Why do I need to perform server side validation?

Question

Thanks to everyone who commented or posted an answer! I've kept my original question and update below for completeness.

[Feb 16, 2011 - Update 2] As some people point out - my question should have been: Given a standard asp.net 4 form, if I don't have any server side validation, what types of malicious attacks am I susceptible to?

Here is my take away on this issue.

• If data isn't sensitive (comments on a page) - from an asp.net security standpoint, following standard best practices (SqlParameters, request validation enabled, etc) will protect you from malicious attacks.
• For sensitive data/applications - it's up to you to decide what type of server side validation is appropriate for your application. You need to think the end to end solution (webservices, other systems, etc). You can view a number of suggestions below - whitelist validation, etc.
• If you are using ajax (xhr requests) to post user input you need to reproduce the protection from the other bullets in your code on the server. Again, lots of solutions below – like ensuring that the data does not contain any html/code, etc. (side note: the .net framework requestValidationMode="4.0" does afford some protection in this regard - but I can't speak to how complete a solution it is)

Please feel free to continue to comment...if any of the above is incorrect please let me know. Thanks!

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[Feb 3, 2011 - Update 1] I want to thank everyone for their answers! Perhaps I should ask the reverse question:

Assume a simple asp.net 4.0 web form (formview + datasource with request validation enabled) that allows logged in users to post comments to a public page (comments stored in sql server db table). What type of data validation or cleansing should I perform on the new "comments" on the server side?

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[Jan 19, 2011 - Original Question] Our asp.net 4 website has a few forms where users can submit data and we use jquery validate on the client side. Users have to be logged in with a valid account to access these forms.

I understand that our client side validation rules could easily be bypassed and clients could post data without required fields, etc. This doesn’t concern me very much - users have to be logged in and I don’t consider our data very “sensitive” nor would I say any of our validation is “critical”. The input data is written to the database using SqlParameters (to defend against sql injection) and we depend on asp.net request validation to defend against potentially dangerous html input.

Is it really worth our time to rewrite the various jquery validation rules on the server? Specifically how could a malicious user compromise our server or what specific attacks could we be open to?

I apologize as it appears that this question has been discussed a few times on this site – but I have yet to find an answer that cites specific risks or issues with not performing server side validation. Thanks in advance

Answer 1

Hypothetical situation:

Let's say you have a zip code field. On the client-side you validate that it must be in a "00000" or "00000-0000" pattern. Since you're allowing a hyphen, you decide to store the field as a varchar in the database.

So, some evil user comes along and decides to bypass all of your client-side validation and submit something that's not in the correct format and makes it past the request validation.

Ok, no big deal..., you're encoding it before displaying it back to the user later anyway.

But what else are you doing with that zip code? Are you submitting it to web service for some sort of lookup? Are you uploading it to a GPS device? Will it ever be interpreted by something else in the future? Does your zipcode field now contain some JSON or something else weird?

Or something like this: http://www.businessinsider.com/livingsocial-server-flaw-2011-1

Answer 2

Security is a dependability attribute that is defined as the probability that the system resists to an attack, or else the probability a fault is not maliciously activated.

In order to implement security, you must perform a threat analysis. Complex computer systems are subject to deeper analyses (think about an aircraft's o a control tower's equipment) as they become more critical and threats pose business or human life at risk.

You can perform your own threat analysis by questioning yourself what happens if a user bypasses validation?.

Two groups of answers, by examples:

Group 1 (critical)

1. The user can buy articles paying less than their price
2. The user can be revealed information about other users
3. The user obtains privileges he/she is not supposed to have

Group 2 (non critical)

1. The user is displayed inconsistent data in the next page
2. Processing continues, but the inconsistency leads to an error that requires human intervention
3. The user's data (but only of that user, not others) get compromised
4. A strange error page is returned to the user, with lots of technical information that cannot be used anyway

In the first case, you must definitely fix your validation problem, because you could lose money after an attack, or lose the trust of your public (think about forging Facebook URLs and showing someone's photos even if you are not mutually friends).

In the second case, if you are sure that an inconsistent field doesn't put your business or the data at risk, you may still avoid fixing

The real problem is

How do you prove that any inconsistent data sent to your website is never supposed to have any consequence over the system that may pose a threat?

So that's why you lose less time fixing your validation rather than thinking about it

Answer 3

Honestly, users don't care what you consider "sensitive" or "critical" data. Those criteria are up to them to decide.

I know that if I was a user of your application and I saw my data change without me directly doing something to cause the change...I would close my account up as fast as possible. It would be readily apparent that your system wasn't secure and none of my data was safe.

Keep in mind that you're forcing people to log in so you at least have their passwords somewhere. Whether or not they are easily accessed, a breach is a breach and I have lost my trust.

So...while you may not consider an input injection attack important, your users will and that is why you should still do server side input validation.

Answer 4

Your data may not be worth much, that's fine by me.

BUT, attackers could inject CSRF "cross site request forgery" attack code into your application; users of your site may have their data at other sites compromised. Yes, it would require those 'other sites' to have bugs, but that happens. Yes, it would require that users not use the 'logout' buttons on those sites, but not enough people use them. Think of all the tasty data your users have stored at other web sites. You wouldn't something bad to happen to your users.

Attackers could inject HTML that invites users to download and install 'plugins necessary for viewing this content' -- plugins that are keyloggers, or search hard drives for credit card numbers or tax filings. Maybe a plugin to become spambots or porn hosts. Your users trust your site to not recommend plugins that are owned by the Yakuza, right? They might not feel friendly if your site recommends installing evil things.

