Input validation and output encoding are two of the most important defenses you can use to protect your applications from security threats.

What is input validation?

Input validation is the process of checking any data that users send to your application. This includes form fields, URLs, API requests, and even data from other systems. You use input validation to make sure that the data is what you expect — for example, that a user’s age is a number, or that an email address has the right format. If the data does not meet your rules, you reject it or handle it safely.

What is output encoding?

Output encoding is the process of converting data before you display it or send it somewhere else, such as a web page or a database. This step ensures that any special characters in the data do not get interpreted as code. For instance, if a user enters <script>, output encoding changes it so that it will not run as code in a browser.

Why are these practices essential for security?

Without input validation and output encoding, attackers can send harmful data to your application. This can lead to serious attacks, such as:

• Injection attacks; for example, SQL injection, where attackers trick your database into running their commands.
• Cross-site scripting (XSS); where attackers inject malicious scripts into web pages viewed by other users.
• Application crashes or unexpected behavior; caused by unexpected or malformed data.

By validating input and encoding output, you stop attackers from slipping malicious data through your application. These practices form the foundation of secure application development and help keep your users and data safe.

Best Practices for Validating User Input and Encoding Output

Validating User Input

• Accept only the data you expect; reject everything else;
• Use allowlists (permitted values) instead of blocklists (forbidden values);
• Check input type, length, format, and range before processing;
• Validate input on the server side, even if you also check it on the client side;
• Sanitize input by removing or escaping dangerous characters;
• Never trust data from external sources, including users and APIs.

Safely Encoding Output

• Encode all output before displaying it in a web page or application;
• Use context-specific encoding (HTML, JavaScript, URL, etc.) based on where the data will appear;
• Always encode user-generated content to prevent cross-site scripting (XSS) attacks;
• Avoid constructing output using string concatenation with untrusted data;
• Use built-in encoding functions provided by your framework or language whenever possible.

By following these practices, you reduce the risk of security vulnerabilities such as injection attacks and ensure your application handles user data safely.