Cross site scripting (or XSS) is #2 on the 2010 OWASP Top Ten web security risks and it pops up nearly everywhere.

XSS occurs when a user-manipulatable value is displayed on a web page without escaping it, allowing someone to inject Javascript or HTML into the page.

In Rails 2.x, values need to be explicitly escaped (e.g., by using the h method). In Rails 3.x, auto-escaping in views is enabled by default. However, one can still use the raw method to output a value directly.

See the Ruby Security Guide for more details.

Query Parameters and Cookies

Rails 2.x example in ERB:

<%= params[:query] %>

Brakeman looks for several situations that can allow XSS. The simplest is like the example above: a value from the params or cookies is being directly output to a view. In such cases, it will issue a warning like:

Unescaped parameter value near line 3: params[:query]

By default, Brakeman will also warn when a parameter or cookie value is used as an argument to a method, the result of which is output unescaped to a view.

For example:

<%= some_method(cookie[:name]) %>

This raises a warning like:

Unescaped cookie value near line 5: some_method(cookies[:oreo])

However, the confidence level for this warning will be weak, because it is not directly outputting the cookie value.

Some methods are known to Brakeman to either be dangerous (link_to is one) or safe (escape_once). Users can specify safe methods using the --safe-methods option. Alternatively, Brakeman can be set to only warn when values are used directly with the --report-direct option.

Model Attributes

Because (many) models come from database values, Brakeman mistrusts them by default.

For example, if @user is an instance of a model set in an action like

def set_user
  @user = User.first
end

and there is a view with

<%= @user.name %>

Brakeman will raise a warning like

Unescaped model attribute near line 3: User.first.name

If you trust all your data (although you probably shouldn’t), this can be disabled with --ignore-model-output.

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