Using Referer Field for Authentication

Background

The referer field in HTML requests can be simply modified by malicious users, rendering it useless as a means of checking the validity of the request in question.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

The following code samples check a packet's referer in order to decide whether or not an inbound request is from a trusted host.

String trustedReferer = "http://www.example.com/"
while(true){
  n = read(newsock, buffer, BUFSIZE);
  requestPacket = processPacket(buffer, n);
  if (requestPacket.referer == trustedReferer){
    openNewSecureSession(requestPacket);
  }
}

boolean processConnectionRequest(HttpServletRequest request){
  String referer = request.getHeader("referer")
  String trustedReferer = "http://www.example.com/"
  if(referer.equals(trustedReferer)){
    openPrivilegedConnection(request);
    return true;
  }
  else{
    sendPrivilegeError(request);
    return false;
  }
}

These examples check if a request is from a trusted referer before responding to a request, but the code only verifies the referer name as stored in the request packet. An attacker can spoof the referer, thus impersonating a trusted client.

See Also