries may abuse double extensions to attempt to conceal dangerous file types of payloads. A very common usage involves tricking a user into opening what they think is a benign file type but is actually executable code. Such files often pose as email attachments and allow an adversary to gain Initial Access into a user’s system via Spearphishing Attachment then User Execution. For example, an executable file attachment named Evil.txt.exe may display as Evil.txt to a user. The user may then view it as a benign text file and open it, inadvertently executing the hidden malware.

Common file types, such as text files (.txt, .doc, etc.) and image files (.jpg, .gif, etc.) are typically used as the first extension to appear benign. Executable extensions commonly regarded as dangerous, such as .exe, .lnk, .hta, and .scr, often appear as the second extension and true file type.

Kill Chain Phase

• Defense Evasion (TA0005)

Platforms: Windows

What to Check

• Identify if Double File Extension technique is applicable to target environment
• Check Windows systems for indicators of Double File Extension
• Verify mitigations are bypassed or absent (2 known mitigations)
• Assess detection coverage (1 detection strategies)

How to Test

Atomic Red Team Tests

The following tests are from Atomic Red Team and provide actionable ways to test this technique:

Atomic Test 1: File Extension Masquerading

download and execute a file masquerading as images or Office files. Upon execution 3 calc instances and 3 vbs windows will be launched.

e.g SOME_LEGIT_NAME.[doc,docx,xls,xlsx,pdf,rtf,png,jpg,etc.].[exe,vbs,js,ps1,etc] (Quartelyreport.docx.exe)

Supported Platforms: windows