Sangoma SBC Remote Command Execution - CVE-2017–17430

[Security Team Appsecco <security () appsecco com>]

## Description

Sangoma NetBorder / Vega Session Controller before 2.3.12-80-GA allows remote unauthenticated attackers to execute 
arbitrary commands via the web interface.

## Technical Details

The `ShellExec` class implemented in `api/ShellExec.class.php` is extensively used by various PHP scripts in the 
management web application to invoke external command line programs. The `Execute` method in this class invokes 
external programs, optionally with `sudo` for privileged execution. Unfortunately this method does not implement any 
input validation or parameter escaping which can be exploited to inject and execute arbitrary shell commands.

During our research, we exploited the `login` functionality in the web app to inject and execute arbitrary shell 
commands when `/usr/local/sng/bin/sng-user-mgmt` was invoked to verify provided credentials.

Following code is executed to authenticate a user from web ui:

```
1197  $shell = new ShellExec();
1198  $cmd = '/usr/local/sng/bin/sng-user-mgmt';
   [...]
1204  $args =' --action=login --user='.$username.' --encrypted-password="' . $string.'"';
1205  $rc = $shell->Execute($cmd, $args, true, array('log'));
1206  if(0 == $rc){
   [...]
1217     $_SESSION['user_login'] = $username;
1218     $_SESSION['system_login'] = "root";
1219     WebSetSessionAuthenticated();
1220     return true;
```

Example payload during login to achieve command execution:

```
username: a;echo A > /tmp/test.txt;
password: anything
```

This results in creation of `/tmp/test.txt` on the server. 

## Vendor Response

This issue has been responsibly disclosed to the vendor for which a patch has been released.
ftp://ftp.sangoma.com/nsc/2.3/Changelog

## Credits

Appsecco Security Team
http://www.appsecco.com

## Timeline

2017-12-05: Discovered and reported to vendor
2017-12-06: Vendor confirmation
2017-12-07: Fixed version (2.3.12) released


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