Hidden Files and Directories

What the technique is

Adversaries may set files and directories to be hidden to evade detection mechanisms. To prevent normal users from accidentally changing special files on a system, most operating systems have the concept of a ‘hidden’ file. These files don’t show up when a user browses the file system with a GUI or when using normal commands on the command line. Users must explicitly ask to show the hidden files either via a series of Graphical User Interface (GUI) prompts or with command line switches (dir /a for Windows and ls –a for Linux and macOS). On Linux and Mac, users can mark specific files as hidden simply by putting a “.” as the first character in the file or folder name. Files and folders that start with a period, ‘.’, are by default hidden from being viewed in the Finder application and standard command-line utilities like “ls”. Users must specifically change settings to have these files viewable. Files on macOS can also be marked with the UF_HIDDEN flag which prevents them from being seen in Finder.app, but still allows them to be seen in Terminal.app. On Windows, users can mark specific files as hidden by using the attrib.exe binary. Many applications create these hidden files and folders to store information so that it doesn’t clutter up the user’s workspace. For example, SSH utilities create a.ssh folder that’s hidden and contains the user’s known hosts and keys. […]

What MITRE says you would watch

• AN0091

  Suspicious use of attrib.exe or PowerShell commands to set hidden attributes on files/directories. Defender view: processes modifying file attributes to 'hidden' or creating files with ADS (alternate data streams).

• AN0092

  Creation of files or directories with a leading '.' in privileged directories (/etc, /var, /usr/bin). Defender view: monitoring auditd logs for file creations where name begins with '.' and correlated with unusual user/process context.

• AN0093

  Use of chflags hidden or SetFile -a V commands to hide files, or creation of hidden files with leading '.'. Defender view: monitoring process execution and file metadata changes setting UF_HIDDEN attribute.