sigstore CSRF possibility in OIDC authentication during signing

Summary

The sigstore-python OAuth authentication flow is susceptible to Cross-Site Request Forgery.

Details

_OAuthSession creates a unique "state" and sends it as a parameter in the authentication request but the "state" in the server response seems not not be cross-checked with this value.

Fix should be fairly trivial.

Impact

This should be low impact: A man-in-the middle attacker could trick a sigstore-python user into signing something with an identity controlled by the attacker (by returning the response to an authentication request they created). This would be quite confusing but not dangerous.