code development NMandelbrot programming

The three rules of network security

who’s watching you?

I realise for most of the audience, this will be stating the obvious, but I want to cover these rules now so I can refer to them later in the series.

I’m going to do this as a series of 3 posts, so I can refer to each rule separately.

The three rules of network security:

  1. Don’t trust the client;
  2. Don’t trust the server(s);
  3. Don’t trust the network.

In short, don’t trust anything you don’t fully control. I list them separately here since the way we mitigate each is very different.

Troy Hunt covers most of the mitigation strategies and the mechanics of this better than me though, so if you’re interested in this topic, go check him out – Hack Yourself First : (or listen to the .Net Rocks podcast – )

Don’t trust the client

If you’re running a server, and you don’t validate any user supplied content, please shut down your server now before you put the rest of the Internet at risk. Depending on what you’re processing, that includes any POSTed content, any query string, HTTP headers, the content hosted at any provided URL if you retrieve it, and many other possible inputs.

Even if you trust the content is not harmful to your IT security, you still can’t necessarily trust it. Your survey results will contain untrue data, none of your IE11 users will show up as IE users, and if you’re doing any calculations on the client, they may give the wrong answer due to misguided assumptions (the pixel density of an iPhone just before the retina display was announced) or malice.

One way to adjust for the effects of wrong answers is to aggregate results across many inputs such as the majority voting system employed by the Apollo computers to minimise the effects of computer failure. You can also check for inappropriate behaviour such as a high rate of submissions that indicate gaming or a DoS style attack. There are so many possible attacks that I can’t list them all here.

Don’t trust the server

As a client, you also need to validate what you receive. Any recent browser will sandbox and restrict any code by default and the recent web standards also include well-defined Chinese walls to prevent code from one site intercepting data from another (see, for example, CORS and compare to the old method of JSONP in terms of validation and verification of incoming requests. Of course, you should also be checking that what you are receiving is from the right source ( instead of

In addition though, you also need to trust what the server will do with the data you send it. Will the owners respect your privacy (and remember, if they’re outside the EU, the Data Protection Act does not apply) or will they sell your data? Will they protect your account (by hashing passwords, and only storing what they need, rather than keeping your credit card details on file long after they need them)? If they receive a government request for your data, will they honour it, and will they let you know?

Don’t trust the network

Even if you write both server and client, the data can be changed or lost in the middle. Any public WiFi can be compromised and your traffic intercepted, and there’s only so much HTTP-only and SSL-only cookies can protect you from when your attacker controls your DNS server. Beyond WiFi, agencies such as NSA and GCHQ are watching end points and can intercept some SSL traffic. The padlock is only as secure as the lockmaker. If you’re Google, you can’t even trust your “internal” network between sites. Expect everything that you do not own or you cannot physically trace to be compromised and secure your data and communications appropriately.