Correctly handle submodule view and avoid throwing 500 error

routers/web/repo/view_home.go

@@ -20,6 +20,8 @@ import (
 	unit_model "code.gitea.io/gitea/models/unit"
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/git"
+	giturl "code.gitea.io/gitea/modules/git/url"
+	"code.gitea.io/gitea/modules/httplib"
 	"code.gitea.io/gitea/modules/log"
 	repo_module "code.gitea.io/gitea/modules/repository"
 	"code.gitea.io/gitea/modules/setting"
@@ -302,8 +304,33 @@ func handleRepoEmptyOrBroken(ctx *context.Context) {
 	ctx.Redirect(link)
 }
 
+func handleRepoViewSubmodule(ctx *context.Context, submodule *git.SubModule) {
+	submoduleRepoURL, err := giturl.ParseRepositoryURL(ctx, submodule.URL)
+	if err != nil {
+		HandleGitError(ctx, "prepareToRenderDirOrFile: ParseRepositoryURL", err)
+		return
+	}
+	submoduleURL := giturl.MakeRepositoryWebLink(submoduleRepoURL)
+	if httplib.IsCurrentGiteaSiteURL(ctx, submoduleURL) {
+		ctx.RedirectToCurrentSite(submoduleURL)
+	} else {
+		// don't auto-redirect to external URL, to avoid open redirect or phishing
+		ctx.Data["NotFoundPrompt"] = submoduleURL
+		ctx.NotFound(nil)
+	}
+}
+
 func prepareToRenderDirOrFile(entry *git.TreeEntry) func(ctx *context.Context) {
 	return func(ctx *context.Context) {
+		if entry.IsSubModule() {
+			submodule, err := ctx.Repo.Commit.GetSubModule(entry.Name())
+			if err != nil {
+				HandleGitError(ctx, "prepareToRenderDirOrFile: GetSubModule", err)
+				return
+			}
+			handleRepoViewSubmodule(ctx, submodule)
+			return
+		}
 		if entry.IsDir() {
 			prepareToRenderDirectory(ctx)
 		} else {

routers/web/repo/view_home_test.go

@@ -0,0 +1,32 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package repo
+
+import (
+	"net/http"
+	"testing"
+
+	"code.gitea.io/gitea/models/unittest"
+	git_module "code.gitea.io/gitea/modules/git"
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/services/contexttest"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestViewHomeSubmoduleRedirect(t *testing.T) {
+	unittest.PrepareTestEnv(t)
+
+	ctx, _ := contexttest.MockContext(t, "/user2/repo1/src/branch/master/test-submodule")
+	submodule := &git_module.SubModule{Path: "test-submodule", URL: setting.AppURL + "user2/repo-other.git"}
+	handleRepoViewSubmodule(ctx, submodule)
+	assert.Equal(t, http.StatusSeeOther, ctx.Resp.WrittenStatus())
+	assert.Equal(t, "/user2/repo-other", ctx.Resp.Header().Get("Location"))
+
+	ctx, _ = contexttest.MockContext(t, "/user2/repo1/src/branch/master/test-submodule")
+	submodule = &git_module.SubModule{Path: "test-submodule", URL: "https://other/user2/repo-other.git"}
+	handleRepoViewSubmodule(ctx, submodule)
+	// do not auto-redirect for external URLs, to avoid open redirect or phishing
+	assert.Equal(t, http.StatusNotFound, ctx.Resp.WrittenStatus())
+}