Handling Special Characters and Encoding

Understanding how to handle special characters is crucial for preventing XSS while maintaining functionality. Some applications need to accept and display special characters for legitimate purposes – mathematical formulas, code snippets, or international characters. The key is distinguishing between data and code, ensuring special characters in data aren't interpreted as code.

Implement proper Unicode handling throughout your application. Normalize Unicode input to prevent homograph attacks where visually similar characters are used to bypass validation. Be aware of different Unicode normalization forms (NFC, NFD, NFKC, NFKD) and choose appropriately for your use case. Consider right-to-left override characters and other Unicode features that can be abused for spoofing attacks.

When storing data, maintain it in its original form rather than storing encoded versions. This preserves data integrity and allows you to apply context-appropriate encoding when displaying it. Storing HTML-encoded data can lead to double-encoding issues and makes it difficult to use the data in different contexts. Instead, store raw data and encode it appropriately at output time.