diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index a98cbd0..4b69a9a 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -7,3 +7,13 @@
 **Vulnerability:** Contact forms using client-side services (like EmailJS) without backend middleware are vulnerable to spam and quota exhaustion.
 **Learning:** While true rate limiting requires a backend, client-side throttling via `localStorage` provides a necessary friction layer for legitimate users and simple bots, protecting external service quotas.
 **Prevention:** Implement reusable rate-limit hooks for all public-facing form submissions in static/serverless applications.
+
+## 2026-02-13 - State Leakage in Tests masking Security Failures
+**Vulnerability:** Flaky tests caused by `localStorage` state leakage (e.g. rate limits persisting between tests) can prevent security features from being properly verified, leading to false negatives or untested paths.
+**Learning:** Global state like `localStorage` must be explicitly cleared in `afterEach` blocks in test environments (jsdom). Failing to do so can cause subsequent tests to fail or behave unpredictably, especially for rate-limiting logic.
+**Prevention:** Always include `localStorage.clear()` in `afterEach` (or `beforeEach`) when testing components that rely on local storage.
+
+## 2026-02-13 - Strict Email Validation vs HTML5 Validation
+**Vulnerability:** Standard email regexes and HTML5 validation are often too permissive, allowing XSS vectors (like `<script>`) in email fields if not properly sanitized/rejected.
+**Learning:** While HTML5 browsers block some invalid emails, relying solely on them is insufficient for defense-in-depth. Application-level validation should explicitly reject dangerous characters (`<`, `>`) to prevent stored XSS or injection if the data is processed by less-secure backends.
+**Prevention:** Implement strict, reusable validation functions (`isValidEmail`) that reject XSS vectors, and ensure tests verify this logic by bypassing browser validation if necessary.
diff --git a/src/pages/Contact.tsx b/src/pages/Contact.tsx
index d11befb..708d7c8 100644
--- a/src/pages/Contact.tsx
+++ b/src/pages/Contact.tsx
@@ -5,7 +5,7 @@ import { useTranslation } from "../i18n";
 import { useRateLimit } from "../hooks";
 import { config } from "../config";
 import { Button, Input, Textarea } from "../components/ui";
-import { sanitizeInput } from "../utils/security";
+import { sanitizeInput, isValidEmail } from "../utils/security";
 import styles from "./Contact.module.css";
 
 const NAME_MAX_LENGTH = 100;
@@ -53,7 +53,7 @@ export function Contact() {
 
     if (!formData.email.trim()) {
       newErrors.email = "Required";
-    } else if (!/^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(formData.email)) {
+    } else if (!isValidEmail(formData.email)) {
       newErrors.email = "Invalid email";
     }
 
diff --git a/src/pages/__tests__/Contact.test.tsx b/src/pages/__tests__/Contact.test.tsx
index 45780de..6e9a170 100644
--- a/src/pages/__tests__/Contact.test.tsx
+++ b/src/pages/__tests__/Contact.test.tsx
@@ -63,6 +63,7 @@ describe('Contact Page', () => {
   afterEach(() => {
     cleanup();
     document.body.innerHTML = '';
+    localStorage.clear();
   });
 
