When to filter/sanitize data: before database insertion or before display?

When it comes to displaying user submitted data, the generally accepted mantra is to "Filter input, escape output."

I would recommend against escaping things like html entities, etc, before going into the database, because you never know when HTML will not be your display medium. Also, different types of situations require different types of output escaping. For example, embedding a string in Javascript requires different escaping than in HTML. Doing this before may lull yourself into a false sense of security.

So, the basic rule of thumb is, sanitize before use and specifically for that use; not pre-emptively.

(Please note, I am not talking about escaping output for SQL, just for display. Please still do escape data bound for an SQL string).

i like to have/store the data in original form.i only escape/filter the data depending on the location where i'm using it.

• on a webpage - encode all html
• on sql - kill quotes
• on url - urlencoding
• on printers - encode escape commands
• on what ever - encode it for that job

There are at least two types of filtering/sanitization you should care about :

• SQL
• HTML

Obviously, the first one has to be taken care of before/when inserting the data to the database, to prevent SQL Injections.
But you already know that, as you said, so I won't talk about it more.

The second one, on the other hand, is a more interesting question :

• if your users must be able to edit their data, it is interesting to return it to them the same way they entered it at first ; which means you have to store a "non-html-specialchars-escaped" version.
• if you want to have some HTML displayed, you'll maybe use something like HTMLPurifier : very powerful... But might require a bit too much resources if you are running it on every data when it has to be displayed...

So :

• If you want to display some HTML, using a heavy tool to validate/filter it, I'd say you need to store an already filtered/whatever version into the database, to not destroy the server, re-creating it each time the data is displayed
  • but you also need to store the "original" version (see what I said before)
  • In that case, I'd probably store both versions into database, even if it takes more place... Or at least use some good caching mecanism, to not-recreate the clean version over and over again.
• If you don't want to display any HTML, you will use htmlspecialchars or an equivalent, which is probably not that much of a CPU-eater... So it probably doesn't matter much
  • you still need to store the "original" version
  • but escaping when you are outputing the data might be OK.

BTW, the first solution is also nice if users are using something like bbcode/markdown/wiki when inputting the data, and you are rendering it in HTML...
At least, as long as it's displayed more often than it's updated -- and especially if you don't use any cache to store the clean HTML version.