Implementation of robust data validation in the source code
import play.api.mvc._
import scala.util.matching.Regex
class HomeController @Inject()(val controllerComponents: ControllerComponents) extends BaseController {
def index() = Action { implicit request: Request[AnyContent] =>
val data = request.body.asText.getOrElse("")
val regex = new Regex("([a-z]+)*")
val matches = regex.findAllIn(data).toList
Ok("Matches found: " + matches)
}
}
The above Scala code is a simple Play Framework controller with a single action,
index(), that accepts any request and attempts to extract text from the request body. This extracted text is then processed by a regular expression that matches any string composed of lowercase letters. The resulting matches are returned in the response.
The issue here lies in the regular expression used:
new Regex("([a-z]+)*"). This regular expression is considered dangerous due to its potential for causing catastrophic backtracking. This happens because the regular expression uses complex operations (
+ and
*) that can match the same input in multiple ways.
If an attacker sends a string that doesn't match the regular expression but is close enough to cause the regular expression engine to attempt many different paths, the server can become overloaded with the computational effort and eventually crash. This is known as a Regular Expression Denial of Service (ReDoS) attack.
In this case, a string composed of lowercase letters followed by a single non-lowercase letter could potentially cause this issue. The regular expression engine will attempt to match the entire string, backtrack when it hits the non-lowercase letter, and then attempt to match again. This back and forth can consume significant resources, leading to a server crash.
import play.api.mvc._
import scala.util.matching.Regex
class HomeController @Inject()(val controllerComponents: ControllerComponents) extends BaseController {
def index() = Action { implicit request: Request[AnyContent] =>
val data = request.body.asText.getOrElse("")
val safeData = data.take(1000) // Limit the size of the input data to prevent DoS attacks
val regex = new Regex("^[a-z]+$") // Use a more efficient and secure regular expression pattern
if (regex.pattern.matcher(safeData).matches) {
val matches = regex.findAllIn(safeData).toList
Ok("Matches found: " + matches)
} else {
BadRequest("Invalid input data") // Implement proper error handling
}
}
}
The original code was vulnerable due to the use of a dangerous regular expression that could lead to catastrophic backtracking. This could potentially cause the server to crash if an attacker sends a specific string of data.
The fixed code addresses these issues in the following ways:
1. Avoid using dangerous regular expressions: The regular expression pattern has been changed to
^[a-z]+$, which is more efficient and secure. This pattern only matches strings that consist entirely of lowercase letters, and it does not allow for any complex operations that could lead to catastrophic backtracking.
2. Validate the input data: Before applying the regular expression, the input data is limited to the first 1000 characters. This prevents a potential Denial of Service (DoS) attack where an attacker could send a very large string of data to crash the server.
3. Implement proper error handling: If the input data does not match the regular expression pattern, the server responds with a
BadRequest status code and a message indicating that the input data is invalid. This allows the server to gracefully handle any unexpected input or errors.