Sanitizer

Introduction

This is a middleware that addresses cross-site scripting concerns. It encodes the header and body according to the configuration. As body encoding depends on Body middleware, it has to be plugged into the request/response chain after the body handler.

Configuration

Here is the default configuration file sanitizer.yml

# Sanitize request for cross site scripting during runtime

# indicate if sanitizer is enabled or not
enabled: true

# if it is enabled, does body need to be sanitized
sanitizeBody: true

# if it is enabled, does header need to be sanitized
sanitizeHeader: false

If the enabled flag is false, this middleware won’t be loaded during server startup.

sanitizeBody and sanitizeHeader control if the body and/or the header need to be sanitized. In most of the cases, sanitizing the body makes sense and sanitizing the header is not necessary.

When to use Sanitizer

This handler should only be used when you are collecting user input from Web/Mobile UI and later on, using the input data to generate web pages, for example, a forum or blog application.

For services where user input will never be used to construct web pages, don’t use this handler.

Query Parameters

In other platforms especially JEE containers, query parameters need to be sanitized as well. However, I have found that Undertow does sanitize special characters in query parameters. This is why this handler doesn’t do anything about query parameters.

Encode Library

The library used for cross-site scripting sanitization is from https://www.owasp.org/index.php/Cross-site_Scripting_(XSS) and the library can be found at https://github.com/OWASP/owasp-java-encoder

Encode Level

The encoding level we are using for both header and body is forJavaScriptSource. It gives us a certain level of confidence, and it won’t mess up the header and body in most cases.