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T1546.004 - Unix Shell Configuration Modification

T1546.004 - Unix Shell Configuration Modification#

Adversaries may establish persistence through executing malicious commands triggered by a user’s shell. User Unix Shells execute several configuration scripts at different points throughout the session based on events. For example, when a user opens a command-line interface or remotely logs in (such as via SSH) a login shell is initiated. The login shell executes scripts from the system (/etc) and the user’s home directory (~/) to configure the environment. All login shells on a system use /etc/profile when initiated. These configuration scripts run at the permission level of their directory and are often used to set environment variables, create aliases, and customize the user’s environment. When the shell exits or terminates, additional shell scripts are executed to ensure the shell exits appropriately.

Adversaries may attempt to establish persistence by inserting commands into scripts automatically executed by shells. Using bash as an example, the default shell for most GNU/Linux systems, adversaries may add commands that launch malicious binaries into the /etc/profile and /etc/profile.d files.(Citation: intezer-kaiji-malware)(Citation: bencane blog bashrc) These files typically require root permissions to modify and are executed each time any shell on a system launches. For user level permissions, adversaries can insert malicious commands into ~/.bash_profile, ~/.bash_login, or ~/.profile which are sourced when a user opens a command-line interface or connects remotely.(Citation: anomali-rocke-tactics)(Citation: Linux manual bash invocation) Since the system only executes the first existing file in the listed order, adversaries have used ~/.bash_profile to ensure execution. Adversaries have also leveraged the ~/.bashrc file which is additionally executed if the connection is established remotely or an additional interactive shell is opened, such as a new tab in the command-line interface.(Citation: Tsunami)(Citation: anomali-rocke-tactics)(Citation: anomali-linux-rabbit)(Citation: Magento) Some malware targets the termination of a program to trigger execution, adversaries can use the ~/.bash_logout file to execute malicious commands at the end of a session.

For macOS, the functionality of this technique is similar but may leverage zsh, the default shell for macOS 10.15+. When the Terminal.app is opened, the application launches a zsh login shell and a zsh interactive shell. The login shell configures the system environment using /etc/profile, /etc/zshenv, /etc/zprofile, and /etc/zlogin.(Citation: ScriptingOSX zsh)(Citation: PersistentJXA_leopitt)(Citation: code_persistence_zsh)(Citation: macOS MS office sandbox escape) The login shell then configures the user environment with ~/.zprofile and ~/.zlogin. The interactive shell uses the ~/.zshrc to configure the user environment. Upon exiting, /etc/zlogout and ~/.zlogout are executed. For legacy programs, macOS executes /etc/bashrc on startup.

Atomic Tests#

Atomic Test #1 - Add command to .bash_profileAdds a command to the .bash_profile file of the current user#

Supported Platforms: macos, linux#### Attack Commands: Run with sh

echo 'echo "Hello from Atomic Red Team T1546.004" > /tmp/T1546.004' >> ~/.bash_profile

Invoke-AtomicTest T1546.004 -TestNumbers 1

Cleanup:#

head -n '-2' ~/.bash_profile > /tmp/T1546.004
mv /tmp/T1546.004 ~/.bash_profile

Invoke-AtomicTest T1546.004 -TestNumbers 1 -Cleanup

Atomic Test #2 - Add command to .bashrcAdds a command to the .bashrc file of the current user#

Supported Platforms: macos, linux#### Attack Commands: Run with sh

echo 'echo "Hello from Atomic Red Team T1546.004" > /tmp/T1546.004' >> ~/.bashrc

Invoke-AtomicTest T1546.004 -TestNumbers 2

Cleanup:#

head -n '-2' ~/.bashrc > /tmp/T1546.004
mv /tmp/T1546.004 ~/.bashrc

Invoke-AtomicTest T1546.004 -TestNumbers 2 -Cleanup

Atomic Test #3 - Add command to .shrcAdds a command to the .shrc file of the current user#

