[FD] Sangoma SBC bypass authentication via argument injection - CVE-2019-12148

[Security Team Appsecco via Fulldisclosure <fulldisclosure@xxxxxxxxxxxx>]

## 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/>

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