Two SSRF findings in Gitea 1.26.2

Affected: Go: code.gitea.io/gitea

| --- | --- | | Versions tested | `gitea/gitea:1.26.2` (digest `sha256:7d13848af12645600a5f9d93ee2560daa9c6fa6b5b859b7bff3a5e1c0b661031`); `gitea/gitea:latest` resolves to the same digest at time of writing | | Source review | `git checkout v1.26.2` (commit `2c749ce`) | | Reproduction | `bash run_poc.sh` (single shot: brings up containers, runs three PoCs, prints captured evidence, tears down on exit) | | Files touched by the fixes | `modules/hostmatcher/hostmatcher.go`, `modules/auth/openid/openid.go` |

## Summary

Gitea guards outbound HTTP from webhooks and repo migration with `net.Dialer.Control`, the correct hook point. The IP classifier behind it misses ten address families, of which CGNAT (100.64.0.0/10) is the practically important one because it is plain IPv4 and is used today by Tailscale, AWS VPC secondary CIDRs, and several Kubernetes pod-CIDR conventions. Any logged-in user can create a webhook pointing at an internal CGNAT host. The full HTTP response from that host (status, headers, body up to 1 MB) is stored in the webhook delivery log and rendered to the webhook owner on the hook detail page. The same gap applies to repo migration.

Separately, the OpenID sign-in form at `/user/login/openid` fetches the user-supplied provider URL server-side via `openid-go`, which uses `http.DefaultClient`. No `hostmatcher`, no IP filter, no CSRF, no authentication. When OpenID sign-in is enabled, anyone on the internet can drive Gitea into making arbitrary GET requests aga…