Technical Tip: SSL VPN and Firewall SAML authentication fails when IdP requires assertions signed with SHA256 algorithm

[CarlosColombini]

Description

This article describes a SAML authentication issue when IdP is configured to accept only SHA256 Signature Algorithm.

FortiGate leverages the SAML Authentication method for several features, such as SSL VPN, Firewall Policies, Wireless, Web Proxy Policies, and Access Proxy Policies.

Service Provider (SP) certificate can be validated by the Identity Provider (IdP), and the Signature Algorithm can be enforced.

A common example is ADFS, which has a Certificate check enabled by default, which also enforces Signature Algorithm SHA256 by default.

Starting with FortiOS 7.0, Signature Algorithm can be configured to use SHA1 or SHA256.

However, due to a known issue reported under ID 850473, even though it is configured as SHA256, the assertions are signed with SHA1.

From ADFS Event Logs, an Event ID 364 will be recorded with the Exception below:

Exception details:

Microsoft.IdentityServer.Protocols.Saml.SamlProtocolSignatureAlgorithmMismatchException: MSIS7093:
The message is not signed with expected signature algorithm.
Message is signed with signature algorithm http://www.w3.org/2000/09/xmldsig#rsa-sha1.
Expected signature algorithm http://www.w3.org/2001/04/xmldsig-more#rsa-sha256.

From FortiGate SAML debug log entries, the Signature Algorithm can also be found as the incorrect one being used.

SigAlg=http%3A%2F%2Fwww.w3.org%2F2000%2F09%2Fxmldsig%23rsa-sha1&

Which causes IdP to respond with the error message below.

__samld_sp_login_resp [826]: Failed to process response message. ret=-432(Status code is not success)

Scope

FortiGate v7.0.12 or older, and v7.2.5 or lower.

Solution

FortiOS 7.0.13, 7.2.6, and 7.4.0 include the fix for this issue as per the Release Notes linked below:
FortiOS 7.0.13 - Release Notes
FortiOS 7.2.6 - Release Notes

FortiOS 7.4.0 - Release Notes

After the firmware upgrade to one of the versions that contain the fix, FortiGate signs the assertions with the SHA256 Algorithm as expected:

SigAlg=http%3A%2F%2Fwww.w3.org%2F2001%2F04%2Fxmldsig-more%23rsa-sha256&amp

Workaround:

Configure the IdP Signature algorithm to accept assertions signed with SHA1 as per the ADFS example below.

Alternatively, a PowerShell command can be used by replacing '<RelyingParty_Name>' with the Relying Party name configured.

Get-AdfsRelyingPartyTrust <RelyingParty_Name> | set-AdfsRelyingPartyTrust -SignatureAlgorithm "http://www.w3.org/2000/09/xmldsig#rsa-sha1"

Relying on Party names can be retrieved with the command 'Get-AdfsRelyingPartyTrust | select Name'.