Sangoma SBC 2.3.23-119-GA Authentication Bypass

`## Introduction  
  
### Description  
  
A remotely exploitable vulnerability exists in the 2.3.23-119-GA version of Sangoma SBC that would allow an unauthenticated user to bypass authentication and login as a non-existen user but with complete access to the dashboard including additional privileged user creation capabilities.  
  
### Vulnerability Type  
  
- Argument Injection or Modification (https://cwe.mitre.org/data/definitions/88.html)  
  
## Product Overview  
  
A Sangoma SBC protects both your data and voice network and is designed to handle every aspect of phone calls that travel over the internet (or voice-over-ip phone calls).  
  
## Background  
  
The Sangoma SBC web application heavily relies on the python script `/usr/local/sng/bin/sng-user-mgmt` for various user operations including authenticating the user that is supplied on the login screen of the web application.  
  
When a username and password is provided to the application, it is processed by `/var/webconfig/gui/Webconfig.inc.php` which uses the `Execute` function from `/var/webconfig/api/ShellExec.class.php` to pass the credentials to `/usr/local/sng/bin/sng-user-mgmt` as arguments. The `Execute` function applies the `escapeshellcmd` function to convert any shell characters as literals, however there is no verification that the variables passed do not contain strings that can be interpreted as additional arguments to `/usr/local/sng/bin/sng-user-mgmt`.  
  
The `/var/webconfig/gui/Webconfig.inc.php` calls the `WebSetSessionAuthenticated()` function if the return value of the `$shell->Execute($cmd, $args, true, array('log','escape'=>true))` is 0 as shown below.  
  
```  
$rc = $shell->Execute($cmd, $args, true, array('log','escape'=>true));  
if(0 == $rc){  
...  
...  
WebSetSessionAuthenticated();  
}else{  
Logger::SysLog("webconfig", "login - ${username} login failed");  
...  
...  
}  
```  
  
The Operating System returns a 0 if the `/usr/local/sng/bin/sng-user-mgmt` program exits successfully.  
  
This is true for all arguments of the program unless an explicit status code is sent back to the Operating System. Invoking the help menu, for example is also a successful execution of the program as can be seen from the below two commands  
  
```  
# /usr/local/sng/bin/sng-user-mgmt -h  
Usage: sng-user-mgmt [options] arg  
  
Options:  
-h, --help show this help message and exit  
-a ACTION, --action=ACTION  
Action to perform.  
-u USER, --user=USER User Name  
-s, --syslog Log to syslog  
-p PASSWORD, --password=PASSWORD  
Password  
-f FORCE, --force=FORCE  
Force to remove a user  
-n NAME, --name=NAME User Name  
-e ENCRYPTEDPASSWORD, --encrypted-password=ENCRYPTEDPASSWORD  
Encrypted Password  
-d HASHEDPASSWORD, --hashed-password=HASHEDPASSWORD  
Hashed Password  
-l ACCESS, --access=ACCESS  
Toggle user login access, ie. Enable / Disable  
-o, --sudoer Add to sudoer list  
root@sangoma-test ~  
# echo $?  
0  
```  
  
As the status code is 0, the check in `/var/webconfig/gui/Webconfig.inc.php` passes and a new user session is created.  
  
Passing additional arguments through the username field that would cause the `sng-user-mgmt` to return a 0 would result in a session being created without any valid credentials being supplied. This session provides complete access to the application, including the ability to create additional sudo privilged users.  
  
## Proof of Concept Exploit  
  
1. Pass a username with the value `adam -h`  
2. The password field can be set to anything as this will be ignored  
3. Click login  
4. The `-h` invokes the help menu for `sng-user-mgmt`, returning a 0 and causing `Webconfig.inc.php` to create a new session.  
5. You are now logged in  
  
## Versions Tested  
  
- 2.3.23-119-GA  
  
## Vendor Response  
  
This issue has been responsibly disclosed to the vendor for which a patch has been released in version 2.3.24  
  
https://wiki.sangoma.com/display/SBC/SBC+Downloads  
  
## Credits  
  
Appsecco Security Team  
http://www.appsecco.com  
  
## Timeline  
  
18th May 2019: Discovered and reported to vendor  
21st May 2019: Vendor confirmation  
23rd July 2019: Fixed version (2.3.24) released  
  
## Reference  
  
- [https://www.sangoma.com/products/sbc/](https://www.sangoma.com/products/sbc/)  
  
  
Riyaz Walikar  
  
+91 9886042242  
  
<http://www.appsecco.com/>www.appsecco.com<http://www.appsecco.com/>  
  
Appsecco is a registered trademark of Appsecco Ltd. Appsecco Limited: Registration Number: 9500721. Registered office: Kemp House, 152 to 160 City Road, London EC1V 2NX, United Kingdom. This email message is intended for the named recipient only. It may be privileged and/or confidential. If you are not the named recipient of this email please notify us immediately and do not copy it or use it for any purpose, nor disclose its contents to any other person.  
  
  
  
`