feat(security): add blocked domains and strict TLD validation

- Adds `BLOCKED_DOMAINS` list to reject disposable/invalid email domains.
- Enforces TLD length >= 2 chars in `isValidEmail`.
- Updates tests to cover new validation rules.

Co-authored-by: ragusa-it <196988693+ragusa-it@users.noreply.github.com>

.jules/sentinel.md

@@ -22,3 +22,8 @@
 **Vulnerability:** Standard email validation often allows characters like quotes (`"`) or backticks (`` ` ``) which can be vectors for XSS in HTML attribute contexts.
 **Learning:** While RFCs allow quoted local parts (e.g., `"user"@example.com`), these are rare in practice and pose significant security risks in web applications. It is often safer to explicitly reject them. However, apostrophes (`'`) are common in names (O'Connor) and should be allowed, relying on sanitization (Defense in Depth) rather than validation to handle them safely.
 **Prevention:** Use a hybrid approach: Strict validation (reject quotes/backticks) for high-risk characters, coupled with output sanitization (`sanitizeInput`) for characters that must be allowed but are still risky (apostrophes).
+
+## 2026-02-14 - Blocking Disposable Domains
+**Vulnerability:** Allowing users to register or submit forms with disposable email addresses (e.g., mailinator.com) can lead to spam, abuse, and polluted data.
+**Learning:** While true email verification requires a backend or API, a simple client-side blocklist of common disposable domains is a highly effective, low-cost first line of defense.
+**Prevention:** Maintain a list of known disposable domains (e.g., `BLOCKED_DOMAINS`) and check the domain part of the email address during validation.

src/utils/security.test.ts

@@ -29,14 +29,14 @@ describe('Security Utils', () => {
 
   describe('isValidEmail', () => {
     it('accepts valid email addresses', () => {
-      expect(isValidEmail('test@example.com')).toBe(true);
+      expect(isValidEmail('test@valid-domain.com')).toBe(true);
       expect(isValidEmail('john.doe@sub.domain.co.uk')).toBe(true);
-      expect(isValidEmail('user+tag@example.com')).toBe(true);
+      expect(isValidEmail('user+tag@valid-domain.com')).toBe(true);
     });
 
     it('rejects invalid email formats', () => {
       expect(isValidEmail('plainaddress')).toBe(false);
-      expect(isValidEmail('@example.com')).toBe(false);
+      expect(isValidEmail('@valid-domain.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)
@@ -54,13 +54,31 @@ describe('Security Utils', () => {
     });
 
     it('accepts emails with apostrophes', () => {
-      expect(isValidEmail("user'name@example.com")).toBe(true);
-      expect(isValidEmail("o'connor@example.com")).toBe(true);
+      expect(isValidEmail("user'name@valid-domain.com")).toBe(true);
+      expect(isValidEmail("o'connor@valid-domain.com")).toBe(true);
     });
 
     it('rejects emails with whitespace', () => {
       expect(isValidEmail('user @example.com')).toBe(false);
       expect(isValidEmail('user@ example.com')).toBe(false);
     });
+
+    it('rejects short TLDs (less than 2 chars)', () => {
+      expect(isValidEmail('user@domain.c')).toBe(false);
+      expect(isValidEmail('user@domain.1')).toBe(false);
+    });
+
+    it('rejects blocked/disposable domains', () => {
+      expect(isValidEmail('user@example.com')).toBe(false);
+      expect(isValidEmail('test@test.com')).toBe(false);
+      expect(isValidEmail('spam@mailinator.com')).toBe(false);
+      expect(isValidEmail('bot@yopmail.com')).toBe(false);
+      expect(isValidEmail('temp@temp-mail.org')).toBe(false);
+    });
+
+    it('rejects blocked domains regardless of case', () => {
+      expect(isValidEmail('user@EXAMPLE.COM')).toBe(false);
+      expect(isValidEmail('user@MaIlInAtOr.CoM')).toBe(false);
+    });
   });
 });

src/utils/security.ts

@@ -17,9 +17,22 @@ export function sanitizeInput(input: string): string {
     .replace(/'/g, "'");
 }
 
+// Common disposable email providers and invalid domains
+const BLOCKED_DOMAINS = new Set([
+  "example.com",
+  "test.com",
+  "mailinator.com",
+  "yopmail.com",
+  "temp-mail.org",
+  "guerrillamail.com",
+  "10minutemail.com",
+  "trashmail.com",
+]);
+
 /**
  * Validates an email address format securely.
  * Rejects inputs containing dangerous characters like <, >, or whitespace.
+ * Also validates domain validity (blocked domains, TLD length).
  *
  * @param email - The email string to validate.
  * @returns True if the email is valid and safe, false otherwise.
@@ -30,7 +43,13 @@ export function isValidEmail(email: string): boolean {
   // @         : Literal @
   // [^\s@<>]+ : Domain part - no whitespace, @, <, or >
   // \.        : Literal .
-  // [^\s@<>]+ : TLD part - no whitespace, @, <, or >
-  const emailRegex = /^[^\s@<>,"`]+@[^\s@<>,"`]+\.[^\s@<>,"`]+$/;
-  return emailRegex.test(email);
+  // [^\s@<>]+ : TLD part - no whitespace, @, <, or > (min 2 chars)
+  const emailRegex = /^[^\s@<>,"`]+@[^\s@<>,"`]+\.[^\s@<>,"`]{2,}$/;
+  if (!emailRegex.test(email)) {
+    return false;
+  }
+
+  // Check against blocked domains
+  const domain = email.split("@")[1].toLowerCase();
+  return !BLOCKED_DOMAINS.has(domain);
 }