Adversaries can hide a program's true filetype by changing the extension of a file. With certain file types (specifically this does not work with .app extensions), appending a space to the end of a filename will change how the file is processed by the operating system.
For example, if there is a Mach-O executable file called evil.bin, when it is double clicked by a user, it will launch Terminal.app and execute. If this file is renamed to evil.txt, then when double clicked by a user, it will launch with the default text editing application (not executing the binary). However, if the file is renamed to evil.txt (note the space at the end), then when double clicked by a user, the true file type is determined by the OS and handled appropriately and the binary will be executed [1].
Adversaries can use this feature to trick users into double clicking benign-looking files of any format and ultimately executing something malicious.
| ID | Name | Description |
|---|---|---|
| G0082 | APT38 |
APT38 has put several spaces before a file extension to avoid detection and suspicion.[2] |
| S0276 | Keydnap |
Keydnap puts a space after a false .jpg extension so that execution actually goes through the Terminal.app program.[3] |
This type of attack technique cannot be easily mitigated with preventive controls since it is based on the abuse of system features.
| ID | Name | Analytic ID | Analytic Description |
|---|---|---|---|
| DET0292 | Masquerading via Space After Filename - Behavioral Detection Strategy | AN0812 |
Detection of file execution where the file name contains a trailing space to masquerade as a known executable. Adversaries may exploit the way command line interpreters handle file names with trailing whitespace. |
| AN0813 |
Execution of renamed or dropped files with a trailing space to deceive users or analysts, especially in LaunchAgents or LaunchDaemons. |