Nebula exploit exercises walkthrough – level07

The flag07 user was writing their very first perl program that allowed them to ping hosts to see if they were reachable from the web server.

The code of the CGI script is provided (and can be viewed in /home/flag07):



# check if Host set. if not, display normal page, etc


Immediately you can see this is not sanitising or validating the input parameter Host that it passes to a command – ping. We can therefore pass it another command for it to execute.

Let’s test the script out, from the command line to start with:

(I’ve stripped out HTML as I am lazy and can’t be bothered getting it to format correctly).

It just runs ping against localhost, as expected.

Run it without parameters, and we get the help:

And then let’s check we can inject a command:


The challenge now is that, for the first time, this script isn’t set to run suid. If I try running getflag, it isn’t going to work.

That thttpd.conf file in flag07’s home directory looks interesting. Could he be running a test web server?

Excellent – a web server on port 7007.

So, we need to:

  • Connect to the web server running on localhost at port 7007
  • Request a index.cgi
  • Pass a Host parameter with a command being careful to URL escape all of the special chars

wget is a simple utility present on nearly all Linux boxes that allows us to get a webpage.

We just need to escape the semi-colon to be %3B.

Check the content of the file and we have run getflag as a flag07.

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