Prevention of server-side template injection attacks
import play.api.mvc._
import play.twirl.api.Html
class ApplicationController extends Controller {
def renderTemplate(userInput: String) = Action {
val template = s"Welcome, $userInput!
"
Ok(Html(template))
}
}
The above Scala code represents a simple Play Framework application with a single action
renderTemplate
that takes a user input string, interpolates it into an HTML template, and then serves this template as a response.
The vulnerability lies in the fact that the user input is directly interpolated into the HTML template without any sanitization or escaping. This allows an attacker to inject arbitrary HTML or script code into the template. This code will then be executed when the template is served and rendered in the user's browser.
For example, an attacker could provide the following input:
Welcome, !
import play.api.mvc._
import play.twirl.api.Html
import org.apache.commons.lang3.StringEscapeUtils
class ApplicationController extends Controller {
def renderTemplate(userInput: String) = Action {
val sanitizedInput = StringEscapeUtils.escapeHtml4(userInput)
val template = s"Welcome, $sanitizedInput!
"
Ok(Html(template))
}
}
The updated code now includes a step to sanitize the user input before it is used in the template. This is done using the
escapeHtml4
method from the Apache Commons Lang library, which will escape any HTML characters in the user input. This prevents any potential HTML injection attacks, as any HTML tags in the user input will be treated as literal text rather than being interpreted as HTML by the browser.
The
escapeHtml4
method will replace characters like
<
and
>
with their HTML escaped equivalents,
<
and
>
respectively. This means that if a user tries to input something like
<script>malicious code</script>
, it will be rendered in the browser as
<script>malicious code</script>
, which will be displayed as literal text rather than being executed as a script.
This is a simple and effective way to prevent server-side template injection attacks, as it ensures that user input cannot be used to inject malicious code into the template. However, it's important to note that this is just one layer of defense, and other security measures should also be implemented, such as using parameterized templates, implementing a strict content security policy, regularly updating dependencies, and conducting regular security testing.