cyberstrikeus T1546.004 Unix Shell Configuration Modification
Sub-technique of: T1546
High-Level Description
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. 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. 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. 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. 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.
Kill Chain Phase
- Privilege Escalation (TA0004)
- Persistence (TA0003)
Platforms: Linux, macOS
What to Check
- Identify if Unix Shell Configuration Modification technique is applicable to target environment
- Check Linux systems for indicators of Unix Shell Configuration Modification
- Check macOS systems for indicators of Unix Shell Configuration Modification
- Verify mitigations are bypassed or absent (1 known mitigations)
- Assess detection coverage (1 detection strategies)
How to Test
Atomic Red Team Tests
The following tests are from Atomic Red Team and provide actionable ways to test this technique:
Atomic Test 1: Add command to .bash_profile
Adds a command to the .bash_profile file of the current user
Supported Platforms: macos, linux
echo '#{command_to_add}' >> ~/.bash_profile
Atomic Test 2: Add command to .bashrc
Adds a command to the .bashrc file of the current user
Supported Platforms: macos, linux
echo '#{command_to_add}' >> ~/.bashrc
Atomic Test 3: Add command to .shrc
Adds a command to the .shrc file of the current user
Supported Platforms: linux
echo '#{command_to_add}' >> ~/.shrc
Atomic Test 4: Append to the system shell profile
An 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: Yes
echo '#{text_to_append}' >> /etc/profile
Atomic Test 5: Append commands user shell profile
An adversary may wish to establish persistence by executing malicious commands from the users ~/.profile every time the "user" logs in.
Supported Platforms: linux
echo '#{text_to_append}' >> ~/.profile
Manual Testing
If Atomic Red Team tests are not applicable, manually verify the technique by:
Identify Attack Surface: Determine if the target environment is susceptible to Unix Shell Configuration Modification by examining the target platforms (Linux, macOS).
Assess Existing Defenses: Review whether mitigations for T1546.004 are in place. If defenses are absent or misconfigured, this technique may be exploitable.
Execute Test: Use tools and methods described in the MITRE ATT&CK page and external references below.
Remediation Guide
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
Detect Shell Configuration Modification for Persistence via Event-Triggered Execution
Risk Assessment
| Finding | Severity | Impact |
|---|---|---|
| Unix Shell Configuration Modification technique applicable | Low | Privilege Escalation |
CWE Categories
| CWE ID | Title |
|---|---|
| CWE-269 | Improper Privilege Management |