The four rules of data safety

firefly-gunOne of the most dangerous objects to handle is guns. No wonder there are strict and understandable rules how to handle them safely. The Canadians have The Four Firearm ACTS, but for this blog entry, I will cite the Four Rules stated by Captain Ira L. Reeves right before the first world war and restated by Colonel Jeff Cooper:

  1. All guns are always loaded
  2. Never let the muzzle (the business end of a gun) cover anything you are not willing to destroy
  3. Keep you finger off the trigger until your sights are on the target
  4. Be sure of your target and what is beyond it

Even if you accidentally break one rule (for example, rule 3 is often blatantly disobeyed on television), there are still enough precautions in place to keep you (and everybody around you) relatively safe. The rules are meant to instill a certain amount of respect for the gun into the owner so that offloading of responsibility isn’t possible any more, as in the line “I know this gun is unloaded, so it’s probably mighty fun to point it at somebody”.

The guns of software development

In software development, the most dangerous objects we can handle is user-created data or inputs. To mitigate the risks we take when we accept inputs from our users (and most software would be pretty useless otherwise), we have the concept of validation: Before anything other may happen with the data, it needs to be validated, meaning “proved to be free of danger”. Improper input validation is so prevalent in software development that it has its own CWE number (CWE-20) and ranked number 1 on the Top 25 list of “most dangerous programming errors”.

There are some concepts ready to help us tackle this task. The most promising is the Taint checking that treats all input as dangerous and therefore unworthy of further usage unless proven otherwise. Taint checking reminds you of validation, but not how to validate and isn’t available in most programming languages, unfortunately. What we need is a language agnostic set of rules that shape our behaviour in a way that we can’t make the most common mistakes of validation. It seems that gun owners have tried the same and succeeded. So Let’s formulate our Four Rules of data safety, inspired by the gun rules.

Our four rules

  1. All data always contains malicious aspects
  2. Never accept input for modules you cannot afford to have hacked
  3. Leave input data alone until you actually want to use it
  4. Be sure what aspects to validate and how to do it properly

This is just a starting ground for discussion, let’s call it the first version of the Four Rules. Here is my motivation for each rule:

All data always contains malicious aspects

Most users of most systems are in no way harmful. But if they attempt to harm a system, it better stands prepared. Problem is, even with a thorough validation in your current context, there is always the possibility that your attacker plays a rail shot, entering the system here, but causing damage somewhere else. A good example of this practice were images with Javascript code in their metadata. An adequate validation of uploaded images would check for a valid image format, but don’t mind the “dead content” in the meta tags. A browser would later discover the Javascript and execute it – a classic cross-site scripting attack. Never treat any data as fully validated. If you know that your particular code is vulnerable to a specific threat, let’s say a zero value in a variable used as a divisor, validate once more against this threat. This practice is also contained in the idea of Defensive programming.

Never accept input for modules you cannot afford to have hacked

Behind this rule lies a simple truth: Everything that can be hacked will be hacked, given enough time. The only protection against any hack is no access at all (like in “some air between network cable and network card”). If for example you run a certificate authority and absolutely cannot risk losing your secret private key, the machine using this key must not be connected to any network. If your database contains data much too valuable to be “stolen”, the database shouldn’t be accessible directly – and all access need to be validated beforehand. You need to think about a pragmatic compromise for your scenario when following this rule, but you’ve always been warned.

Leave input data alone until you actually want to use it

This was the most difficult rule for me to decide on. The rationale is that even the slightest bit of validation is actually usage of the input. Given enough knowledge about the validation, an attacker could possibly attack the system by abusing weaknesses in the validation itself (see rule 1 for inspiration). Any contact with input data is dangerous, even when it happens with the best intentions. The downside is that you won’t have a stronghold security architecture, where a mighty wall separates the danger zone from friendly territory (or tainted from cleaned data). Remember that even persisting the input data is using it in some form.

Be sure what aspects to validate and how to do it properly

If the time has come to use the input and to validate it right before, you need to think deep about the threats you want to eliminate. Just like with guns, where real bullets (as opposed by “television bullets”) won’t stop at the shooter’s convenience, your validation has consequences beyond an immediate gain of security. A common error is the rushed countermeasure, when you think of a specific threat and immediately try to abolish it. Take your time and think deep! For example, if your users can enter way too high values, it’s of no use to constrain the input field length, because direct web requests and notations like “1E9” are still possible. But converting an input string to a number to check its value might not be the smartest idea, too. Not long ago, you could crash nearly every application by entering a certain “number of death”. Following this rule requires experience and lots of reading, learning and thinking. And even then, there’s always somebody smarter than you, so ultimately, you should plan your system under the impression of rule 2.

As stated, this is just a starting point to try to formulate rules for data validation that provide a behaviour framework that avoids the most common mistakes and pitfalls. I’m highly interested to hear your thoughts about this topic. Please leave a comment below – but be gentle with the comment validation algorithm.

