This article assumes the attacker have already gained root access over a machine and now is planning to plant a hidden backdoor. There are lots of methods to achieve the same like installing a rootkit, port knocking, etc but in this article we focus on simple technique that will achieve the same and bypass results from native tools like

  • netstat : Prints network connections, routing tables, interface statistics, masquerade connections, and multicast memberships
  • ps : Report a snapshot of the current processes

  • lsof : List open files

 

For our simple demonstration attacker creates a netcat backdoor and leaves a port open. The open port can be easily listed by above tools. The attacker creates a script and hijacks the execution of the native tools by utilising the PATH environment variable precedence.

 

Once the attacker plants his netcat backdoor by nc command,

nc_start.png, May 2021

 

 

 

this will be detected as shown in below image

nc_trace.png, May 2021

 

 

 

In order to hide the detection attacker creates a simple script as below:

#!/bin/bash
/bin/netstat \[email protected] | grep -Ev '1234|nc'

The above script upon execution:

  • Calls the original netstat command located at /bin/ folder
  • [email protected] : captures all the command line arguement passed to netstat command 
  • grep -Ev '1234|nc' : Removes every line from output matching the strings 1234 and nc

 

Attacker saves the above script titled netstat in /usr/local/bin and makes the script executable by

chmod +x /usr/local/bin/netstat

 

The point that the attacker leverages here is the implementation of PATH variable. The attacker checks the output of

echo $PATH

and notices that /bin comes after /usr/local/bin, this means the binary placed at /usr/local/bin will get executed first due to PATH order preference. He notes that the original netstat binary is placed at /bin/netcat by typing which netstat

loc_netstat.png, May 2021

 

Once the attacker has placed the script at /usr/local/bin, now when the local user of infected machine calls netstat -ltp

he wont be able to detect the presence of a listening netcat instance.

no_nc.png, May 2021

 

Attacker repeats the same process for ps and lsof commands by placing binaries with same name