Archive for the 'Security' Category

Friday, December 17th, 2010

Will it Blend?

I had the honor of presenting at RuxCon and BayThreat this year.  Both were great conferences with great people.  I’m always humbled when I learn of what others are doing in the security community and even more humbled when asked to present.  I gave a presentation called Will It Blend.  The title of the talk is based on a series of videos from Blendtec (I could watch these videos all day).  The content of the talk however is about “blended threats”.  During the talk I presented a set of bugs I discovered in various browser plug-ins.  Independently, these bugs are pretty lame.  However, if we chain the bugs together, we get something that’s actually pretty interesting.  If you’re interested in taking a look at the slides, you can find them here (PPTPLEX format) or on the RuxCon/Baythreat websites.  The vuln chaining is a little difficult to visualize by looking at the slides, so at the end of my talk I gave a live demo of the bugs being chained together.  For those who were unable to attend my talk live, I’ve created a video to help understand how the exploit would be pulled off (http://www.youtube.com/watch?v=fMFVVNE8ytQ).  It will help to go over the slides first, then watch the video.

Most of the relevant code is available in the slide deck (its really simple).  There are around 5 different bugs in play here, involving a variety of vendors.  All the vendors involved have been contacted.  The oldest bug here is over a year old, the youngest is about five months old.  Kudos to Adobe.  Adobe X has changed its caching behavior, so this specific attack cannot be used against Adobe X users. 

I’m not sure where the blame lies for fixing these issues.  On one hand, if a single vendor addresses their portion of the attack, the entire chain of vulnerabilities is broken.  On the other hand, if only one vendor addresses their issue, all we have to do is find some other software/plugin that buys us the same capability and its game on again.

I hope someone finds the presentation useful.  Happy hunting.

Posted by xssniper | Filed in Security | 4 Comments »

 

Monday, October 18th, 2010

PDF RCE et al. (CVE-2010-3625, CVE-2010-0191, CVE-2010-0045)

A few weeks ago, Adobe released an advisory for a ton of Acrobat Reader bugs.  Buried in the long list of Acrobat Reader bugs was a patch for a vulnerability I reported to Adobe.  Taking a look at the entry in the advisory, we see the following description:

(CVE-2010-3625)

This update resolves a prefix protocol handler vulnerability that could lead to code execution

What’s interesting is many months ago (in April 2010), Adobe released a similar patch for a different bug I had reported to them.  The description from April’s advisory is as follows:

(CVE-2010-0191).

This update resolves a prefix protocol handler vulnerability that could lead to code execution

Going back even further, there is an Apple advisory that has a bug with a description similar to the Adobe advisories:

CVE-2010-0045

Description: An issue in Safari’s handling of external URL schemes may cause a local file to be opened in response to a URL encountered on a web page. Visiting a maliciously crafted website may lead to arbitrary code execution.

I’ll walk you through the latest PDF bug, but the symptoms for all the bugs are very similar.  As you know, PDF Reader supports the use of JavaScript.  One of the JavaScript APIs supported by Acobat Reader (>7.0) is app.launchURL().  app.launchURL() takes two parameters, the URL to be opened and a flag that specifies whether the URL should be opened in a new window.  Typical usage of app.launchURL() looks something like this:

app.lauchURL(“http://www.google.com” , false);

Simple enough.  Naturally, when a string that looks like URI is encountered one of the first things that’s attempted is to point the URI value to a file:// location and observe whether the local file is opened.  In this case, access to file:// is blocked by Adobe reader.  Next up are arbitrary protocol handlers.  Tests for mailto://, foo://, bar:// all work, however JavaScript:// seems to be blocked.  This feels like a protocol handler blacklist.  I think there was a SouthPark episode about using blacklists last year…

There is a simple way to bypass most protocol handler blacklists.  This bypass was the key to CVE-2010-3625, CVE-2010-0191, and CVE-2010-0045.  The trick is to simply append a “URL:” prefix protocol handler to your URI.  You can test this by opening Internet Explorer (IE8 on Win7) and typing “url:javascript:alert(1)”.  I must give credz where credz are due.  I first learned of this prefix protocol handler when looking at the source code for HTMLer (which is a port of MangleMe).

