diff --git a/spec/PagesRouter.spec.js b/spec/PagesRouter.spec.js
index 4864ceb42d..0d0afbc39e 100644
--- a/spec/PagesRouter.spec.js
+++ b/spec/PagesRouter.spec.js
@@ -1396,15 +1396,31 @@ describe('Pages Router', () => {
       expect(response.text).toContain('<img');
     });
 
-    it('should escape XSS in locale parameter', async () => {
+    it('should reject XSS payload in locale parameter', async () => {
       const xssLocale = '"><svg/onload=alert(1)>';
       const response = await request({
         url: `http://localhost:8378/1/apps/choose_password?locale=${encodeURIComponent(xssLocale)}&appId=test`,
       });
 
       expect(response.status).toBe(200);
+      // Invalid locale is rejected by format validation, so the XSS
+      // payload never reaches the page content
       expect(response.text).not.toContain('<svg/onload=alert(1)>');
-      expect(response.text).toContain('&quot;&gt;&lt;svg');
+      expect(response.text).not.toContain('&quot;&gt;&lt;svg');
+    });
+
+    it('should reject non-ASCII characters in locale parameter', async () => {
+      // Non-ASCII characters like ğ (U+011F) would cause ERR_INVALID_CHAR
+      // when set as HTTP header value if not rejected by locale validation
+      const nonAsciiLocale = 'ğ';
+      const response = await request({
+        url: `http://localhost:8378/1/apps/choose_password?locale=${encodeURIComponent(nonAsciiLocale)}&appId=test`,
+      });
+
+      expect(response.status).toBe(200);
+      // Non-ASCII locale is rejected by format validation;
+      // no ERR_INVALID_CHAR error occurs
+      expect(response.headers['x-parse-page-param-locale']).toBeUndefined();
     });
 
     it('should handle legitimate usernames with quotes correctly', async () => {
diff --git a/src/Routers/PagesRouter.js b/src/Routers/PagesRouter.js
index 7b6ddf8b6c..49c3aab138 100644
--- a/src/Routers/PagesRouter.js
+++ b/src/Routers/PagesRouter.js
@@ -555,6 +555,16 @@ export class PagesRouter extends PromiseRouter {
       (req.body || {})[pageParams.locale] ||
       (req.params || {})[pageParams.locale] ||
       (req.headers || {})[pageParamHeaderPrefix + pageParams.locale];
+
+    // Validate locale format to prevent path traversal and invalid
+    // HTTP header characters; only allow standard locale patterns
+    // like "en", "en-US", "de-AT", "zh-Hans-CN"
+    if (locale !== undefined && typeof locale !== 'string') {
+      return undefined;
+    }
+    if (typeof locale === 'string' && !/^[a-zA-Z]{2,3}(-[a-zA-Z0-9]{2,8})*$/.test(locale)) {
+      return undefined;
+    }
     return locale;
   }