URL encoding, also known as percent-encoding, is a fundamental mechanism for converting characters into a format that can be safely transmitted over the internet within Uniform Resource Locators (URLs). This process replaces unsafe or reserved characters with a percent sign (%) followed by their two-digit hexadecimal representation.

How URL Encoding Works

When data is transmitted via URLs, certain characters have special meanings or are not permitted. URL encoding addresses this by transforming these characters into a universally recognized format. For example, a space character becomes %20, an ampersand (&) becomes %26, and a question mark (?) becomes %3F.

Characters That Require Encoding

• Reserved characters: Characters with special URL meanings, such as ?, &, =, /, and #
• Unsafe characters: Spaces, quotation marks, and angle brackets
• Non-ASCII characters: International characters and special symbols

Security Implications

URL encoding serves as a critical security control in application security. Proper implementation helps prevent several attack vectors:

• Cross-Site Scripting (XSS): Prevents malicious scripts from being injected through URL parameters
• SQL Injection: Blocks specially crafted queries embedded in URLs
• Path Traversal: Stops attackers from manipulating file paths using encoded sequences

Best Practices

For robust security, applications should consistently encode user-supplied data before incorporating it into URLs, decode data only when necessary, and implement proper input validation alongside encoding mechanisms. This ensures data integrity and protects against exploitation attempts that rely on improper character handling.