You probably forget too much, too soon and way too definite

eraser_1If you felt spoken to by the blog title, you might relax a bit: I didn’t mean you, but your application. And I don’t suggest that your application forgets things, but rather removes them deliberately. My point is that it shouldn’t be able to do so.

A disaster waiting to happen

Try to imagine a child that is given a sharp scissors to play with by its parents. It runs around the house, scissors in hand, cutting away things here and there. Inevitably, as mandated by Murphy’s law, it will stumble and fall, probably hurting itself in the process. This scenario is a disaster waiting to happen. It is a perfect analogy of your application as long as it is able to perform the delete operation.

A safe environment

Now imagine an application that is forbidden to delete data. The database user used by the program is forbidden to issue the SQL DELETE command. This is like the child in the analogy before, but with the scissors taken away. It will leave a mess behind while playing that needs an adult to clean up periodically and it will fall down, but it won’t stab itself. If you can run your application in such a restricted environment, you can guarantee a “no-vanish” data safety: No data element that is stored will disappear ever.

Data safety

In case you wonder, this isn’t the maximum data safety level you can (and perhaps should) have. There is still the danger of accidental alteration, where existing data is replaced or overwritten by other data. To achieve the highest “no-loss” data safety level, you need to have a journaling database system that tracks every change ever made in an ever-growing transaction log. If that sounds just like a version control system to you, it’s probably because it essentially will be such a thing.
But “no-vanish” data safety is the first and most important step on the data safety ladder. And it’s easy to accomplish if you incorporate it into your system right from the start, implementing everything around the concept that no deletion will occur on behalf of the system.

Why data safety?

But why should you attempt to adhere to such a restriction? The short answer is: because the data in your system is worth it. We recently determined the immediate monetary worth of primary data entries in one of our systems and found out that every entry is worth several thousand euros. And we have several hundred entries in this system alone. So accidentally deleting two or three entries in this system is equivalent to wrecking your car. Who wouldn’t buy a car when the manufacturer guarantees that wreckages cannot happen, by design? That’s what data safety tries to achieve: Giving a guarantee that no matter how badly the developers wreck their code, the data will not be affected (at least to a degree, depending on the safety level).

No deletion

The best way to give a guarantee and hold onto it is to eliminate the root cause of all risks. In our digital world, this is surprisingly easy to accomplish in theory: If you don’t want to lose data (by accidental removal), prohibit usage of the delete operation on the lowest layer (probably the database). If your developers still try to delete things, they only get some kind of runtime error and their application will likely crash, but the data remains intact.

Implementation using a RDBMS

If you are using a relational database system, you should be aware that “no-vanish” safety comes with a cost. Every time you fetch a list of something from your database, you need to add the constraint that the result must only contain “non-obsolete” entries. Every row in your main tables will adopt some sort of “obsolete” column, housing a boolean flag that indicates that this row was marked as deleted by the application. Remember, you cannot delete a row, you can only mark it deleted using your own mechanisms.
The tables in your database holding “derived data”, like join tables or data only referenced by foreign keys, don’t necessarily need the obsolete flag, but will clutter up over time. That’s not a problem as long as nobody thinks that all entries in these tables are “living data”. The entries that are referenced by obsolete entries in the main tables will simply be forgotten because they are inaccessible by normal means of data retrieval.

The dedicated hitman

Using this approach, your database size will grow constantly and never shrink. There will come the moment when you really want to compact the whole thing and weed out the unused data. Don’t give the delete right back to your application’s database user! You basically don’t trust your application with this (remember the child analogy). You want this job done by a professional. You need a dedicated hitman. Create a second database user that doesn’t has the right to alter the data, but can delete it. Now run a separate job (another application) on your database within this new context and remove everything you want removed. The key here is to separate your normal application and the removal task as far as possible to prohibit accidental usage. If you think you’ve heard this concept somewhere before: It basically boils down to a “garbage collector”. The term “hitman” is just a dramatization of the bleak reality, trying to remind you to be very careful what data you really want to assasinate.

Implementation using a graph database

If this seems like a lot of effort to you, perhaps the approach used by graph databases suits you better. In a graph database, you group your data using “graph connections” or “edges”. If you have a node “persons” in your database, every person node in the database will be associated to this node using such a connection. If you want to remove a person node (without throwing the data away), you remove the edge to the “persons” node and add a new edge to the “deleted_persons” node. You can probably see how this will be easier to handle in your application code than ensuring that the obsolete flag is considered everywhere.


This isn’t a bashing of relational database systems, and it isn’t a praise of graph databases. In fact, the concept of “no deletion” is agnostic towards your actual persistence technology. It’s a requirement to ensure some basic data safety level and a great way to guarantee to your customer that his business assets are safe with your system.

If you have thoughts on this topic, don’t hesitate to share them!