Detect Replay Attacks In to Your PHP Web Service

Replay attack is a common kind of attack, the hackers are using to break the security of a web service.

If you can intercept one soap message while it is transferring through the wire, you can replay that message to the server again and again. Since the original message may have already encrypted, signed and contain valid authentication credentials the replaying messages will also be able to pass all the security tests and fool the server and do enormous damages to the business. For an example think of replying a soap message that a client is conducting a payment with a e-commerce service. The service may tend to charge the client multiple times for each request which break the integrity of the business.

So it is no doubt when ever you thinking of designing an enterprise web service application, you should give attention to ‘Replay Detection’ more seriously.

One solution is you can handle the replay detection inside the business logic itself. If you do this for the above mentioned e-commerce like services, you will keep all the session ids and make sure only one payment is possible for one session. But this may need some really careful design of the application logic.

The other solution is to let your web service framework to handle the ‘Replay Detection’. That will clearly separate the security aspects from the business aspects of your service. And it will give you more flexibility in configuring your security requirements. And the other advantage is it will detect ‘Replay Attacks’ well before hitting the business logic, making the web service perform well.

WSF/PHP allows you to detect replay attacks using WS-Addressing and WS-Username token headers. WS-Addressing headers contains a message id which can be considered as unique to a soap message and ws-security headers contains created time of the message which can be used to calculate the age of the message and derive its validity.

WSF/PHP provide web service developer a callback with the ‘message id’ and the ‘message created time’  per each message. In the callback you can store this message id and created time in a database, and check them against all the incoming soap messages. If it found duplicate entries, you can consider it as a replay attack.

Here is an example draft of the above scenario written using WSF/PHP.

/* replay detection callback */
function replay_detect_callback($msg_id, $time_created) {
    /* Here is the pseudo code of the logic

    query for the $msg_id and $time_created for the database.
    if already exist
      return FALSE;
    else
      Insert message id and time created to the database
    return TRUE */
}

$security_token = new WSSecurityToken(array( 
                      "replayDetectionCallback" => "replay_detect_callback",
                      "enableReplayDetect" => TRUE,
                      /* Other tokens */ ));

$svr = new WSService(array("securityToken" => $security_token,
                      /* Other options*/ ));

$svr->reply()

So if you use this code in the service you can happily detect any replays of an old message and avoid possible damages. But remember this security check will work only if the replaying message contain the same message id and the created time as the original one. It is possible that an intruder not only intercept the message, but also change these fields and replay it in to your server. Such replays will not be detected from this code.

The solution is to sign each SOAP requests. If a client sign a message with his private key, the server can confirm that the message is not altered while it is on the wire. So if intruder replay a signed message, either it will fail the replay detection test (if it is replaying without changing the message id and created time) or it will fail validating the signature (if the message id and the created time is altered).

So if you implement a replay detection test with a signature test, you can eliminate all the replay attacks to your service (at least theoretically :).