Troubleshooting Tip: TLS 1.0 Applications get blocked after upgrading to FortiOS v7.6.7 and v8.0.0

Description

This article describes an expected behavior where the FortiGate starts blocking connections that use TLS version 1.0 when SSL certificate-inspection profiles are applied to Firewall Policies.

Scope

FortiGate in v7.6.7 and v8.0.0.

Solution

In FortiOS v7.6.6 and earlier, the FortiGate would allow HTTPS user traffic to utilize any standard version of TLS, even if it was deprecated (SSL 3.0, TLS 1.0/1.1/1.2/1.3). Any versions not on this list would otherwise be disallowed via the default set unsupported-ssl-version block setting:

config firewall ssl-ssh-profile
    edit "certificate-inspection"
        config https
            set ports 443
            set status certificate-inspection
            set client-certificate bypass
            set unsupported-ssl-version block
            set cert-probe-failure allow
            set encrypted-client-hello 
        end
    next
end

However, starting from FortiOS v7.6.7 and v8.0.0, the FortiGate is now treating TLS 1.0 and SSL 3.0 as unsupported. This means that any connections using these deprecated versions will be dropped by the FortiGate when TLS certificate- or deep-inspection is being performed.

Notably, FortiOS v8.0.0 also introduced a new setting called min-allowed-ssl-version as part of Change #1219167 that controls what the minimum supported version of TLS/SSL is for user traffic being inspected by the FortiGate. This setting is set to tls-1.1 by default:

config firewall ssl-ssh-profile
    edit "certificate-inspection"
        config https
            set ports 443
            set status certificate-inspection
            set client-certificate bypass
            set unsupported-ssl-version block
            set cert-probe-failure allow
            set encrypted-client-hello
            set min-allowed-ssl-version tls-1.1
        end
    next
end

Finally, Bug #1280944 has identified that even when unsupported-ssl-version is set to allow, it is still not possible to establish TLS 1.0 or SSL 3.0 connections through the FortiGate when TLS inspection is enabled, and the firmware version is FortiOS v7.6.7 or v8.0.0.

Resolution and workaround:

Bug #1280944 will introduce a change such that setting unsupported-ssl-version from block to allow will allow unsupported TLS versions (which now include TLS 1.0 and SSL 3.0) to be allowed to pass through the FortiGate. This means that the issue will have at least one resolution added to the upcoming FortiOS v7.6.8 and v8.0.1 firmware versions.

For administrators of FortiOS v8.0.0 and later, another solution currently available is to create a custom TLS inspection profile and set min-allowed-ssl-version to allow the older, deprecated TLS standards to be accepted and inspected:

config firewall ssl-ssh-profile
    edit <name>
        config https
            set ports 443
            set status certificate-inspection
            set min-allowed-ssl-version [ ssl-3.0 | tls-1.0 | tls-1.1 | tls-1.2 | tls-1.3 ]
        end
    next
end

However, there is currently no workaround available for FortiOS v7.6.7 since min-allowed-ssl-version is not an available option. Devices/applications that must use the deprecated TLS 1.0 standard should instead be placed into separate firewall policies that do not have any form of SSL/TLS inspection.

Related documents:

Technical Tip: Difference between min-allowed-ssl-version and unsupported-ssl-version in firewall ssl-ssh-profile section

RFC 8996 - Deprecating TLS 1.0 and TLS 1.1