Query: how to detect Double File Extension
Matching passage: T1036.007 Double File Extension. Tactics: stealth. 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: <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.
Near-miss passage: T1036.002 Right-to-Left Override. Tactics: stealth. Description: Adversaries may abuse the right-to-left override (RTLO or RLO) character (U+202E) to disguise a string and/or file name to make it appear benign. RTLO is a non-printing Unicode character that causes the text that follows it to be displayed in reverse. For example, a Windows screensaver executable named <code>March 25 \u202Excod.scr</code> will display as <code>March 25 rcs.docx</code>. A JavaScript file named <code>photo_high_re\u202Egnp.js</code> will be displayed as <code>photo_high_resj.png</code>.(Citation: Infosecinstitute RTLO Technique) Adversaries may abuse the RTLO character as a means of tricking a user into executing what they think is a benign file type. A common use of this technique is with [Spearphishing Attachment](https://attack.mitre.org/techniques/T1566/001)/[Malicious File](https://attack.mitre.org/techniques/T1204/002) since it can trick both end users and defenders if they are not aware of how their tools display and render the RTLO character. Use of the RTLO character has been seen in many targeted intrusion attempts and criminal activity.(Citation: Trend Micro PLEAD RTLO)(Citation: Kaspersky RTLO Cyber Crime) RTLO can be used in the Windows Registry as well, where regedit.exe displays the reversed characters but the command line tool reg.exe does not by default.