When is it best to sanitize user input?

Question

User equals untrustworthy. Never trust untrustworthy user\'s input. I get that. However, I am wondering when the best time to sanitize input is. For example, do you blindly

Answer 1

Clean the data before you store it. Generally you shouldn't be preforming ANY SQL actions without first cleaning up input. You don't want to subject yourself to a SQL injection attack.

I sort of follow these basic rules.

1. Only do modifying SQL actions, such as, INSERT, UPDATE, DELETE through POST. Never GET.
2. Escape everything.
3. If you are expecting user input to be something make sure you check that it is that something. For example, you are requesting an number, then make sure it is a number. Use validations.
4. Use filters. Clean up unwanted characters.

Answer 2

My opinion is to sanitize user input as soon as posible client side and server side, i'm doing it like this

1. (client side), allow the user to enter just specific keys in the field.
2. (client side), when user goes to the next field using onblur, test the input he entered against a regexp, and notice the user if something is not good.
3. (server side), test the input again, if field should be INTEGER check for that (in PHP you can use is_numeric() ), if field has a well known format check it against a regexp, all others ( like text comments ), just escape them. If anything is suspicious stop script execution and return a notice to the user that the data he enetered in invalid.

If something realy looks like a posible attack, the script send a mail and a SMS to me, so I can check and maibe prevent it as soon as posible, I just need to check the log where i'm loggin all user inputs, and the steps the script made before accepting the input or rejecting it.

Answer 3

Perl has a taint option which considers all user input "tainted" until it's been checked with a regular expression. Tainted data can be used and passed around, but it taints any data that it comes in contact with until untainted. For instance, if user input is appended to another string, the new string is also tainted. Basically, any expression that contains tainted values will output a tainted result.

Tainted data can be thrown around at will (tainting data as it goes), but as soon as it is used by a command that has effect on the outside world, the perl script fails. So if I use tainted data to create a file, construct a shell command, change working directory, etc, Perl will fail with a security error.

I'm not aware of another language that has something like "taint", but using it has been very eye opening. It's amazing how quickly tainted data gets spread around if you don't untaint it right away. Things that natural and normal for a programmer, like setting a variable based on user data or opening a file, seem dangerous and risky with tainting turned on. So the best strategy for getting things done is to untaint as soon as you get some data from the outside.

And I suspect that's the best way in other languages as well: validate user data right away so that bugs and security holes can't propagate too far. Also, it ought to be easier to audit code for security holes if the potential holes are in one place. And you can never predict which data will be used for what purpose later.

Answer 4

Users are evil!

Well perhaps not always, but my approach is to always sanatize immediately to ensure nothing risky goes anywhere near my backend.

The added benefit is that you can provide feed back to the user if you sanitize at point of input.

Answer 5

The most important thing is to always be consistent in when you escape. Accidental double sanitizing is lame and not sanitizing is dangerous.

For SQL, just make sure your database access library supports bind variables which automatically escapes values. Anyone who manually concatenates user input onto SQL strings should know better.

For HTML, I prefer to escape at the last possible moment. If you destroy user input, you can never get it back, and if they make a mistake they can edit and fix later. If you destroy their original input, it's gone forever.

Answer 6

Unfortunately, almost no one of the participants ever clearly understand what are they talking about. Literally. Only @Kibbee managed to make it straight.

This topic is all about sanitization. But the truth is, such a thing like wide-termed "general purpose sanitization" everyone is so eager to talk about is just doesn't exist.

There are a zillion different mediums, each require it's own, distinct data formatting. Moreover - even single certain medium require different formatting for it's parts. Say, HTML formatting is useless for javascript embedded in HTML page. Or, string formatting is useless for the numbers in SQL query.

As a matter of fact, such a "sanitization as early as possible", as suggested in most upvoted answers, is just impossible. As one just cannot tell in which certain medium or medium part the data will be used. Say, we are preparing to defend from "sql-injection", escaping everything that moves. But whoops! - some required fields weren't filled and we have to fill out data back into form instead of database... with all the slashes added.

On the other hand, we diligently escaped all the "user input"... but in the sql query we have no quotes around it, as it is a number or identifier. And no "sanitization" ever helped us.

On the third hand - okay, we did our best in sanitizing the terrible, untrustworthy and disdained "user input"... but in some inner process we used this very data without any formatting (as we did our best already!) - and whoops! have got second order injection in all its glory.

So, from the real life usage point of view, the only proper way would be

• formatting, not whatever "sanitization"
• right before use
• according to the certain medium rules
• and even following sub-rules required for this medium's different parts.