Beware of html_safe in Rails !!!

Software Engineer

Ruby on Rails

2024-08-26 19:27:07 +0700 · 2 mins read

Modern web frameworks provide a secure way to develop new applications. However, if we do not understand it well, it still leads to security issues.
html_safe in Rails is an example. It seems "safe", but is it safe?

Understanding html_safe

Ref: https://api.rubyonrails.org/classes/String.html#method-i-html_safe

In Rails, the html_safe method is used to mark a string as safe for HTML output. By default, Rails escapes HTML to prevent injection attacks. When you use html_safe on a string, you are telling Rails that the string is safe to insert into HTML without escaping.

That DOESN'T mean html_safe will make the parsed HTML safe for Cross-Site Scripting (XSS). It's equivalent to a raw helper in the view

For example:

// language: ruby
<%= "<h1>Hello, World!</h1>".html_safe %>

This will render as:

// language: markup
<h1>Hello, World!</h1>

Without `html_safe`, the output would be escaped:

// language: markup
&lt;h1&gt;Hello, World!&lt;/h1&gt;

The Danger of html_safe

The primary risk of using html_safe is that it can inadvertently introduce XSS vulnerabilities. XSS occurs when an attacker can inject malicious scripts into a web page, potentially leading to data theft, session hijacking, or other malicious activities.

Consider the following scenario: you receive user input and mark it as html_safe:

// language: ruby
<%= params[:user_input].html_safe %>

If an attacker submits the following input:

// language: markup
<script>alert('Hacked!');</script>

The output will be:

// language: markup
<script>alert('Hacked!');</script>

This script will execute in the user's browser, leading to an XSS attack.

Best Practices and Safer Alternatives

To avoid the pitfalls of html_safe, consider the following best practices:

Avoid Using html_safe on user input: Never mark user-generated content as html_safe. Always assume that user input could be malicious and must be sanitized.
Sanitize User Input: If you must allow some HTML content from user input, use the sanitize helper to remove potentially dangerous tags and attributes:

// language: ruby
<%= sanitize(params[:user_input]) %>

Rails' sanitizing method allows a whitelist of safe tags and attributes, reducing the risk of XSS attacks. Ref: https://api.rubyonrails.org/classes/ActionView/Helpers/SanitizeHelper.html

Use Rubocop to follow Rails best practices and Brakeman to find security vulnerabilities. No one can know everything, let these tools help you.

Happy coding!

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