FortiGate allows Ping from "not trusted hosts" since FortiOS 6.0

Recently I discovered something after updating a FortiGate cluster, which I intensively monitor, not only via working monitoring queries, but also doing some negative monitoring*: Since FortiOS 6.0 the Fortinet FortiGate firewall answers to source addresses via ICMP Type:8 EchoRequest "Ping", which are not included in the trustedhosts (config system admin -> edit admin -> set trusthost1). This is due to a change in FortiOS, which allows ping from more addresses. To limit ping, you have to use LocalIn-Policies (which always should be used, too).

*negative monitoring = I recommend not only to monitor what is allowed (like your SNMPv3 AuthPriv queries, your SSH or HTTPS-Calls), but also to monitor what is forbidden, in order to find some possible bugs or misconfigurations, if suddenly something is answering what really shouldn't. This can be done by trying to use insecure protocols like Telnet, SNMPv1/v2c, HTTP, etc.. or by sending requests from not allowed source ips or by trying to login using deleted/altered accounts or with wrong credentials.


Fortinet documentation:

In versions 5.x and below, trusted hosts configured by an administrator user only allow access from certain IP addresses configured in trusted hosts, to all services configured on the interface, including ping.

From version 6.0 onwards ping service on management interfaces are not included within the scope of trusted hosts. This means that you will be able to ping the interface from an IP that is not included within trusted hosts.

In order to only allow trusted hosts to be able to ping the interface and deny everyone else, you will need to configure a Local In Policy as below.

CLI configuration:

System > Administrators >
config system admin
    edit "admin"
        set trusthost1 172.26.73.48 255.255.255.255
        set accprofile "super_admin"
        set vdom "root"
    next
end

Configuring address and address group as per the trusted hosts:

config firewall address
    edit "trusted-1"
        set type ipmask
        set comment ''
        set visibility enable
        set associated-interface ''
        set color 0
        set allow-routing disable
        set subnet 172.26.73.48 255.255.255.255
    next
end

config firewall addrgrp
    edit "trusted_grp"
        set member "trusted-1"
        set comment ''
        set visibility enable
        set color 0
    next
end

Configuring Firewall local in policies:

config firewall local-in-policy
    edit 2
        set intf "port1"
        set srcaddr "trusted_grp"
        set dstaddr "all"
        set action accept
        set service "PING"
        set schedule "always"
        set status enable
        set comments ''
    next
    edit 1
        set intf "port1"
        set srcaddr "all"
        set dstaddr "all"
        set action deny
        set service "PING"
        set schedule "always"
        set status enable
        set comments ''
    next
end

Before configuring the local in policy:

diagnose sniffer packet any 'host 172.26.73.78 and icmp' 4
interfaces=[any]
filters=[host 172.26.73.48 and icmp]
3.647787 port1 in 172.26.73.78 -> 10.5.22.114: icmp: echo request
3.647850 port1 out 10.5.22.114 -> 172.26.73.78: icmp: echo reply
4.651341 port1 in 172.26.73.78 -> 10.5.22.114: icmp: echo request
4.651383 port1 out 10.5.22.114 -> 172.26.73.78: icmp: echo reply
5.657949 port1 in 172.26.73.78 -> 10.5.22.114: icmp: echo request
5.657992 port1 out 10.5.22.114 -> 172.26.73.78: icmp: echo reply

After configuring the local in policies:

diagnose sniffer packet any 'host 172.26.73.78 and icmp' 4
interfaces=[any]
filters=[host 172.26.73.48 and icmp]
4.264950 port1 in 172.26.73.78 -> 10.5.22.114: icmp: echo request
8.904217 port1 in 172.26.73.78 -> 10.5.22.114: icmp: echo request
13.906576 port1 in 172.26.73.78 -> 10.5.22.114: icmp: echo request
18.893835 port1 in 172.26.73.78 -> 10.5.22.114: icmp: echo request

trace_id=22 func=print_pkt_detail line=5497 msg="vd-root:0 received a packet(proto=1, 172.26.73.78:1->10.5.22.114:2048) from port1. type=8, code=0, id=1, seq=335."
id=20085 trace_id=22 func=init_ip_session_common line=5657 msg="allocate a new session-00874fe6"
id=20085 trace_id=22 func=vf_ip_route_input_common line=2591 msg="find a route: flag=80000000 gw-10.5.22.114 via root"
id=20085 trace_id=22 func=fw_local_in_handler line=409 msg="iprope_in_check() check failed on policy 1, drop

Reply for the trusted host:

diagnose sniffer packet any 'host 172.26.73.48 and icmp' 4
interfaces=[any]
filters=[host 172.26.73.48 and icmp]
7.239647 port1 in 172.26.73.48 -> 10.5.22.114: icmp: echo request
7.239743 port1 out 10.5.22.114 -> 172.26.73.48: icmp: echo reply
8.261081 port1 in 172.26.73.48 -> 10.5.22.114: icmp: echo request
8.261122 port1 out 10.5.22.114 -> 172.26.73.48: icmp: echo reply
9.276261 port1 in 172.26.73.48 -> 10.5.22.114: icmp: echo request
9.276321 port1 out 10.5.22.114 -> 172.26.73.48: icmp: echo reply
10.294536 port1 in 172.26.73.48 -> 10.5.22.114: icmp: echo request
10.294588 port1 out 10.5.22.114 -> 172.26.73.48: icmp: echo reply

Diag debug flow on FortiOS 6.0.9
Source IP of ping is not configured in trustedhosts

2020-02-26 08:38:47 id=20085 trace_id=255 func=init_ip_session_common line=5684 msg="allocate a new session-00018a90"
2020-02-26 08:38:47 id=20085 trace_id=255 func=vf_ip_route_input_common line=2591 msg="find a route: flag=80000000 gw-10.128.36.35 via root"
2020-02-26 08:38:47 id=20085 trace_id=256 func=print_pkt_detail line=5519 msg="vd-root:0 received a packet(proto=1, 10.128.36.35:1->10.240.161.178:0) from local. type=0, code=0, id=1, seq=1156."


Diag debug flow on FortiOS 5.6.12
Source IP of ping is not configured in trustedhosts

2020-02-26 08:46:05 id=20085 trace_id=10 func=print_pkt_detail line=5375 msg="vd-root received a packet(proto=1, 10.240.161.178:1->10.128.36.4:2048) from mgmt1. type=8, code=0, id=1, seq=1166."
2020-02-26 08:46:05 id=20085 trace_id=10 func=init_ip_session_common line=5534 msg="allocate a new session-fd0ad6af"
2020-02-26 08:46:05 id=20085 trace_id=10 func=vf_ip_route_input_common line=2574 msg="find a route: flag=80000000 gw-10.128.36.4 via root"
2020-02-26 08:46:05 id=20085 trace_id=10 func=fw_local_in_handler line=402 msg="iprope_in_check() check failed on policy 0, drop"

Source: https://kb.fortinet.com/kb/documentLink.do?externalID=FD44156