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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.12 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.34 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.5367 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.891011 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.

Item Value
ID T1546.004
Sub-techniques T1546.001, T1546.002, T1546.003, T1546.004, T1546.005, T1546.006, T1546.007, T1546.008, T1546.009, T1546.010, T1546.011, T1546.012, T1546.013, T1546.014, T1546.015, T1546.016
Tactics TA0004, TA0003
Platforms Linux, macOS
Permissions required Administrator, User
Version 2.1
Created 24 January 2020
Last Modified 20 August 2021

Procedure Examples

ID Name Description
S0690 Green Lambert Green Lambert can establish persistence on a compromised host through modifying the profile, login, and run command (rc) files associated with the bash, csh, and tcsh shells. 1314
S0362 Linux Rabbit Linux Rabbit maintains persistence on an infected machine through rc.local and .bashrc files. 15

Mitigations

ID Mitigation Description
M1022 Restrict File and Directory Permissions Making these files immutable and only changeable by certain administrators will limit the ability for adversaries to easily create user level persistence.

Detection

ID Data Source Data Component
DS0017 Command Command Execution
DS0022 File File Creation
DS0009 Process Process Creation

References

  1. Paul Litvak. (2020, May 4). Kaiji: New Chinese Linux malware turning to Golang. Retrieved December 17, 2020. ↩

  2. Benjamin Cane. (2013, September 16). Understanding a little more about /etc/profile and /etc/bashrc. Retrieved February 25, 2021. ↩

  3. Anomali Threat Research. (2019, October 15). Illicit Cryptomining Threat Actor Rocke Changes Tactics, Now More Difficult to Detect. Retrieved December 17, 2020. ↩↩

  4. ArchWiki. (2021, January 19). Bash. Retrieved February 25, 2021. ↩

  5. Claud Xiao and Cong Zheng. (2017, April 6). New IoT/Linux Malware Targets DVRs, Forms Botnet. Retrieved December 17, 2020. ↩

  6. Anomali Threat Research. (2018, December 6). Pulling Linux Rabbit/Rabbot Malware Out of a Hat. Retrieved December 17, 2020. ↩

  7. Cesar Anjos. (2018, May 31). Shell Logins as a Magento Reinfection Vector. Retrieved December 17, 2020. ↩

  8. Armin Briegel. (2019, June 5). Moving to zsh, part 2: Configuration Files. Retrieved February 25, 2021. ↩

  9. Leo Pitt. (2020, August 6). Persistent JXA - A poor man’s Powershell for macOS. Retrieved January 11, 2021. ↩

  10. Leo Pitt. (2020, November 11). Github - PersistentJXA/BashProfilePersist.js. Retrieved January 11, 2021. ↩

  11. Cedric Owens. (2021, May 22). macOS MS Office Sandbox Brain Dump. Retrieved August 20, 2021. ↩

  12. Patrick Wardle. (2019, September 17). Writing a File Monitor with Apple’s Endpoint Security Framework. Retrieved December 17, 2020. ↩

  13. Sandvik, Runa. (2021, October 1). Made In America: Green Lambert for OS X. Retrieved March 21, 2022. ↩

  14. Sandvik, Runa. (2021, October 18). Green Lambert and ATT&CK. Retrieved March 21, 2022. ↩

  15. Anomali Labs. (2018, December 6). Pulling Linux Rabbit/Rabbot Malware Out of a Hat. Retrieved March 4, 2019. ↩