Supported Platforms: linux#### Attack Commands: Run with sh

echo 'echo "Hello from Atomic Red Team T1546.004" > /tmp/T1546.004' >> ~/.shrc

Invoke-AtomicTest T1546.004 -TestNumbers 3

Cleanup:#

head -n '-2' ~/.shrc > /tmp/T1546.004
mv /tmp/T1546.004 ~/.shrc

Invoke-AtomicTest T1546.004 -TestNumbers 3 -Cleanup

Atomic Test #4 - Append to the system shell profileAn adversary may wish to establish persistence by executing malicious commands from the systems /etc/profile every time “any” user logs in.#

Supported Platforms: linux Elevation Required (e.g. root or admin)#### Attack Commands: Run with sh

echo '# Hello from Atomic Red Team T1546.004' >> /etc/profile

Invoke-AtomicTest T1546.004 -TestNumbers 4

Cleanup:#

sed -i "s/# Atomic Red Team was here! T1546.004//" /etc/profile

Invoke-AtomicTest T1546.004 -TestNumbers 4 -Cleanup

Atomic Test #5 - Append commands user shell profileAn adversary may wish to establish persistence by executing malicious commands from the users ~/.profile every time the “user” logs in.#

Supported Platforms: linux Elevation Required (e.g. root or admin)#### Attack Commands: Run with sh

echo '# Atomic Red Team was here... T1546.004' >> ~/.profile

Invoke-AtomicTest T1546.004 -TestNumbers 5

Cleanup:#

sed -i "s/# Atomic Red Team was here... T1546.004//" ~/.profile

Invoke-AtomicTest T1546.004 -TestNumbers 5 -Cleanup

Atomic Test #6 - System shell profile scriptsAn adversary may wish to establish persistence by adding commands into any of the script files in the /etc/profile.d/ directory, which are executed every time “any” user logs in.#

Supported Platforms: linux Elevation Required (e.g. root or admin)#### Attack Commands: Run with sh

echo '# Atomic Red Team was here... T1546.004' >> /etc/profile.d/bash_completion.sh

Invoke-AtomicTest T1546.004 -TestNumbers 6

Cleanup:#

sed -i "s/# Atomic Red Team was here... T1546.004//" /etc/profile.d/bash_completion.sh

Invoke-AtomicTest T1546.004 -TestNumbers 6 -Cleanup

Atomic Test #7 - Create/Append to .bash_logoutThe Bash shell runs ~/.bash_logout “if it exists” to run commands on user logout. An adversary may create or append to a .bash_logout to clear history, start processes etc. Note the ~/.bash_logout is only run if you explicitly exit or log out of an “interactive login shell session” i.e. via the console, SSH, /bin/bash -l or su -l .#

This test creates the art user, logs in, creates a .bash_logout which will echo some text into the art.txt file on logout and logs out and the /home/art/art.txt is created. Supported Platforms: linux Elevation Required (e.g. root or admin)#### Attack Commands: Run with bash

useradd --create-home --shell /bin/bash art
su --login art
echo 'echo "Atomic Red Team was here... T1546.004" >> $HOME/art.txt' >> $HOME/.bash_logout

Invoke-AtomicTest T1546.004 -TestNumbers 7

Cleanup:#

userdel -fr art

Invoke-AtomicTest T1546.004 -TestNumbers 7 -Cleanup

Detection#

While users may customize their shell profile files, there are only certain types of commands that typically appear in these files. Monitor for abnormal commands such as execution of unknown programs, opening network sockets, or reaching out across the network when user profiles are loaded during the login process.

Monitor for changes to /etc/profile and /etc/profile.d, these files should only be modified by system administrators. MacOS users can leverage Endpoint Security Framework file events monitoring these specific files.(Citation: ESF_filemonitor)

For most Linux and macOS systems, a list of file paths for valid shell options available on a system are located in the /etc/shells file.

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