fix: various security fixes (#38406)

Addresses a batch of privately reported security issues, grouped by
area:

- **SSRF** - migration PR-patch/asset fetches, OAuth2 avatar & OpenID
discovery, pull-mirror URL re-validation, and the outbound proxy path.
- **Access-token scope** - prevent scope escalation on token creation;
keep public-only tokens confined (feeds, packages, Actions listings,
star/watch lists, limited/private owners).
- **Access control / disclosure** - go-get default-branch leak, webhook
authorization-header leak, watch clearing on private transitions,
label/attachment scoping.
- **Denial of service** - input bounds for npm dist-tags, Debian control
files, Arch file lists, and SSH keys.

### 📌 Attention for site admins

Not breaking - existing configs keep working - but two changes are worth
a look:

- **New SSRF protection** Outbound requests (migrations, OAuth2 avatars,
OpenID discovery, pull mirrors, proxy path) are now validated against
the allow/block host lists. If your instance legitimately reaches
internal hosts, you may need to add them to
`[security].ALLOWED_HOST_LIST` (and the relevant `ALLOW_LOCALNETWORKS`
settings).
- **Deprecation** `[webhook].ALLOWED_HOST_LIST` is deprecated and will
be removed in a future release. Use `[security].ALLOWED_HOST_LIST`
instead; the old key still works for now.

---------

Co-authored-by: TheFox0x7 <thefox0x7@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>

modules/auth/openid/openid.go

@@ -4,8 +4,14 @@
 package openid
 
 import (
+	"net/http"
+	"sync"
 	"time"
 
+	"gitea.dev/modules/hostmatcher"
+	"gitea.dev/modules/proxy"
+	"gitea.dev/modules/setting"
+
 	"github.com/yohcop/openid-go"
 )
 
@@ -19,11 +25,23 @@ import (
 var (
 	nonceStore     = openid.NewSimpleNonceStore()
 	discoveryCache = newTimedDiscoveryCache(24 * time.Hour)
+
+	// openIDInstance does discovery/verification via an SSRF-protected client, so a user-supplied
+	// OpenID identifier can't reach internal/loopback/reserved addresses. It honors the operator's
+	// [security] ALLOWED_HOST_LIST (empty defaults to "external"), matching the avatar/webhook/migration
+	// clients, and validates the proxy path too. Lazy: reads proxy/settings once.
+	openIDInstance = sync.OnceValue(func() *openid.OpenID {
+		allowList := hostmatcher.ParseHostMatchList("security.ALLOWED_HOST_LIST", setting.Security.AllowedHostList)
+		return openid.NewOpenID(&http.Client{
+			Timeout:   30 * time.Second,
+			Transport: hostmatcher.NewHTTPTransport("openid", allowList, nil, proxy.Proxy(), setting.Proxy.ProxyURLFixed, nil),
+		})
+	})
 )
 
 // Verify handles response from OpenID provider
 func Verify(fullURL string) (id string, err error) {
-	return openid.Verify(fullURL, discoveryCache, nonceStore)
+	return openIDInstance().Verify(fullURL, discoveryCache, nonceStore)
 }
 
 // Normalize normalizes an OpenID URI
@@ -33,5 +51,5 @@ func Normalize(url string) (id string, err error) {
 
 // RedirectURL redirects browser
 func RedirectURL(id, callbackURL, realm string) (string, error) {
-	return openid.RedirectURL(id, callbackURL, realm)
+	return openIDInstance().RedirectURL(id, callbackURL, realm)
 }

modules/auth/openid/openid_test.go

@@ -0,0 +1,29 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package openid
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"sync/atomic"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestOpenIDDiscoveryBlocksInternalHost(t *testing.T) {
+	var reached atomic.Bool
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		reached.Store(true)
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer srv.Close()
+
+	// RedirectURL performs server-side discovery of the identifier URL; a loopback URL
+	// must be refused at dial time instead of reaching the internal server
+	_, err := RedirectURL(srv.URL, "http://example.com/callback", "http://example.com/")
+	require.Error(t, err)
+	assert.False(t, reached.Load(), "OpenID discovery must not reach an internal/loopback host")
+}