OWASP - Input Validation Cheat Sheet

Introduction

• The OWASP Input Validation Cheat Sheet provides comprehensive guidelines on how to validate and sanitize inputs effectively to prevent security vulnerabilities.
• Input validation is a critical part of securing applications, as improper handling of user inputs can lead to various types of attacks, including SQL injection, cross-site scripting (XSS), and buffer overflows.

Key Principles of Input Validation

1. Accept Known Good (Whitelist):
   • Define and accept only known good input.
   • This is often done using whitelists of acceptable input values or patterns.
2. Reject Known Bad (Blacklist):
   • While less preferred, blacklisting involves defining patterns or values that are known to be harmful and rejecting inputs that match these patterns.
3. Sanitize Input:
   • Sanitize input by removing or encoding characters that can be potentially harmful.
   • This helps mitigate injection attacks and other vulnerabilities.
4. Canonicalization:
   • Normalize inputs to a standard format before validation. This ensures that equivalent forms of input are consistently validated.

Best Practices for Input Validation

1. Use Built-in Validation Functions:
   • Leverage built-in validation functions provided by programming languages or frameworks whenever possible.
2. Validate Inputs on Both Client and Server Side:
   • Perform input validation on both client-side (for usability) and server-side (for security). Never rely solely on client-side validation.
3. Define a Clear Set of Input Requirements:
   • Specify and document the expected format, length, type, and range of all inputs.
4. Use Strong Data Typing:
   • Use the correct data types for inputs (e.g., integers, dates) to enforce proper validation at the language or framework level.
5. Limit Input Length:
   • Define and enforce maximum length constraints for all inputs to prevent buffer overflows and denial-of-service attacks.
6. Encode Outputs:
   • Encode outputs to ensure that any potentially harmful characters are rendered harmless.
   • This is particularly important for web applications to prevent XSS attacks.

Examples of Input Validation

• Example in Python ```python import re

def is_valid_username(username): # Define a regex pattern for a valid username (alphanumeric, 3-20 characters) pattern = re.compile(r’^[a-zA-Z0-9]{3,20}$’) return bool(pattern.match(username))

Example usage

username = “validUser123” if is_valid_username(username): print(“Username is valid”) else: print(“Username is invalid”)

- Example in JavaScript (Client-side)
```javascript
function isValidEmail(email) {
  // Define a regex pattern for a valid email address
  var pattern = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/;
  return pattern.test(email);
  // Example usage
  var email = "user@example.com";
  if (isValidEmail(email)) {
  console.log("Email is valid");
  } else {
  console.log("Email is invalid");
}
}

OWASP Input Validation Cheat Sheet Summary

1. Input Validation Strategy:
   • Prefer whitelisting over blacklisting.
   • Perform canonicalization before validation.
2. Data Validation:
   • Validate input length, format, type, and range.
   • Use regular expressions for complex patterns.
   • Implement context-specific validation (e.g., email, URL).
3. Error Handling:
   • Provide clear error messages for validation failures without revealing sensitive information.
4. Output Encoding:
   • Encode outputs to prevent injection attacks, such as XSS.
5. Regular Reviews and Updates:
   • Regularly review and update validation rules and patterns to address new security threats.