Technical Tip: Prevent Malicious IP addresses from accessing SSL VPN with brute-force attacks on FortiGate

[Kush_Patel]

Description	This article describes how to prevent malicious actors from using brute-force attacks on the FortiGate to access SSL VPN.
Scope	FortiGate.
Solution	The utilization of the TOR network by attackers offers an elevated level of anonymity, which frequently drives them to heavily rely on brute-force attacks from within this network. By employing ISDB objects, the FortiGate can be configured to block SSLVPN login attempts from known databases of IP addresses, for example: VPN-Anonymous.VPN, Tor-Relay.Node, Tor-Exit.Node, Malicious-Malicious.Server. In v7.6.0+, there is the option to configure a local-in policy via the GUI. ISDB objects are also directly selectable within the local-in policy (starting with v7.6.1). Below v7.6.1, ISDB objects are only selectable within Firewall Policies. Pre Requisite: Ensure that the FortiGate is set to use the 'full' version of the Internet Service DataBase: config sys global     set internet-service-database full end execute update-now It is required for the SSL VPN to instead listen in on a loopback, so a Firewall Policy is needed to reach the VPN. Create a loopback interface with a dummy IP address that will not be reachable: Create a service for a TCP port that will be used to listen for SSL VPN connections: Create a VIP to forward the traffic from WAN to the loopback interface as follows (here, 0.0.0.0 is an example WAN IP): Configure SSL VPN to listen on the loopback interface and port 14144 as follows: Configure firewall policies to block the traffic coming from TOR IPs but allow access to valid users as follows: The following ISDB objects can be added to the blocked list: VPN-Anonymous.VPN Tor-Relay.Node Tor-Exit.Node Malicious-Malicious.Server Further, to check if any of the IPs from which the login attempts are originating belong to known databases (ISDB), follow this process: Navigate to Log & Report -> System Events -> VPN Events and add a column for 'Remote IP'. Apply the filter 'SSL user failed to log in' under Message. The IPs listed under 'Remote IP' can be cross-referenced with existing ISDB databases on the CLI:  'diagnose internet-service match root <Remote IP> 255.255.255.255' This will show if the 'Remote IP' is part of any known databases. Note: when using 'all' as the destination, the traffic may sometimes fail to match and subsequently get allowed, so it is recommended to enable the 'match-vip' option using the CLI as follows: config firewall policy edit "8" set match-vip enable end The match-vip option is disabled by default until v7.2.3. In versions after v7.2.3, the option is enabled by default. Alternatively, try selecting the specific VIP as the destination to match the policy: Related articles: Technical Tip: Virtual IP (VIP) port forwarding configuration Troubleshooting Tip: VIP traffic not matching the firewall policy with an 'all' destination SSL VPN security best practices