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Attack: Impair Defenses: Impair Command History Logging

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’ve 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.

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 PowerShell Command History)(Citation: Sophos PowerShell command audit)(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).

MITRE

Tactic

technique

Test : Setting the HISTCONTROL environment variable

OS

Description:

An attacker may exploit the space before a command (e.g. “ ls”) or the duplicate command suppression feature in Bash history to prevent their commands from being recorded in the history file or to obscure the order of commands used.

In this test we 1. sets $HISTCONTROL to ignoreboth 2. clears the history cache 3. executes ls -la with a space in-front of it 4. confirms that ls -la is not in the history cache 5. sets $HISTCONTROL to erasedups 6. clears the history cache 7..9 executes ls -la $HISTFILE 3 times 10. confirms that their is only one command in history

Executor

bash

Sigma Rule

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