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Reflected cross-site scripting (XSS)


Implementation of input validation and output encoding to prevent the execution of malicious scripts in user-generated content.


  • Usage of Ruby 2.7 for Ruby programming and development
  • Usage of Rails framework for building web applications


Non compliant code

def show
@user = User.find(params[:id])
@message = params[:message]

The above code snippet is an action method from a controller in a Ruby on Rails application. This method is vulnerable to Reflected Cross-Site Scripting (XSS) attacks.

Here, the application is taking user input directly through the params[:message] and storing it in @message variable without any validation or sanitization. The @message variable is then likely used somewhere in the view, which is sent back to the user's browser.

If an attacker manipulates the 'message' parameter in the HTTP request to include malicious JavaScript code, the server will reflect this script back in the HTTP response. When this response is rendered by the victim's browser, the malicious script will be executed, leading to an XSS attack.

For instance, an attacker could send a URL like<script>malicious_code_here</script> to a victim. When the victim clicks on this link, the malicious script would be executed in their browser under the security context of

This vulnerability can lead to various malicious activities, such as stealing session cookies, performing actions on behalf of the user, or even delivering malware.


  • Implement input validation and sanitization to ensure that the 'message' parameter does not contain any malicious code.
  • Use appropriate encoding or escaping techniques when displaying the 'message' parameter in the web content.
  • Consider using a Content Security Policy (CSP) to restrict the types of content that can be loaded by the web application.
  • Regularly update and patch the Ruby on Rails framework to benefit from the latest security fixes and enhancements.

Compliant code

def show
@user = User.find(params[:id])
@message = sanitize(params[:message])

The updated code now includes a call to the sanitize method for the 'message' parameter. This method is provided by the Ruby on Rails framework and it helps to prevent cross-site scripting (XSS) attacks by ensuring that any potentially harmful HTML tags are escaped before they are included in the web content.

The sanitize method works by parsing the input string and removing any tags that are not included in a whitelist of allowed tags. This means that even if a user tries to include malicious code in the 'message' parameter, this code will be safely escaped and will not be executed by the browser.

In addition to this, it's also recommended to implement a Content Security Policy (CSP) for the web application. A CSP is a security measure that helps to prevent a variety of attacks, including XSS attacks, by specifying the domains that the browser should consider to be valid sources of executable scripts.

Finally, it's important to regularly update and patch the Ruby on Rails framework to benefit from the latest security fixes and enhancements. This can help to protect the web application from known vulnerabilities that could be exploited by attackers.