With the prefix protocol handler in hand, we’re all set to bypass the protocol handler blacklist:

app.launchURL(“url:file://c:/windows/system32/calc.exe”, true);

There is some weird shell behavior here (which I won’t get into), but the key pieces are (as far as this bug is concerned):  the url: prefix protocol handler and setting the “New Window” flag to true.  A link to a simple PoC is provided below.  This bug worked on Win7 with no prompts.  For some users, this bug will not work if IE is already running (must be launched from a browser other than IE).  For users without Adobe’s April patch, this bug should work on all browsers in most configurations.

http://xs-sniper.com/sniperscope/Adobe/calc.pdf

There you go, a simple yet effective way to bypass a protocol handler blacklist.  I hope that knowledge of this prefix protocol handler provides that missing piece you needed.  Happy hunting.

Posted by xssniper | Filed in Security | 2 Comments »

 

Monday, September 27th, 2010

Turning the Tables – Part I

Boom… I’ve just taken over a Zeus C&C.  I fire up a second, clean VM just to verify… yup it works.  Ok, now what?

A while back, I came across a kit for setting up a Zeus botnet.  It was an interesting package.  Looking at the C&C, bot builder, the actual bot, and user manual was pretty cool (yes, it comes with a user manual).  You have to admire the some of the tricks used by the bot, these guys are clever.  I set up a mini-botnet on a testing network and began to examine how the botnet worked.  Eventually, I came across some bugs (even a blind squirrel finds a nut every once in a while).  There are some fascinating things to consider when finding bugs in software that is used primarily by criminals, but I won’t bore you with that now.  Instead I’d like to share with you some of the more interesting parts of my research.

Before I proceed, there are a few things I’d like to state:

  1. This research was done on my own time on my own equipment.  The thoughts on my blog are my own.
  2. Disclosure of this issue is a bit tricky.  I’ll cover some of the issues I came across in a future post.
  3. I’m releasing the details of my work because I felt it was important for the public to have this knowledge to better defend their networks.
  4. All the work presented here is for academic and research purposes only.

In the spirit of responsible disclosure I contacted security@zeus.com and informed them that I may have discovered a security issue with their C&C server software.  The Zeus.com team informed me that they were a cloud service provider and didn’t have C&C software.  Zeus.com then proceeded to spam me with advertisements for their latest products.  I then contacted security@botnets-r-us.ru but received no response.  Botnets-r-us.ru then proceeded to spam me with Viagra ads and executables for me to download.  With no other alternative and an email inbox full of spam, I have no choice but to provide full disclosure of the vulnerability to the public.

Taking a look at the documentation that accompanies the Zeus package, I see change log indicates that I’m working with a recent version of Zeus (likely released earlier this year).

Examining the source code from the C&C confirms that I’m working with version 1.3.2.1, which was released on January 15th of this year.

I haven’t tested this exploit against newer versions of the C&C, but this post should provide everything you need to check yourself.  If you do happen to have a newer version of the C&C code (or kits from other botnets), please contact me (xssniper  -at- gmail) I’d love to have a look.  I looked on the Internetz to see if someone else had discovered this, but I found nothing.  If this bug was previously disclosed and I failed to credit you, please let me know (I don’t follow the bot scene very closely).

The C&C software has a PHP based web application that provides a control panel for botmasters and also serves as a gateway for bot communication.  There are several websites that have described the C&C so I won’t spend much time on that here, but I do feel it’s important to touch on a few things.  When the C&C web application is installed, very little attack surface is exposed to unauthenticated users.  The two most interesting pages available to unauthenticated users are the login page and the gateway.  By default, the login page is located at /cp.php.  By default, the gateway is located at /gate.php.  Some botmasters rename the gate.php file, however if you’ve managed to capture a live Zeus bot it will phone home to a php file.  The php file that the bot phones home to is the gateway (gate.php).  For clarity, let’s assume the gateway is at /gate.php (the default).  The gateway will only respond to requests from bots.  For example, if you point your browser to /gate.php, you’ll get a blank page back:

Luckily, we have both bot samples and the source for the C&C, allowing us to reverse the protocol needed for communication to the gateway.  Let’s walk through a couple key pieces of the gate.php source.  First, the gateway requires a POST request.

if(@$_SERVER['REQUEST_METHOD'] !== ‘POST’)die();

require_once(‘system/global.php’);

require_once(‘system/config.php’);

If the gateway receives a POST request, it grabs the POST body, performs some basic validation, and then decrypts the data using the RC4 algorithm.

