Information
The tokenEndpoint is used to verify the authenticity of the RP (relying party).
When the OpenID Connect server provider requires a token endpoint authentication method the provider can verify the authenticity of the relying party.
Solution
Add the tokenEndpointAuthMethodsSupported attribute to the openidConnectProvider element to ${server.config.dir}/configDropins/overrides/*.xml and set it to a valid list of authentication methods, such as client_secret_post to ensure that an authentication method is required for the token endpoint.
<openidConnectProvider tokenEndpointAuthMethodsSupported="client_secret_post, client_secret_basic" />