SAML ACS Endpoint should ignore any unknown parameters

[azasypkin]

Currently our SAML ACS Endpoint expects only SAMLResponse and RelayState parameters and rejects requests that provide anything else that we don't expect. This may become a problem for IdPs that used to send some additional parameters for whatever reason.

It shouldn't harm if we stop rejecting such requests and just ignore unknown parameters. We could potentially use allowUnknown of schema.object validation scheme we use for the request body, but I don't like that after validation we'll still have these unknown properties in the object that someone can potentially enumerate through. After a conversation with @restrry we think that there is a chance we may add an additional schema.object mode that may allow unknown properties, but would strip them out.

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)

[legrego]

After a conversation with @restrry we think that there is a chance we may add an additional schema.object mode that may allow unknown properties, but would strip them out.

I like this idea. Something along the lines of allowUnknown and ignoreUnknown would be pretty helpful I think.

[azasypkin]

Fixed in #69401