Masquerading: Double File Extension

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.[1][2]

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.[2]

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.

ID: T1036.007
Sub-technique of:  T1036
Tactic: Defense Evasion
Platforms: Windows
Version: 1.0
Created: 04 August 2021
Last Modified: 14 October 2021

Procedure Examples

ID Name Description
S0534 Bazar

The Bazar loader has used dual-extension executable files such as PreviewReport.DOC.exe.[3]

G0129 Mustang Panda

Mustang Panda has used an additional filename extension to hide the true file type.[4][5]


ID Mitigation Description
M1028 Operating System Configuration

Disable the default to "hide file extensions for known file types" in Windows OS.[6][7]

M1017 User Training

Train users to look for double extensions in filenames, and in general use training as a way to bring awareness to common phishing and spearphishing techniques and how to raise suspicion for potentially malicious events.


ID Data Source Data Component
DS0022 File File Creation
File Metadata

Monitor for files written to disk that contain two file extensions, particularly when the second is an executable.[6]