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Lack of data validation - Headers


Implementation of proper data validation for HTTP headers


  • Usage of Ruby 2.5.0 for Ruby programming and development
  • Usage of Ruby for building dynamic and object-oriented applications
  • Usage of Rails framework for building web applications


Non compliant code

class ApplicationController < ActionController::Base
def show
@user = User.find(params[:id])
response.headers["X-Username"] = @user.username

In the above code snippet, the show method in the ApplicationController is retrieving a user's information based on the id parameter passed in the request. It then sets the X-Username header to the username of the found user.

The vulnerability lies in the fact that there is no validation being performed on the @user.username before it's being set in the header. This lack of validation means that an attacker could potentially manipulate the username to include malicious scripts or commands, leading to a Cross-Site Scripting (XSS) attack or other security breaches.

For instance, if the username field in the database is not properly sanitized and allows for special characters, an attacker could inject scripts or commands like "><script>malicious_code_here</script> or "; rm -rf /*. When this username is used and set in the X-Username header, the injected code could be executed leading to potential security risks.


  • Implement input validation for the 'X-Username' header to ensure it only contains safe characters.
  • Use appropriate encoding or escaping techniques to prevent any potential injection attacks.
  • Consider using a whitelist approach to validate the 'X-Username' header value against a predefined set of allowed characters or patterns.
  • Sanitize the 'X-Username' header value before using it to prevent any potential XSS attacks.
  • Regularly update and patch the Ruby on Rails framework to benefit from the latest security enhancements and fixes.

Compliant code

class ApplicationController < ActionController::Base
def show
@user = User.find(params[:id])
username = sanitize(@user.username)
response.headers["X-Username"] = username if valid_username?(username)


def sanitize(input)

def valid_username?(username)
# Define a regex for a valid username
valid_username_regex = /\\A[a-zA-Z0-9_]*\\z/

The updated code includes two new methods: sanitize and valid_username?.

The sanitize method uses the sanitize helper method provided by the ActionController::Base.helpers module to sanitize the username. This method strips out all HTML tags and attributes, ensuring that no malicious scripts can be injected via the username.

The valid_username? method checks if the username matches a predefined regular expression (valid_username_regex). This regular expression only allows alphanumeric characters and underscores, which are typically safe and commonly used in usernames. This is a whitelist approach to input validation, which is generally more secure than a blacklist approach.

In the show method, the username is first sanitized and then validated. The X-Username header is only set if the username is valid. This ensures that only safe, valid usernames are included in the HTTP headers, preventing potential XSS attacks and preserving the integrity of the stored information.

Remember to regularly update and patch the Ruby on Rails framework to benefit from the latest security enhancements and fixes.