   it('submits the form with correct parameters', async () => {
@@ -170,4 +171,26 @@ describe('Contact Page', () => {
     // EmailJS should NOT be called
     expect(emailjs.send).not.toHaveBeenCalled();
   });
+
+  it('shows error when email contains invalid characters', async () => {
+    const { container } = render(<Contact />);
+
+    // Fill out the form with invalid email (XSS vector)
+    fireEvent.change(screen.getByLabelText('Name'), { target: { value: 'John Doe' } });
+    fireEvent.change(screen.getByLabelText('Email'), { target: { value: '<script>@example.com' } });
+    fireEvent.change(screen.getByLabelText('Subject'), { target: { value: 'Test Subject' } });
+    fireEvent.change(screen.getByLabelText('Message'), { target: { value: 'Hello world' } });
+
+    // Submit via form submit event to bypass browser validation (jsdom/browser would block this otherwise)
+    // This ensures our application-level validation logic (isValidEmail) is tested
+    const form = container.querySelector('form');
+    if (form) fireEvent.submit(form);
+
+    // Validation error should appear
+    const errorMessage = await screen.findByText('Invalid email');
+    expect(errorMessage).toBeTruthy();
+
+    // EmailJS should NOT be called
+    expect(emailjs.send).not.toHaveBeenCalled();
+  });
 });
diff --git a/src/utils/security.test.ts b/src/utils/security.test.ts
new file mode 100644
index 0000000..bdcaab9
--- /dev/null
+++ b/src/utils/security.test.ts
@@ -0,0 +1,56 @@
+
+// @vitest-environment jsdom
+import { describe, it, expect } from 'vitest';
+import { sanitizeInput, isValidEmail } from './security';
+
+describe('Security Utils', () => {
+  describe('sanitizeInput', () => {
+    it('escapes special HTML characters', () => {
+      expect(sanitizeInput('<script>')).toBe('&lt;script&gt;');
+      expect(sanitizeInput('foo & bar')).toBe('foo &amp; bar');
+      expect(sanitizeInput('"quotes"')).toBe('&quot;quotes&quot;');
+      expect(sanitizeInput("'single quotes'")).toBe('&#039;single quotes&#039;');
+      expect(sanitizeInput('>')).toBe('&gt;');
+    });
+
+    it('returns non-string input as is', () => {
+      // @ts-ignore
+      expect(sanitizeInput(123)).toBe(123);
+      // @ts-ignore
+      expect(sanitizeInput(null)).toBe(null);
+    });
+
+    it('handles mixed content correctly', () => {
+      const input = '<script>alert("XSS")</script>';
+      const expected = '&lt;script&gt;alert(&quot;XSS&quot;)&lt;/script&gt;';
+      expect(sanitizeInput(input)).toBe(expected);
+    });
+  });
+
+  describe('isValidEmail', () => {
+    it('accepts valid email addresses', () => {
+      expect(isValidEmail('test@example.com')).toBe(true);
+      expect(isValidEmail('john.doe@sub.domain.co.uk')).toBe(true);
+      expect(isValidEmail('user+tag@example.com')).toBe(true);
+    });
+
+    it('rejects invalid email formats', () => {
+      expect(isValidEmail('plainaddress')).toBe(false);
+      expect(isValidEmail('@example.com')).toBe(false);
+      expect(isValidEmail('user@')).toBe(false);
+      expect(isValidEmail('user@.com')).toBe(false);
+      expect(isValidEmail('user@com')).toBe(false); // Missing dot in domain part (simple regex might allow, but strict one requires dot)
+    });
+
+    it('rejects emails with dangerous characters (<, >)', () => {
+      expect(isValidEmail('<script>@example.com')).toBe(false);
+      expect(isValidEmail('user@<script>.com')).toBe(false);
+      expect(isValidEmail('user<name>@example.com')).toBe(false);
+    });
+
+    it('rejects emails with whitespace', () => {
+      expect(isValidEmail('user @example.com')).toBe(false);
+      expect(isValidEmail('user@ example.com')).toBe(false);
+    });
+  });
+});
diff --git a/src/utils/security.ts b/src/utils/security.ts
index 3d04c49..2ea9768 100644
--- a/src/utils/security.ts
+++ b/src/utils/security.ts
@@ -16,3 +16,21 @@ export function sanitizeInput(input: string): string {
     .replace(/"/g, "&quot;")
     .replace(/'/g, "&#039;");
 }
+
+/**
+ * Validates an email address format securely.
+ * Rejects inputs containing dangerous characters like <, >, or whitespace.
+ *
+ * @param email - The email string to validate.
+ * @returns True if the email is valid and safe, false otherwise.
+ */
+export function isValidEmail(email: string): boolean {
+  // Basic format check + rejection of XSS vectors (<, >)
+  // [^\s@<>]+ : Local part - no whitespace, @, <, or >
+  // @         : Literal @
+  // [^\s@<>]+ : Domain part - no whitespace, @, <, or >
+  // \.        : Literal .
+  // [^\s@<>]+ : TLD part - no whitespace, @, <, or >
+  const emailRegex = /^[^\s@<>]+@[^\s@<>]+\.[^\s@<>]+$/;
+  return emailRegex.test(email);
+}