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: <code>File.txt.exe</code> may render in some views as just <code>File.txt</code>). 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.(Citation: PCMag DoubleExtension)(Citation: SOCPrime DoubleExtension) 

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](https://attack.mitre.org/tactics/TA0001) into a user’s system via [Spearphishing Attachment](https://attack.mitre.org/techniques/T1566/001) then [User Execution](https://attack.mitre.org/techniques/T1204). For example, an executable file attachment named <code>Evil.txt.exe</code> may display as <code>Evil.txt</code> to a user. The user may then view it as a benign text file and open it, inadvertently executing the hidden malware.(Citation: SOCPrime DoubleExtension)

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.
Double File Extension
source_name: mitre-attack  url: https://attack.mitre.org/techniques/T1036/007  external_id: T1036.007     source_name: PCMag DoubleExtension  description: PCMag. (n.d.). Encyclopedia: double extension. Retrieved August 4, 2021.  url: https://www.pcmag.com/encyclopedia/term/double-extension     source_name: SOCPrime DoubleExtension  description: Eugene Tkachenko. (2020, May 1). Rule of the Week: Possible Malicious File Double Extension. Retrieved July 27, 2021.  url: https://socprime.com/blog/rule-of-the-week-possible-malicious-file-double-extension/     source_name: Seqrite DoubleExtension  description: Seqrite. (n.d.). How to avoid dual attack and vulnerable files with double extension?. Retrieved July 27, 2021.  url: https://www.seqrite.com/blog/how-to-avoid-dual-attack-and-vulnerable-files-with-double-extension/     