How it the illegal user-agent header used?

Akka Libraries
1

How it the illegal user-agent header used?

This topic is based on a conversation between an anonymous user of Akka and our “Ask AI about Akka” bot in the documentation.

2

The User-Agent header in HTTP requests can sometimes contain illegal or malformed values. Akka HTTP has a mechanism to handle such scenarios. If a request contains a User-Agent header with deeply nested comments, Akka HTTP may fail with a stack overflow in the parser. Stack overflows are handled as fatal errors in Akka leading to a complete shutdown of the application. This vulnerability is documented in the Akka HTTP security advisory CVE-2021-42697.

To prevent certain types of attack, it is not recommended to include arbitrary invalid user input in the response. However, sometimes it can be useful to include it in the exception and logging for diagnostic reasons. In such cases, you can use exceptions that extend ExceptionWithErrorInfo, such as IllegalHeaderException. Here is an example in Scala:

import akka.http.scaladsl.model.IllegalHeaderException

val route = get {
  throw IllegalHeaderException("Value of header Foo was illegal", "Found illegal value \"<script>alert('evil_xss_or_xsrf_reflection')</script>\"")
}


// Test:
Get("/") ~> route ~> check {
  responseAs[String] should include("header Foo was illegal")
  responseAs[String] shouldNot include("evil_xss_or_xsrf_reflection")
}

This code will throw an IllegalHeaderException when the route is accessed, and the exception message will include the illegal header value for diagnostic purposes. However, the response sent to the client will not include the illegal value, preventing potential security issues. This is documented in the Akka HTTP Exception Handling documentation.

3

What header to use to force illegal user-agent by using a header with the withheaders method?

4

The respondWithHeaders method in Akka HTTP allows you to add custom headers to the HTTP response. If you want to simulate an illegal User-Agent header, you can create a custom HttpHeader with the name “User-Agent” and an illegal value. Here is an example in Scala:

import akka.http.scaladsl.model.HttpHeader
import akka.http.scaladsl.model.headers.RawHeader

val illegalUserAgent: HttpHeader = RawHeader("User-Agent", "<illegal value>")
val route = respondWithHeaders(illegalUserAgent) {
  complete("Hello, world!")
}

In this example, the RawHeader function is used to create a new HttpHeader with the name “User-Agent” and an illegal value. The respondWithHeaders directive is then used to add this header to the response for the route.

Please note that this is just an example and the actual illegal value for the User-Agent header would depend on the specific requirements or constraints of your application or the HTTP specification.

You can find more information about the respondWithHeaders method in the Akka HTTP documentation.