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/uri/uri.go

@@ -23,13 +23,21 @@ func (e ErrURISchemeNotSupported) Error() string {
 
 // Open open a local file or a remote file
 func Open(uriStr string) (io.ReadCloser, error) {
+	return OpenWithClient(uriStr, http.DefaultClient)
+}
+
+// OpenWithClient opens a local file or a remote file, using the given (non-nil) HTTP client
+// for http/https URLs. Callers that must confine remote access (e.g. to defeat SSRF via
+// redirects) should pass a client whose transport validates the peer at dial time; Open
+// passes http.DefaultClient.
+func OpenWithClient(uriStr string, client *http.Client) (io.ReadCloser, error) {
 	u, err := url.Parse(uriStr)
 	if err != nil {
 		return nil, err
 	}
 	switch strings.ToLower(u.Scheme) {
 	case "http", "https":
-		f, err := http.Get(uriStr)
+		f, err := client.Get(uriStr)
 		if err != nil {
 			return nil, err
 		}

modules/uri/uri_test.go

@@ -4,10 +4,18 @@
 package uri
 
 import (
+	"context"
+	"errors"
+	"net"
+	"net/http"
+	"net/http/httptest"
 	"path/filepath"
+	"strings"
+	"sync/atomic"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestReadURI(t *testing.T) {
@@ -17,3 +25,45 @@ func TestReadURI(t *testing.T) {
 	assert.NoError(t, err)
 	defer f.Close()
 }
+
+// TestOpenWithClientValidatesRedirectTarget verifies OpenWithClient routes the
+// whole request chain (including redirects) through the provided client, so a
+// client whose transport refuses to dial an internal target blocks a redirect to
+// it — whereas the default client (old Open behavior) follows it.
+func TestOpenWithClientValidatesRedirectTarget(t *testing.T) {
+	var internalHit atomic.Bool
+	internal := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		internalHit.Store(true)
+		_, _ = w.Write([]byte("secret"))
+	}))
+	defer internal.Close()
+
+	front := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Redirect(w, r, internal.URL, http.StatusFound)
+	}))
+	defer front.Close()
+
+	internalAddr := strings.TrimPrefix(internal.URL, "http://")
+
+	// a client that refuses to dial the internal target, mimicking the migration
+	// hostmatcher dialer that re-validates every hop
+	blockingClient := &http.Client{Transport: &http.Transport{
+		DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
+			if addr == internalAddr {
+				return nil, errors.New("blocked internal address")
+			}
+			return (&net.Dialer{}).DialContext(ctx, network, addr)
+		},
+	}}
+
+	_, err := OpenWithClient(front.URL, blockingClient)
+	require.Error(t, err)
+	assert.False(t, internalHit.Load(), "the redirect target must not be reached through the validating client")
+
+	// the default client (the previous behavior) follows the redirect to the internal target
+	internalHit.Store(false)
+	rc, err := Open(front.URL)
+	require.NoError(t, err)
+	_ = rc.Close()
+	assert.True(t, internalHit.Load(), "sanity check: the default client follows the redirect")
+}