When we do indata validation through value objects we get an application tier that is water-proof. The model describes exactly those data that we think are meaningful and can handle (“username, identifier with which the user present herself to the system; regexp [a-z]+“. Each piece of indata is validated as part of the value object constructor (public Username(String s)). For indata to pass through to application, it cannot avoid validation, as the application methods require the value object type (void authenticate(Username uname…)). What more can you ask for?
- Well, you see, we do not want usernames like “danbj”, we would prefer our real names, like “
No problem, we expand the regexp with uppercase letters and space, getting a regexp like [a-zA-Z\ ]+.
- Nice, but our respected colleague
Fair enough – we stuff Scandinavian letters and hyphen into the regexp as well, giving [a-zåäöA-ZÅÄÖ\ \-]+.
- Now there is only one person left: our highly respected Irish colleague Oliver O’Hehir.
Well, well, we are almost finished then, we only need to put the apostrophe into the regexp ending up with something along [a-zåäöA-ZÅÄÖ\ \-\’]+.
Wait, wait, wait!!! Who the h*** just logged in with username “’ OR ‘a’ is not null --“?
Sure, we might have tightened up the regexp to block out that specific attack string and any other malicious use of the format we can think of. But, it is always those we did not think of that causes the trouble.
Well, as always we can think that “SQL Injection is solved by prepared statements”, but remember that Injection Flaw is much larger than SQL Injection. The same vulnerabilities might be there when doing LDAP access, using parameters to construct file names (e g Directory Traversal), or if you have some Domain Specific Language (DSL) which you interpret. In any of these cases there might be a string that might well be fully legally formatted, but attacks the structure of how the underlying resources are used.
Over to a completely different domain: FM broadcast and music radio. In the FM radio broadcast system you must be able to shut down the transmitters from a remote site. Unfortunately, there is only one way to communicate with the transmitters – via radio. The problem was solved by defining a specific sequence of audio blips (very precise on frequencies, duration, and interval) and denoting that sequence the meaning “shut down the transmitter”.
The pioneering Swedish rap group JustD put that exact sequence as the final beat on one of their songs, without telling anyone. They must have laughed all the way home from the studio. That song has been played on Swedish radio exactly once.
The JustD track hack is a wonderful example of exploiting an Injection Flaw, there is no way to escape it “in band”. I have the same gut feeling about indata validation and SQL Injection.
No matter how we structure the indata model, there might always be some data that actually is valid indata, but causes the system to crash.
So, indata modelling and validation in all its glory: However necessary it is for upholding security, it is not sufficient.