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[FD] Sangoma SBC Remote Command Execution - CVE-2017–17430

## 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.

## Credits

Appsecco Security Team

## 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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