$data      = @file_get_contents(‘php://input’);

$data_size = @strlen($data);

if($data_size < HEADER_SIZE + ITEM_HEADER_SIZE)die();

$data = RC4($data, BOTNET_CRYPTKEY);

This is not a typical POST request with POST parameters in the body.  Instead, this POST request contains a binary blob as its POST body (there are no POST parameter names).  The last line in the code snippet provided above mentions RC4 and a PHP constant named BOTNET_CRYPTKEY.  In case you’re wondering, the RC4 key (BOTNET_CRYPTKEY) is set by the botmaster when setting up the C&C and is stored server side (in the /system/config.php file).  As RC4 is a symmetric algorithm, the bot must also have a representation of the key.  The key is embedded into the bot (supplied via configuration file).  So once you have captured a live bot, you’ll be able to extract the RC4 key.  The key can be extracted from memory or if you are able to decrypt the config.bin file, you’ll see the key passed as part of the configuration for the bot.  If you’re interested in doing this, check out threatexpert.com.  Worst case, you can try brute forcing the key.

Once the data is decrypted, the gateway does a quick sanity check.

if(strcmp(md5(substr($data, HEADER_SIZE), true), substr($data, HEADER_MD5, 16)) !== 0)die();

and proceeds to unpack the data if the sanity check turns out ok

$list = array();

for($i = HEADER_SIZE; $i < $data_size;)

{

$k = @unpack(‘L4′, @substr($data, $i, ITEM_HEADER_SIZE));

$list[$k[1]] = @substr($data, $i + ITEM_HEADER_SIZE, $k[3]);

$i += (ITEM_HEADER_SIZE + $k[3]);

}

unset($data);

Once the data is unpacked, we will have an array ($list[]) populated with various configuration and log data being passed from the bot to the C&C.  Using what we’ve discovered thus far, we can create a fake bot that is capable of communicating with the C&C.  Depending on the values held in the $list array, the gateway executes various functions.  One of the functions I found interesting was this:

else if(!empty($list[SBCID_BOTLOG]) && !empty($list[SBCID_BOTLOG_TYPE]))

{

$type = ToInt($list[SBCID_BOTLOG_TYPE]);

if($type == BLT_FILE)

{

//Расширения, которые представляют возможность удаленного запуска.

$bad_exts = array(‘.php3′, ‘.php4′, ‘.php5′, ‘.php’, ‘.asp’, ‘.aspx’, ‘.exe’, ‘.pl’, ‘.cgi’, ‘.cmd’, ‘.bat’, ‘.phtml’);

$fd_hash  = 0;

$fd_size  = strlen($list[SBCID_BOTLOG]);

//Формируем имя файла.

if(IsHackNameForPath($bot_id) || IsHackNameForPath($botnet))die();

$file_root = REPORTS_PATH.’/files/’.urlencode($botnet).’/’.urlencode($bot_id);

$file_path = $file_root;

$last_name = ”;

$l = explode(‘/’, (isset($list[SBCID_PATH_DEST]) && strlen($list[SBCID_PATH_DEST]) > 0 ? str_replace(‘\\’, ‘/’, $list[SBCID_PATH_DEST]) : ‘unknown’));

foreach($l as &$k)

{

if(IsHackNameForPath($k))die();

$file_path .= ‘/’.($last_name = urlencode($k));

}

if(strlen($last_name) === 0)$file_path .= ‘/unknown.dat’;

unset($l);

//Проверяем расширении, и указываем маску файла.

if(($ext = strrchr($last_name, ‘.’)) === false || in_array(strtolower($ext), $bad_exts) !== false)$file_path .= ‘.dat’;

$ext_pos = strrpos($file_path, ‘.’);

//FIXME: Если имя слишком большое.

if(strlen($file_path) > 180)$file_path = $file_root.’/longname.dat’;

//Добавляем файл.

