OpenVPN Monitor 1.1.3 Command Injection Vulnerability

#############################################################
#
# Product:  openvpn-monitor
# Vendor:   https://github.com/furlongm/openvpn-monitor
# CSNC ID:  CSNC-2021-010
# CVE ID:   CVE-2021-31605
# Subject:  OpenVPN Management Socket Command Injection
# Severity: High
# Effect:   Denial of Service
# Author:   Emanuel Duss <[email protected]>
#           Sylvain Heiniger <[email protected]>
# Date:     2021-09-22
#
#############################################################

Introduction
------------

openvpn-monitor is a simple Python program to generate HTML that displays the
status of an OpenVPN server, including all current connections. It uses the
OpenVPN management console. It typically runs on the same host as the OpenVPN
server. [0][1]

During a security assessment of one of our customers, several security
vulnerabilities were discovered in this software.

This advisory describes an injection vulnerability which allows an attacker to
inject arbitrary commands into the OpenVPN server management interface socket.


Affected
--------

- Vulnerable: openvpn-monitor <= 1.1.3
- Not vulnerable: none

The vulnerability is already fixed in the source code [3], but there is no new
release which contains the fix. Therefore, all currently available releases
contain this vulnerability.


Technical Description
---------------------

The following route is defined which processes user input sent via HTTP POST:

    [CUT BY COMPASS]
        @app.route('/', method='POST')
        def post_slash():
            vpn_id = request.forms.get('vpn_id')
            ip = request.forms.get('ip')
            port = request.forms.get('port')
            client_id = request.forms.get('client_id')
            return render(vpn_id=vpn_id, ip=ip, port=port, client_id=client_id)
    [CUT BY COMPASS]

Depending on the OpenVPN server version, either the `ip` and `port` parameters
or the `client_id` parameter is used in the `self.send_command(command)`
function call as a parameter for a command in the OpenVPN server management
interface socket:

    [CUT BY COMPASS]
    class OpenvpnMgmtInterface(object):

        def __init__(self, cfg, **kwargs):
            self.vpns = cfg.vpns

            if kwargs.get('vpn_id'):
                vpn = self.vpns[kwargs['vpn_id']]
                self._socket_connect(vpn)
                if vpn['socket_connected']:
                    release = self.send_command('version\n')
                    version = semver(self.parse_version(release).split(' ')[1])
                    if version.major == 2 and \
                            version.minor >= 4 and \
                            kwargs.get('client_id'):
                        command = 'client-kill {0!s}\n'.format(kwargs['client_id'])
                    else:
                        command = 'kill {0!s}:{1!s}\n'.format(kwargs['ip'], kwargs['port'])
                    self.send_command(command)
                    self._socket_disconnect()
    [CUT BY COMPASS]

An attacker can use a newline character (`0x0a`) to inject additional commands
into the socket. This allows an attacker for example to stop the OpenVPN server
by sending a SIGTERM signal via the `signal SIGTERM` management command:

    # curl http://openvpn-monitor.example.net -d "vpn_id=UDP&ip=10.5.23.42&port=1194&client_id=5%0asignal%20SIGTERM"

It's also possible to disconnect all clients at once using the `signal SIGHUP`
command or to reconfigure the OpenVPN server or connected clients. See the
OpenVPN documentation [2] for more information about the OpenVPN management
interface socket and all available commands.

This attack can even be performed if the client disconnect feature is disabled,
because of an identified authorization bypass vulnerability (CVE-2021-31606).


Vulnerability Classification
----------------------------

CVSS v3.1 Metrics [3]:

* CVSS Base Score: 9.3
* CVSS Vector: AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:H


Workaround / Fix
----------------

# openvpn-monitor Vendor

The user input should be sanitized so that it is not possible anymore to break
out of the context and inject own commands into the management socket.

# openvpn-monitor Users

Users of the openvpn-monitor should either apply the patch which is available
on GitHub [2] or use the latest version from the source code repository [1].