Alright folks, buckle up! Today I’m spillin’ the beans on somethin’ I was messin’ with last week: a “nude backdoor.” Now, before y’all get any wild ideas, this wasn’t about makin’ anything scandalous. It was purely a security exercise, tryin’ to see how vulnerable a system could be with, let’s say, minimal protection. Think of it like striping away all the fancy security layers to see what’s left exposed underneath.
So, where did I start? First, I needed a target. I spun up a fresh VM – a clean Ubuntu server, nice and vanilla. The goal was simple: create a way to remotely execute commands on this server, completely bypassing any authentication or authorization. Basically, a wide-open backdoor. Nude, if you will. No usernames, no passwords, no nothin’.
I started with the basics: Netcat. Old-school, I know, but it’s a damn useful tool. I crafted a simple script that would listen on a specific port, and anythin’ sent to that port would be executed as a shell command. Here’s roughly what it looked like:
mkfifo /tmp/backpipenc -l -p 12345 0/tmp/backpipe 2>/tmp/backpipe
Pretty straightforward, right? Make a named pipe, use Netcat to listen on port 12345, and pipe the input to `/bin/sh`. Any output or errors get piped back to the same Netcat connection. I ran that as root, and boom! I could now connect to that port and execute commands on the server as root. Scary, huh?
Of course, that’s way too obvious. Netcat listenin’ on a weird port is gonna raise some flags. So, I wanted to hide it better. Next, I decided to try to piggyback off an existing service. I thought about Apache, since it’s almost always runnin’ on servers anyway. I figured I could somehow inject my backdoor into one of Apache’s processes.
This is where things got a bit trickier. I started lookin’ at Apache modules and how they handle requests. The idea was to find a way to create a “secret” URL that, when accessed, would trigger my backdoor. After some digging, I decided to use a PHP script. I know, I know, PHP gets a lot of flak, but it’s readily available and easy to manipulate.
I created a simple PHP script that would check for a specific query parameter. If that parameter was present, it would execute the value as a shell command. Here’s a super-simplified version:
I dropped that script into the Apache web directory and tried it out. Sure enough, if I went to `http://my-server/my_*?secret_command=whoami`, it would execute the `whoami` command on the server and display the result. Nasty!
But, that’s still pretty obvious. Anyone who stumbled across that script could easily figure out what was going on. So, I wanted to obfuscate it further. I started by encoding the query parameter and the command using base64. That way, the URL would look less suspicious.
I modified the PHP script to decode the parameters before executing the command. It looked somethin’ like this:
Now, to execute a command, I’d first base64 encode the PHP code to run the command, then pass that as the ‘data’ parameter.
This was better, but still not great. The fact that a PHP script was executin’ arbitrary commands was still a risk. A good intrusion detection system would probably pick that up. So, the final step was to try and hide the script entirely.
Instead of creatin’ a new PHP file, I decided to append my backdoor code to an existing, legitimate PHP file. I picked a commonly used file, like `*`, and added my obfuscated code to the very end. To make it even harder to find, I wrapped the code in a comment block. Most folks wouldn’t even notice it was there.
The result? A completely “nude” backdoor, hidden in plain sight. No authentication, no authorization, just a hidden trigger that allowed me to remotely execute commands on the server as the web server user. Pretty scary stuff, and a good reminder to always be vigilant about security, even in seemingly well-protected systems.
Remember, this was all for educational purposes. Don’t go using these techniques for malicious activities. Use your powers for good, not evil!
