diff --git a/routers/common/redirect.go b/routers/common/redirect.go
index d64f74ec82..e9f14a6eb1 100644
--- a/routers/common/redirect.go
+++ b/routers/common/redirect.go
@@ -16,11 +16,12 @@ func FetchRedirectDelegate(resp http.ResponseWriter, req *http.Request) {
 	// 2. when use "window.reload()", the hash is not respected, the newly loaded page won't scroll to the hash target.
 	// The typical page is "issue comment" page. The backend responds "/owner/repo/issues/1#comment-2",
 	// then frontend needs this delegate to redirect to the new location with hash correctly.
-	redirect := req.PostFormValue("redirect")
-	if !httplib.IsCurrentGiteaSiteURL(req.Context(), redirect) {
-		resp.WriteHeader(http.StatusBadRequest)
+	redirect := req.FormValue("redirect")
+	if req.Method != http.MethodPost || !httplib.IsCurrentGiteaSiteURL(req.Context(), redirect) {
+		http.Error(resp, "Bad Request", http.StatusBadRequest)
 		return
 	}
-	resp.Header().Add("Location", redirect)
+	// no OpenRedirect, the "redirect" is validated by "IsCurrentGiteaSiteURL" above
+	resp.Header().Set("Location", redirect)
 	resp.WriteHeader(http.StatusSeeOther)
 }
diff --git a/routers/common/redirect_test.go b/routers/common/redirect_test.go
new file mode 100644
index 0000000000..c4e6d2e6c7
--- /dev/null
+++ b/routers/common/redirect_test.go
@@ -0,0 +1,48 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package common
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strings"
+	"testing"
+
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/test"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFetchRedirectDelegate(t *testing.T) {
+	defer test.MockVariableValue(&setting.AppURL, "https://gitea/")()
+
+	cases := []struct {
+		method string
+		input  string
+		status int
+	}{
+		{method: "POST", input: "/foo?k=v", status: http.StatusSeeOther},
+		{method: "GET", input: "/foo?k=v", status: http.StatusBadRequest},
+		{method: "POST", input: `\/foo?k=v`, status: http.StatusBadRequest},
+		{method: "POST", input: `\\/foo?k=v`, status: http.StatusBadRequest},
+		{method: "POST", input: "https://gitea/xxx", status: http.StatusSeeOther},
+		{method: "POST", input: "https://other/xxx", status: http.StatusBadRequest},
+	}
+	for _, c := range cases {
+		t.Run(c.method+" "+c.input, func(t *testing.T) {
+			resp := httptest.NewRecorder()
+			req := httptest.NewRequest(c.method, "/?redirect="+url.QueryEscape(c.input), nil)
+			FetchRedirectDelegate(resp, req)
+			assert.Equal(t, c.status, resp.Code)
+			if c.status == http.StatusSeeOther {
+				assert.Equal(t, c.input, resp.Header().Get("Location"))
+			} else {
+				assert.Empty(t, resp.Header().Get("Location"))
+				assert.Equal(t, "Bad Request", strings.TrimSpace(resp.Body.String()))
+			}
+		})
+	}
+}
diff --git a/web_src/js/external-render-helper.test.ts b/web_src/js/external-render-helper.test.ts
new file mode 100644
index 0000000000..452d7f8f2d
--- /dev/null
+++ b/web_src/js/external-render-helper.test.ts
@@ -0,0 +1,25 @@
+import './external-render-helper.ts';
+
+test('isValidCssColor', async () => {
+  const isValidCssColor = window.testModules.externalRenderHelper!.isValidCssColor;
+  expect(isValidCssColor(null)).toBe(false);
+  expect(isValidCssColor('')).toBe(false);
+
+  expect(isValidCssColor('#123')).toBe(true);
+  expect(isValidCssColor('#1234')).toBe(true);
+  expect(isValidCssColor('#abcabc')).toBe(true);
+  expect(isValidCssColor('#abcabc12')).toBe(true);
+
+  expect(isValidCssColor('rgb(255 255 255)')).toBe(true);
+  expect(isValidCssColor('rgb(0, 255, 255)')).toBe(true);
+
+  // examples from MDN: https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Values/color_value/rgb
+  expect(isValidCssColor('rgb(255 255 255 / 50%)')).toBe(true);
+  expect(isValidCssColor('rgb(from #123456 hwb(120deg 10% 20%) calc(g + 40) b / 0.5)')).toBe(true);
+
+  expect(isValidCssColor('#123 ; other')).toBe(false);
+  expect(isValidCssColor('#123 : other')).toBe(false);
+  expect(isValidCssColor('#rgb(0, 255, 255); other')).toBe(false);
+  expect(isValidCssColor('#rgb(0, 255, 255)} other')).toBe(false);
+  expect(isValidCssColor('url(other)')).toBe(false);
+});
diff --git a/web_src/js/external-render-helper.ts b/web_src/js/external-render-helper.ts
index 3acac8db14..9162d0f550 100644
--- a/web_src/js/external-render-helper.ts
+++ b/web_src/js/external-render-helper.ts
@@ -15,6 +15,17 @@ RENDER_COMMAND = `echo '<div style="width: 100%; height: 2000px; border: 10px so
 
