Indicator Removal on Host: Clear Command History
In addition to clearing system logs, an adversary may clear the command history of a compromised account to conceal the actions undertaken during an intrusion. 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, these command histories can be accessed in a few different ways. While logged in, this 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 benefit of this is that it allows users to go back to commands they've used before in different sessions.
Adversaries may delete their commands from these logs by manually clearing the history (
history -c) or deleting the bash history file
On Windows hosts, PowerShell has two different command history providers: the built-in history and the command history managed by the
PSReadLine module. The built-in history only tracks the commands used in the current session. This command history is not available to other sessions and is deleted when the session ends.
PSReadLine command history tracks the commands used in all PowerShell sessions and writes them to a file (
$env:APPDATA\Microsoft\Windows\PowerShell\PSReadLine\ConsoleHost_history.txt by default). This history file is available to all sessions and contains all past history since the file is not deleted when the session ends.
Adversaries may run the PowerShell command
Clear-History to flush the entire command history from a current PowerShell session. This, however, will not delete/flush the
ConsoleHost_history.txt file. Adversaries may also delete the
ConsoleHost_history.txt file or edit its contents to hide PowerShell commands they have run.
|Environment Variable Permissions||
Making the environment variables associated with command history read only may ensure that the history is preserved.
|Restrict File and Directory Permissions||
Preventing users from deleting or writing to certain files can stop adversaries from maliciously altering their
User authentication, especially via remote terminal services like SSH, without new entries in that user's
~/.bash_history is suspicious. Additionally, the removal/clearing of the
~/.bash_history file can be an indicator of suspicious activity.
Monitor for suspicious modifications or deletion of
ConsoleHost_history.txt and use of the