Depending upon what kinds of bugs invalid data might trigger, you might find yourself a spambot or a porn host. It heavily depends on how defensively you have coded other aspects of your application. Too many applications blindly trust input data.

And the best part: your users aren't human. Your users are browsers, which might be executing attacks supplied by other sites that didn't bother to perform good input validation and output sanitizing. Your users are viruses or worms that happen to find you by chance or by design. You might trust the individuals, but how far do you trust their computers? Me, not very far.

Please write applications to be as secure as you can -- you may put a large button on the front page to drop all users' data if you want -- but please don't intentionally write insecure programs.

Answer 5

This an excellent and brave question. The short (and possibly brave) answer is you don't. If you are aware of all the security vulnerabilities and you still don't believe it's necessary, then that's your choice.

It really depends on who your users are, who the site is exposed to (in terms of intranet or internet) and how easy it is to obtain an account. You say that your data is not sensitive yet you still require users to log in. How bad would it be if an unauthorised user were to access the system by hopping on another user's machine whilst they were elsewhere?

Bear in mind that relying on the request validation to look for malicious input can never be proved to be 100% safe so security is usually done at multiple levels with a fair bit of redundancy.

However it has to be your choice and you are doing the right thing to find out the consequences of leaving this out.

Answer 6

I believe that you need to validate both on the client side and on the server side, and here's why.

On the client side, you are often saving the user from submitting data that is obviously wrong. They have not filled in a required field. They have put letters in a field that is only supposed to contain numbers. They have provided a date in the future when only a date in the past will do (such as date of birth). And so on. By preventing these kinds of mistakes on the client side, you are avoiding user frustration, and also reducing the number of unnecessary hits to your web server.

On the server side, you should generally repeat all of the validation that you did on the client side. That is because, as you have observed, clever users can get around client-side validation and submit invalid data. In addition, there is some validation that is inefficient or impossible to do on the client side. Sometimes, you check that the data entry adheres to business rules. You might check it against existing data in the database. If you just let users enter anything (especially omitting required fields), the website won't function properly for them.

Answer 7

Check out the Tamper Data extension for firefox. You can feed the server anything you want very easily

Answer 8

Anyone performing HTTP POSTs to your server via your web site (with jQuery validation) can also perform HTTP POSTs via some other means that bypasses the jQuery validation. For example, I could use System.Net.HttpWebRequest to POST some data to your server with the appropriate cookies that injects malicious content into the form fields. I'd have to set up the __EVENT_VALIDATION and __VIEWSTATE fields correctly, but if I succeed, I'd be bypassing the validation.

If you don't have server-side data validation, then you are effectively not validating the inputs at all. The jQuery validation is nice for user experience but not a real line of defense.

This is especially so with inputs like a free-form comments field. You definitely want to ensure that the field does not contain HTML or other malicious script. As an extra measure of defense, you should also escape the comment content when it is displayed in your web app with a library like AntiXss (see http://wpl.codeplex.com/).

Answer 9

In terms of client-side vs. server-side validation, my opinion is that client-side validation is just to make sure the form is filled correctly and a user could tamper with the form and bypass the verifications you do in javascript.

On the server-side you could actually make sure that you actually want to store this data and validate it in depth manner and check relative database tables to ensure that your database is always normalized with any data set that you get from the client. I would say even that the server side is more important than the client side in terms of not showing the user what do you look for in the form and how you validate the data.

to summarize, I recommend verification on both sides, but if I had to choose between the two i would recommend server-side validation , but that could mean that your server could potentially performing additional validations that you could have prevented from validating on the client side

Answer 10

To answer your second question:

You need to use a whitelist to keep malicious input out of the incoming comments.

The .NET Framework request validation does a very good job of stopping XSS payloads in incoming POST requests. It may not, however, prevent other malicious or mischevious HTML from getting into the comments (image tags, hyperlinks, etc.).

So if possible I would set up whitelist validation on the server side for allowed characters. A regex should cover this just fine. You should allow A-Za-z0-9, whitespace, and a few punctuation marks. If the regex fails to match, return an error message to the user and stop the transaction. Regarding SQL Injection: I would allow apostrophes through in this case (unless you like terrible grammar in your comments), but put code comments around your parameterized SQL queries to the effect of: "This is the only protection against SQL, so be careful when modifying." You should also lock down the permissions of the database account used by the web process (read/write only, not database owner permissions). What I wouldn't do is try to do blacklist validation on the input, as that is very time consuming to do correctly (see RSnake's XSS Cheat Sheet at http://ha.ckers.org/xss.html for an idea of the number of things you would need to prevent just for XSS).

Between the .NET framework and your own whitelist validation you should be safe from HTML-based attacks such as XSS and CSRF*. SQL injection will be prevented by using parameterized queries. If the comment data touches any other assets you may need to put more controls in place, but those cover the attacks relevant to the basic data submission form you've outlined.

Also, I wouldn't try to "cleanse" the data at all. It is very difficult to do properly and users (as was mentioned above) hate it when their data is modified without their permission. It is more secure and more usable to give user's a clear error message when your data validation fails. If you put their comment back on the page for them to edit, HTML encode the output so you aren't vulnerable to a Reflected XSS attack.

And as always, OWASP.org (http://www.owasp.org) is a good reference for all things webappsec related. Check out their Top Ten and Development Guide projects.

*CSRF may not be a direct concern of yours, as fraudulent posts to your site may not matter to you, but preventing XSS has the side benefit of keeping CSRF payloads targeting other sites from being hosted from your site.