 */
 
+// Check whether the user-provided color value is a valid CSS color format to avoid CSS injection.
+// Don't extract this function to a common module, because this file is an IIFE module for external render
+// and should not have any dependency to avoid potential conflicts.
+function isValidCssColor(s: string | null): boolean {
+  if (!s) return false;
+  // it should only be in format "#hex" or "rgb(...)", because it comes from a computed style's color value
+  const reHex = /^#([0-9a-fA-F]{3}|[0-9a-fA-F]{4}|[0-9a-fA-F]{6}|[0-9a-fA-F]{8})$/;
+  const reRgb = /^rgb\([^{}'";:]+\)$/;
+  return reHex.test(s) || reRgb.test(s);
+}
+
 const url = new URL(window.location.href);
 
 const isDarkTheme = url.searchParams.get('gitea-is-dark-theme') === 'true';
@@ -23,7 +34,7 @@ if (isDarkTheme) {
 }
 
 const backgroundColor = url.searchParams.get('gitea-iframe-bgcolor');
-if (backgroundColor) {
+if (isValidCssColor(backgroundColor)) {
   // create a style element to set background color, then it can be overridden by the content page's own style if needed
   const style = document.createElement('style');
   style.textContent = `
@@ -75,3 +86,7 @@ if (iframeId) {
     }
   });
 }
+
+if (window.testModules) {
+  window.testModules.externalRenderHelper = {isValidCssColor};
+}
diff --git a/web_src/js/features/common-page.ts b/web_src/js/features/common-page.ts
index fd37e307f7..65269fab4a 100644
--- a/web_src/js/features/common-page.ts
+++ b/web_src/js/features/common-page.ts
@@ -156,8 +156,8 @@ export function checkAppUrl() {
   if (curUrl.startsWith(appUrl) || `${curUrl}/` === appUrl) {
     return;
   }
-  showGlobalErrorMessage(`Your ROOT_URL in app.ini is "${appUrl}", it's unlikely matching the site you are visiting.
-Mismatched ROOT_URL config causes wrong URL links for web UI/mail content/webhook notification/OAuth2 sign-in.`, 'warning');
+  showGlobalErrorMessage(`The detected web site URL is "${appUrl}", it's unlikely matching the site config.
+Mismatched app.ini ROOT_URL or reverse proxy "Host/X-Forwarded-Proto" config might cause wrong URL links for web UI/mail content/webhook notification/OAuth2 sign-in.`, 'warning');
 }
 
 export function checkAppUrlScheme() {
diff --git a/web_src/js/globals.d.ts b/web_src/js/globals.d.ts
index 9fa2c11e7f..010dd8f584 100644
--- a/web_src/js/globals.d.ts
+++ b/web_src/js/globals.d.ts
@@ -68,6 +68,15 @@ interface Window {
   turnstile: any,
   hcaptcha: any,
 
+  // Make IIFE private functions can be tested in unit tests, without exposing the IIFE module to global scope.
+  // Otherwise, when using "export" in IIFE code, the compiled JS will inject global "var externalRenderHelper = ..."
+  // which is not expected and may cause conflicts with other modules.
+  testModules: {
+    externalRenderHelper?: {
+      isValidCssColor(s: string | null): boolean,
+    }
+  }
+
   // do not add more properties here unless it is a must
 }
 
diff --git a/web_src/js/vitest.setup.ts b/web_src/js/vitest.setup.ts
index a08325bcba..d79b630be8 100644
--- a/web_src/js/vitest.setup.ts
+++ b/web_src/js/vitest.setup.ts
@@ -24,4 +24,6 @@ window.config = {
   i18n: {},
 };
 
+window.testModules = {};
+
 export {}; // mark as module for top-level await