Reliance on IP Address for Authentication

Description

IP addresses can be easily spoofed. Attackers can forge the source IP address of the packets they send, but response packets will return to the forged IP address. To see the response packets, the attacker has to sniff the traffic between the victim machine and the forged IP address. In order to accomplish the required sniffing, attackers typically attempt to locate themselves on the same subnet as the victim machine. Attackers may be able to circumvent this requirement by using source routing, but source routing is disabled across much of the Internet today. In summary, IP address verification can be a useful part of an authentication scheme, but it should not be the single factor required for authentication.

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

Both of these examples check if a request is from a trusted address before responding to the request.

sd = socket(AF_INET, SOCK_DGRAM, 0);
serv.sin_family = AF_INET;
serv.sin_addr.s_addr = htonl(INADDR_ANY);
servr.sin_port = htons(1008);
bind(sd, (struct sockaddr *) & serv, sizeof(serv));

while (1) {
  memset(msg, 0x0, MAX_MSG);
  clilen = sizeof(cli);
  if (inet_ntoa(cli.sin_addr)==getTrustedAddress()) {
    n = recvfrom(sd, msg, MAX_MSG, 0, (struct sockaddr *) & cli, &clilen);
  }
}

while(true) {

  DatagramPacket rp=new DatagramPacket(rData,rData.length);
  outSock.receive(rp);
  String in = new String(p.getData(),0, rp.getLength());
  InetAddress clientIPAddress = rp.getAddress();
  int port = rp.getPort();

  if (isTrustedAddress(clientIPAddress) & secretKey.equals(in)) {
    out = secret.getBytes();
    DatagramPacket sp =new DatagramPacket(out,out.length, IPAddress, port); outSock.send(sp);
  }

}

The code only verifies the address as stored in the request packet. An attacker can spoof this address, thus impersonating a trusted client.

See Also