html5 pattern versus htmlspecialchars and related questions



Do I need to sanitize data coming from an input element (e.g., text) that has a pattern attribute that blocks script tags, like < and > and quote characters? I am not seeing any reference on the web relating using the html5 pattern attribute to intercept cross site scripting injection attacks.

What about data coming from an input element (e.g., text) that is headed to a database column that's only 11 characters wide? It would seem no matter was injected, 11 characters wouldn't be enough to do anything other than replace the valid data that would have gone there?

If data is coming back from the database, do I need to re-sanitize it? If the anwer is yes, then it should it apply to *every* value= ? even ones with the pattern attribute blocking as I indicated above.
Never trust user input.

Always validate it. (make sure you get what you expect, date, name, number etc..)

Setting HTML attributes cannot make sure you will be sent correct data.

No version of HTML will stop cross site scripting, it cannot.

There is a lot of regurgitated rubbish on the internet along with some good advice so you'll have to do a lot of reading to understand how your site can be attacked and how to prevent it - understand is the key word. htmlspecialchars and using prepared statements when storing data in you database is a start, but never assume that you are completely safe, things change.
When it comes to sanitizing and validating user input, it's important to consider multiple layers of security. Let's answer your specific questions:
Purge data from input elements using the pattern attribute.
The HTML5 pattern attribute is primarily used for client-side validation and by itself provides no protection against cross-site scripting (XSS) attacks. This helps ensure that user input conforms to certain patterns, but it does not sanitize or validate the data against potential security threats. Therefore, server-side validation and sanitization of user input should be performed to prevent XSS attacks and other security vulnerabilities.
Data stored in narrow database columns:
Even with finite-width database columns, it is important to properly clean and validate data before saving it. Limiting column width alone does not prevent all types of attacks. Attackers may still attempt to exploit vulnerabilities or manipulate data within available storage space. Always clean and validate user input according to your specific needs and expected data format, regardless of column width.
Clean up the data retrieved again from the database.
When viewing data retrieved from a database, you should consider the context in which it will be used. When sending data directly into HTML content, such as within HTML tags and attributes, it is important to properly sanitize the data to prevent XSS attacks. Even if the data was previously sanitized during storage, it is recommended to re-sanitize the data on output to avoid introducing unintended vulnerabilities.
Regarding your question about re-sanitizing the value if the pattern attribute blocks certain characters, it's important to note that sanitization and validation should be performed independently of client-side constraints. The HTML5 pattern attribute is useful for directing user input, but should not be used solely for security reasons. Always perform server-side sanitization and validation to ensure data integrity and security.
In summary, it is important to implement both client-side and server-side validation and sanitization techniques to protect against various types of attacks. Client-side validation ensures a better user experience by detecting errors early. However, server-side validation is essential to ensure security and prevent malicious input.