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

I like to sanitize it as early as possible, which means the sanitizing happens when the user tries to enter in invalid data. If there's a TextBox for their age, and they type in anything other that a number, I don't let the keypress for the letter go through.

Then, whatever is reading the data (often a server) I do a sanity check when I read in the data, just to make sure that nothing slips in due to a more determined user (such as hand-editing files, or even modifying packets!)

Edit: Overall, sanitize early and sanitize any time you've lost sight of the data for even a second (e.g. File Save -> File Open)

Answer 2

I sanitize my user data much like Radu...

1. First client-side using both regex's and taking control over allowable characters input into given form fields using javascript or jQuery tied to events, such as onChange or OnBlur, which removes any disallowed input before it can even be submitted. Realize however, that this really only has the effect of letting those users in the know, that the data is going to be checked server-side as well. It's more a warning than any actual protection.

2. Second, and I rarely see this done these days anymore, that the first check being done server-side is to check the location of where the form is being submitted from. By only allowing form submission from a page that you have designated as a valid location, you can kill the script BEFORE you have even read in any data. Granted, that in itself is insufficient, as a good hacker with their own server can 'spoof' both the domain and the IP address to make it appear to your script that it is coming from a valid form location.

3. Next, and I shouldn't even have to say this, but always, and I mean ALWAYS, run your scripts in taint mode. This forces you to not get lazy, and to be diligent about step number 4.

4. Sanitize the user data as soon as possible using well-formed regexes appropriate to the data that is expected from any given field on the form. Don't take shortcuts like the infamous 'magic horn of the unicorn' to blow through your taint checks... or you may as well just turn off taint checking in the first place for all the good it will do for your security. That's like giving a psychopath a sharp knife, bearing your throat, and saying 'You really won't hurt me with that will you".

   And here is where I differ than most others in this fourth step, as I only sanitize the user data that I am going to actually USE in a way that may present a security risk, such as any system calls, assignments to other variables, or any writing to store data. If I am only using the data input by a user to make a comparison to data I have stored on the system myself (therefore knowing that data of my own is safe), then I don't bother to sanitize the user data, as I am never going to us it a way that presents itself as a security problem. For instance, take a username input as an example. I use the username input by the user only to check it against a match in my database, and if true, after that I use the data from the database to perform all other functions I might call for it in the script, knowing it is safe, and never use the users data again after that.

5. Last, is to filter out all the attempted auto-submits by robots these days, with a 'human authentication' system, such as Captcha. This is important enough these days that I took the time to write my own 'human authentication' schema that uses photos and an input for the 'human' to enter what they see in the picture. I did this because I've found that Captcha type systems really annoy users (you can tell by their squinted-up eyes from trying to decipher the distorted letters... usually over and over again). This is especially important for scripts that use either SendMail or SMTP for email, as these are favorites for your hungry spam-bots.

To wrap it up in a nutshell, I'll explain it as I do to my wife... your server is like a popular nightclub, and the more bouncers you have, the less trouble you are likely to have in the nightclub. I have two bouncers outside the door (client-side validation and human authentication), one bouncer right inside the door (checking for valid form submission location... 'Is that really you on this ID'), and several more bouncers in close proximity to the door (running taint mode and using good regexes to check the user data).

I know this is an older post, but I felt it important enough for anyone that may read it after my visit here to realize their is no 'magic bullet' when it comes to security, and it takes all these working in conjuction with one another to make your user-provided data secure. Just using one or two of these methods alone is practically worthless, as their power only exists when they all team together.

Or in summary, as my Mum would often say... 'Better safe than sorry".

UPDATE:

One more thing I am doing these days, is Base64 encoding all my data, and then encrypting the Base64 data that will reside on my SQL Databases. It takes about a third more total bytes to store it this way, but the security benefits outweigh the extra size of the data in my opinion.

Answer 3

Assume all users are malicious. Sanitize all input as soon as possible. Full stop.

Answer 4

User input should always be treated as malicious before making it down into lower layers of your application. Always handle sanitizing input as soon as possible and should not for any reason be stored in your database before checking for malicious intent.

Answer 5

I sanitize my data right before I do any processing on it. I may need to take the First and Last name fields and concatenate them into a third field that gets inserted to the database. I'm going to sanitize the input before I even do the concatenation so I don't get any kind of processing or insertion errors. The sooner the better. Even using Javascript on the front end (in a web setup) is ideal because that will occur without any data going to the server to begin with.

The scary part is that you might even want to start sanitizing data coming out of your database as well. The recent surge of ASPRox SQL Injection attacks that have been going around are doubly lethal because it will infect all database tables in a given database. If your database is hosted somewhere where there are multiple accounts being hosted in the same database, your data becomes corrupted because of somebody else's mistake, but now you've joined the ranks of hosting malware to your visitors due to no initial fault of your own.

Sure this makes for a whole lot of work up front, but if the data is critical, then it is a worthy investment.

Answer 6

I find that cleaning it immediately has two advantages. One, you can validate against it and provide feedback to the user. Two, you do not have to worry about consuming the data in other places.