for($i = 0; $i < 9999; $i++)

{

if($i == 0)$f = $file_path;

else $f = substr_replace($file_path, ‘(‘.$i.’).’, $ext_pos, 1);

if(file_exists($f))

{

if($fd_size == filesize($f))

{

if($fd_hash === 0)$fd_hash = md5($list[SBCID_BOTLOG], true);

if(strcmp(md5_file($f, true), $fd_hash) === 0)break;

}

}

else

{

if(!CreateDir(dirname($file_path)) || !($h = fopen($f, ‘wb’)))die();

flock($h, LOCK_EX);

fwrite($h, $list[SBCID_BOTLOG]);

flock($h, LOCK_UN);

fclose($h);

break;

}

}

}

A quick look at the function above shows that if $list[SBCID_BOTLOG] and $list[SBCID_BOTLOG_TYPE] are set to the correct values, we can trick the C&C into thinking we have a bot that needs to upload a logfile.  Before the C&C accepts our supplied logfile, it first attempts some validation by checking to see if the file extension we’re providing is in a blacklist of “bad extensions” and whether the filepath supplied is “IsHackNameForPath” (a custom validation routine written by the C&C author).

$bad_exts = array(‘.php3′, ‘.php4′, ‘.php5′, ‘.php’, ‘.asp’, ‘.aspx’, ‘.exe’, ‘.pl’, ‘.cgi’, ‘.cmd’, ‘.bat’, ‘.phtml’);

…<snip>…

if(($ext = strrchr($last_name, ‘.’)) === false || in_array(strtolower($ext), $bad_exts) !== false)$file_path .= ‘.dat’;

…<snip>

//Формируем имя файла.
if(IsHackNameForPath($bot_id) || IsHackNameForPath($botnet))die();

We know the web server supports PHP because the C&C web management console is written in PHP.  If we can pretend like we’re a bot, convince the C&C that we have a “BOTLOG” that needs to be uploaded, and instead of uploading a “BOTLOG” we upload a PHP file with our PHP content, we could have arbitrary code execution on the C&C.  It seems the C&C code protects against this attack… or does it?  Unfortunately for the botmaster, the PHP interpreter is very liberal on extensions.  Some examples of the quirky extension madness associated with PHP can be found on slide 23 in this presentation (given by Kuza55 at CCC 2007).  In this case, I want to upload a PHP file to both IIS and Apache (the supported platforms for the C&C) so I use the trailing dot trick.  All I have to do is append a trailing period to the end of the .php extension (.php.), and I can bypass the extension check yet have the file contents run by the PHP interpreter.  Once the extension check is bypassed, the value I supplied for $list[SBCID_BOTLOG] is written as content to the file I specified on the webserver.  Now I just have to guess where my PHP file was written.  This line of PHP in the gateway source gives us a clue.

$file_root = REPORTS_PATH.’/files/’.urlencode($botnet).’/’.urlencode($bot_id);

The default location for the BOT LOG is: C&C-webroot\_reports\files\<Name of the botnet>\<Bot ID>\

I also control (via values passed from my fake bot to the C&C) the two subdirectory names (in this example: “BKs_BOTNET” for <Name of the botnet> and “BK_PWNZ_UR_CnC” for <Bot ID>).  If the botmaster is using a default install and hasn’t relocated the _reports folder, we should be able to simply guess where our PHP file was written to (/_reports/files/BKs_BOTNET/BK_PWNZ_UR_CnC/pwnd.php).

If the botmaster was smart and relocated the _reports folder, guessing where the uploaded PHP file becomes more difficult.  We can take all the guesswork out by using some directory traversal tricks and planting the PHP file directly into the webroot.

Boom… we’ve just taken over a Zeus C&C.  Once we have our own PHP code running on the C&C, we can include the /system/config.php file.  Config.php contains the location of the MySQL database as well as the DB username and password (via connection string), giving us complete control over the management console and all the bots associated with this C&C.

For those interested in “studying” this vulnerability, I’ve put together a Proof of Concept.  All you have to do is provide the location of the gateway (provided by the bot), the RC4 key (provided by the bot), and the PHP code that you want to upload.

Posted by xssniper | Filed in Security, Tools | 30 Comments »