Find file History
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Type Name Latest commit message Commit time
Failed to load latest commit information.
request.txt CTF 2014: Next Global Backdoor

Category: Web Points: 150 Author: reiners Description:

J0nas is a simple man. He specializes in only one thing: Building the most beautiful back doors that you’ve ever seen. This gem was used in multiple saloons, letting only the most skilled c0wb0ys pass. index.phps


Let’s reformat the PHP source first:

	//* DELME */ var_dump(__LINE__, current($GLOBALS)) ? next($GLOBALS) :
	//* DELME */ var_dump(__LINE__, current($GLOBALS[GLOBALS]['GLOBALS'])) ? next($GLOBALS[GLOBALS]['GLOBALS'])[ $next['next'] ] :
	$next['GLOBALS'] = next($next['GLOBALS'])
	//* DELME */ var_dump(__LINE__, $GLOBALS[next]) ? $GLOBALS[next]['next']( $GLOBALS['next']{'GLOBALS'}) :

Basically, a whole bunch of array pointer fun. You can see that there is one “dynamic” function call. At first glance, it looks like we can execute an arbitrary function with an argument of our choice if we can control the contents of $GLOBALS[next]['next'] and $GLOBALS['next']{'GLOBALS'}. However, there is more to it than that. :-)

next moves the internal array pointer by one spot, and $GLOBALS is a built-in array of global variables. By default, it contains the so-called superglobals, i.e. $_GET, $_POST, $_COOKIE, $_FILES and $_SERVER (depending on variables_order), and $GLOBALS itself.

Because I suck at debugging (I have never used Xdebug), I sprinkled in a few /* DELME /* var_dump() lines to see what the current entry in each step was. It was hard for me to parse by reading because of all the variable variables and next’ing, so I also added a source entry to the GET, POST, and cookie arrays.

At the very start, the $GLOBALS array pointer points to $_GET. The first call to next() points it to $_POST, but another call advances it to $_COOKIE. (I found this hard to parse by reading because of the right-to-left interpretation mixed with the code inside the array keys — hence the var_dump.)

It then reads the value for the GLOBALS key, i.e. $_COOKIE['GLOBALS']. This will be the the name of the function to call, and the name of the file input under consideration when doing another next($GLOBALS). After that third next(), we are at the $_FILES array (i.e., $GLOBALS['_FILES_]). The next() call on the $_FILES[$_COOKIE['GLOBALS']] entry advances that file array’s pointer to the second key, which is the content type. That value is used as the argument to the function call.

TL;DR The function in $_COOKIE['GLOBALS'] is called with $_FILE[$_COOKIE['GLOBALS']]['type'] as its only argument.

So, in short: leave $_GET and $_POST empty, set a cookie called GLOBALS with the name of the PHP function to call, add a file whose input name is that function name, and whose content type is the argument. (And ignore all the times next is used as an array key, be it as a string or as an (undefined) constant.)

Note that because eval is not a function but a language construct, you cannot use it as the function to call. Use assert instead.

Finally, here is an example request (note the DOS line endings) which allows you to specify a shell command to run as a query string parameter.

You might need to download the StartSSL root certificate first:

$ wget

Then, perform the request like this:

$ socat,cafile=./ca-bundle.pem stdio < request.txt

Eventually we found the flag in /flag.txt on the server. The flag is flag{backdoor_business_is_hard,_fella}.

Other write-ups and resources

  • none yet