Adversaries may impair command history logging to hide commands they run on a compromised system. Various command interpreters keep track of the commands users type in their terminal so that users can retrace what they have done. On Linux and macOS, command history is tracked in a file pointed to by the environment variable `HISTFILE`. When a user logs off a system, this information is flushed to a file in the user's home directory called `~/.bash_history`. The `HISTCONTROL` environment variable keeps track of what should be saved by the history command and eventually into the `~/.bash_history` file when a user logs out. `HISTCONTROL` does not exist by default on macOS, but can be set by the user and will be respected. The `HISTFILE` environment variable is also used in some ESXi systems.(Citation: Google Cloud Threat Intelligence ESXi VIBs 2022) Adversaries may clear the history environment variable (`unset HISTFILE`) or set the command history size to zero (`export HISTFILESIZE=0`) to prevent logging of commands. Additionally, `HISTCONTROL` can be configured to ignore commands that start with a space by simply setting it to "ignorespace". `HISTCONTROL` can also be set to ignore duplicate commands by setting it to "ignoredups". In some Linux systems, this is set by default to "ignoreboth" which covers both of the previous examples. This means that " ls" will not be saved, but "ls" would be saved by history. Adversaries can abuse this to operate without leaving traces by simply prepending a space to all of their terminal commands. On Windows systems, the `PSReadLine` module tracks commands used in all PowerShell sessions and writes them to a file (`$env:APPDATA\Microsoft\Windows\PowerShell\PSReadLine\ConsoleHost_history.txt` by default). Adversaries may change where these logs are saved using `Set-PSReadLineOption -HistorySavePath {File Path}`. This will cause `ConsoleHost_history.txt` to stop receiving logs. Additionally, it is possible to turn off logging to this file using the PowerShell command `Set-PSReadlineOption -HistorySaveStyle SaveNothing`.(Citation: Microsoft about_History prevent command history)(Citation: Sophos PowerShell Command History Forensics) Adversaries may also leverage a Network Device CLI on network devices to disable historical command logging (e.g. `no logging`).
Detection of environment variable tampering (HISTFILE, HISTCONTROL, HISTFILESIZE) and absence of expected bash history writes. Correlation of unset or zeroed history variables with active shell sessions is indicative of adversarial evasion.
Detection of bash/zsh history suppression via HISTFILE/HISTCONTROL manipulation and absence of ~/.bash_history updates. Observing environment variable changes tied to terminal processes is a strong indicator.
Detection of CLI commands that disable history logging such as 'no logging'. Anomalous lack of new commands in session logs while activity persists is a strong signal.
Operating System Configuration
Operating System Configuration involves adjusting system settings and hardening the default configurations of an operating system (OS) to mitigate adversary exploitation and prevent abuse of system functionality. Proper OS configurations address security vulnerabilities, limit attack surfaces, and ensure robust defense against a wide range of techniques. This mitigation can be implemented through the following measures: Disable Unused Features: - Turn off SMBv1, LLMNR, and NetBIOS where not needed. - Disable remote registry and unnecessary services. Enforce OS-level Protections: - Enable Data Execution Prevention (DEP), Address Space Layout Randomization (ASLR), and Control Flow Guard (CFG) on Windows. - Use AppArmor or SELinux on Linux for mandatory access controls. Secure Access Settings: - Enable User Account Control (UAC) for Windows. - Restrict root/sudo access on Linux/macOS and enforce strong permissions using sudoers files. File System Hardening: - Implement least-privilege access for critical files and system directories. - Audit permissions regularly using tools like icacls (Windows) or getfacl/chmod (Linux/macOS). Secure Remote Access: - Restrict RDP, SSH, and VNC to authorized IPs using firewall rules. - Enable NLA for RDP and enforce strong password/lockout policies. Harden Boot Configurations: - Enable Secure Boot and enforce UEFI/BIOS password protection. - Use BitLocker or LUKS to encrypt boot drives. Regular Audits: - Periodically audit OS configurations using tools like CIS Benchmarks or SCAP tools. *Tools for Implementation* Windows: - Microsoft Group Policy Objects (GPO): Centrally enforce OS security settings. - Windows Defender Exploit Guard: Built-in OS protection against exploits. - CIS-CAT Pro: Audit Windows security configurations based on CIS Benchmarks. Linux/macOS: - AppArmor/SELinux: Enforce mandatory access controls. - Lynis: Perform comprehensive security audits. - SCAP Security Guide: Automate configuration hardening using Security Content Automation Protocol. Cross-Platform: - Ansible or Chef/Puppet: Automate configuration hardening at scale. - OpenSCAP: Perform compliance and configuration checks.
Environment Variable Permissions
Restrict the modification of environment variables to authorized users and processes by enforcing strict permissions and policies. This ensures the integrity of environment variables, preventing adversaries from abusing or altering them for malicious purposes. This mitigation can be implemented through the following measures: Restrict Write Access: - Use Case: Set file system-level permissions to restrict access to environment variable configuration files (e.g., `.bashrc`, `.bash_profile`, `.zshrc`, `systemd` service files). - Implementation: Configure `/etc/environment` or `/etc/profile` on Linux systems to only allow root or administrators to modify the file. Secure Access Controls: - Use Case: Limit access to environment variable settings in application deployment tools or CI/CD pipelines to authorized personnel. - Implementation: Use role-based access control (RBAC) in tools like Jenkins or GitLab to ensure only specific users can modify environment variables. Restrict Process Scope: - Use Case: Configure policies to ensure environment variables are only accessible to the processes they are explicitly intended for. - Implementation: Use containerized environments like Docker to isolate environment variables to specific containers and ensure they are not inherited by other processes. Audit Environment Variable Changes: - Use Case: Enable logging for changes to critical environment variables. - Implementation: Use `auditd` on Linux to monitor changes to files like `/etc/environment` or application-specific environment files.