IdentityServer 3 signing certificate expiry

Question

What happens when the signing certificate (used for signing jwt tokens) expires when using IdentityServer 3?

It's unclear to me and I can't find any documentation, other than that it's possible to get a warning that it has expired. (Ref. https://identityserver.github.io/Documentation/docsv2/configuration/events.html)

Is there any mechanism that stops the use of expired signing certs?
And what happens on the client side (client being the Web API that uses IdentityServer for authentication) when validating a token signed by an expired certificate? (For example if https://github.com/IdentityServer/IdentityServer3.AccessTokenValidation is used as a middleware.)

Answer 1

Well I've just tested this (on IdentityServer4) and it seems to continue to work happily with an expired signing certificate, here's my test cert's validity:

I'm able to login, get an ID token and an access token and then access an API with the access token. What IdentityServer does do however is to log a warning:

2017-07-13 12:15:54.871 +02:00 [Warning] 
    Certificate "CN=test_expired_signing_certificate" has expired on "13/07/2016 14:14:37"

This matches what the IdentityServer (3) docs say here:

IdentityServer raises a number of events at runtime, e.g:

snip...

• Expired/invalid/no signing certificate

By default these events are forwarded to the configured log provider - a custom event service can process or forward them in any way suitable for the environment.

So this would be your way of detecting it when it's already too late. A better option is to rollover signing keys periodically and within the validity of the keys, this is the common approach which also allows for a compromised key to be revoked if necessary. See this issue where the process is discussed, basically IdentityServer can handle two keys:

[Middleware refreshes] the metadata document ... once a day.

The metadata doc can hold 2 keys - primary and secondary and the middleware will load and use both when present (JWTs have a key identifier that allows picking the right one).

When you start to rollover - set both keys and at some point swap primary and secondary.