Gitea blind SSRF through OpenID assertion verification

When OpenID sign-in is enabled, Gitea processes unauthenticated assertion parameters and sends an HTTP POST request to the caller-supplied OpenID provider endpoint during verification. An attacker controls the destination and much of the form body, allowing interaction with internal HTTP services reachable from the Gitea server. The response is not returned to the attacker, so the issue is blind SSRF; its impact depends on the services and network addresses the server can reach.