This is a general question more than a specific problem I have with code. The question came up discussing the pros & cons of using frameworks such as VueJS and performing say form validation on the client vs. submitting the data to the server where a controller does the data validation.
His argument was that since the code (i.e JS code) and the data reside on the client there is always the possibility of someone tampering with the data/code then submitting the data to the server therefor bypassing validation. I'm new to VueJS coming to it from PHP + CodeIgniter and would like to hear your thoughts on the subject.
How valid is this argument?
thank you,
Yes, a malicious user can tamper with client-side code and remove all the security and validation checks. Trying to stop them is a waste of time and effort. Instead, move all such checks out of the client-side code and into your server-side code.
How Valid? Very Valid.
Client side validation are extremely easy to bypass. Which is why you need to check the inputs even at the server side.
That being said client side validation are still useful for a number of reasons. For instance if your password has to be alphanumeric you want to check that at the client side first. Its not fun having bottlenecks in your server just because your password requirement is too hard to reach. In essence without your client side validation, every time somebody enters a less than strong password it goes to your server then comes back which is a delay and unnecessary network traffic.
That being said you should still validate if they are alphanumeric at the server side.
Which is better to do client side or server side validation?
In our situation we are using
jQuery and MVC.
JSON data to pass between our View and Controller.
A lot of the validation I do is validating data as users enter it.
For example I use the the keypress event to prevent letters in a text box, set a max number of characters and that a number is with in a range.
I guess the better question would be, Are there any benefits to doing server side validation over client side?
Awesome answers everyone. The website that we have is password protected and for a small user base(<50). If they are not running JavaScript we will send ninjas. But if we were designing a site for everyone one I'd agree to do validation on both sides.
As others have said, you should do both. Here's why:
Client Side
You want to validate input on the client side first because you can give better feedback to the average user. For example, if they enter an invalid email address and move to the next field, you can show an error message immediately. That way the user can correct every field before they submit the form.
If you only validate on the server, they have to submit the form, get an error message, and try to hunt down the problem.
(This pain can be eased by having the server re-render the form with the user's original input filled in, but client-side validation is still faster.)
Server Side
You want to validate on the server side because you can protect against the malicious user, who can easily bypass your JavaScript and submit dangerous input to the server.
It is very dangerous to trust your UI. Not only can they abuse your UI, but they may not be using your UI at all, or even a browser. What if the user manually edits the URL, or runs their own Javascript, or tweaks their HTTP requests with another tool? What if they send custom HTTP requests from curl or from a script, for example?
(This is not theoretical; eg, I worked on a travel search engine that re-submitted the user's search to many partner airlines, bus companies, etc, by sending POST requests as if the user had filled each company's search form, then gathered and sorted all the results. Those companies' form JS was never executed, and it was crucial for us that they provide error messages in the returned HTML. Of course, an API would have been nice, but this was what we had to do.)
Not allowing for that is not only naive from a security standpoint, but also non-standard: a client should be allowed to send HTTP by whatever means they wish, and you should respond correctly. That includes validation.
Server side validation is also important for compatibility - not all users, even if they're using a browser, will have JavaScript enabled.
Addendum - December 2016
There are some validations that can't even be properly done in server-side application code, and are utterly impossible in client-side code, because they depend on the current state of the database. For example, "nobody else has registered that username", or "the blog post you're commenting on still exists", or "no existing reservation overlaps the dates you requested", or "your account balance still has enough to cover that purchase." Only the database can reliably validate data which depends on related data. Developers regularly screw this up, but PostgreSQL provides some good solutions.
Yes, client side validation can be totally bypassed, always. You need to do both, client side to provide a better user experience, and server side to be sure that the input you get is actually validated and not just supposedly validated by the client.
I am just going to repeat it, because it is quite important:
Always validate on the server
and add JavaScript for user-responsiveness.
The benefit of doing server side validation over client side validation is that client side validation can be bypassed/manipulated:
The end user could have javascript switched off
The data could be sent directly to your server by someone who's not even using your site, with a custom app designed to do so
A Javascript error on your page (caused by any number of things) could result in some, but not all, of your validation running
In short - always, always validate server-side and then consider client-side validation as an added "extra" to enhance the end user experience.
You must always validate on the server.
Also having validation on the client is nice for users, but is utterly insecure.
Well, I still find some room to answer.
In addition to answers from Rob and Nathan, I would add that having client-side validations matters. When you are applying validations on your webforms you must follow these guidelines:
Client-Side
Must use client-side validations in order to filter genuine requests coming from genuine users at your website.
The client-side validation should be used to reduce the errors that might occure during server side processing.
Client-side validation should be used to minimize the server-side round-trips so that you save bandwidth and the requests per user.
Server-Side
You SHOULD NOT assume the validation successfully done at client side is 100% perfect. No matter even if it serves less than 50 users. You never know which of your user/emplyee turn into an "evil" and do some harmful activity knowing you dont have proper validations in place.
Even if its perfect in terms of validating email address, phone numbers or checking some valid inputs it might contain very harmful data. Which needs to be filtered at server-side no matter if its correct or incorrect.
If client-side validation is bypassed, your server-side validations comes to rescue you from any potential damage to your server-side processing. In recent times, we have already heard lot of stories of SQL Injections and other sort of techniques that might be applied in order to gain some evil benefits.
Both types of validations play important roles in their respective scope but the most strongest is the server-side. If you receive 10k users at a single point of time then you would definitely end up filtering the number of requests coming to your webserver. If you find there was a single mistake like invalid email address then they post back the form again and ask your user to correct it which will definitely eat your server resources and bandwidth. So better you apply javascript validation. If javascript is disabled then your server side validation will come to rescue and i bet only a few users might have accidentlly disable it since 99.99% of websites use javascript and its already enabled by default in all modern browsers.
You can do Server side validation and send back a JSON object with the validation results for each field, keeping client Javascript to a minimum (just displaying results) and still having a user friendly experience without having to repeat yourself on both client and server.
Client side should use a basic validation via HTML5 input types and pattern attributes and as these are only used for progressive enhancements for better user experience (Even if they are not supported on < IE9 and safari, but we don't rely on them). But the main validation should happen on the server side..
I will suggest to implement both client and server validation it keeps project more secure......if i have to choose one i will go with server side validation.
You can find some relevant information here
https://web.archive.org/web/20131210085944/http://www.webexpertlabs.com/server-side-form-validation-using-regular-expression/
I came across an interesting link that makes a distinction between gross, systematic, random errors.
Client-Side validation suits perfectly for preventing gross and random errors. Typically a max length for any input. Do not mimic the server-side validation rule; provide your own gross, rule of thumb validation rule (ex. 200 characters on client-side; a specific n chars less than 200 on server-side dictated by a strong business rule).
Server-side validation suits perfectly for preventing systematic errors; it will enforce business rules.
In a project I'm involved in, the validation is done on the server through ajax requests. On the client I display error messages accordingly.
Further reading: gross, systematic, random errors:
https://answers.yahoo.com/question/index?qid=20080918203131AAEt6GO
JavaScript can be modified at runtime.
I suggest a pattern of creating a validation structure on the server, and sharing this with the client.
You'll need separate validation logic on both ends, ex:
"required" attributes on inputs client-side
field.length > 0 server-side.
But using the same validation specification will eliminate some redundancy (and mistakes) of mirroring validation on both ends.
Client side data validation can be useful for a better user experience: for example, I a user who types wrongly his email address, should not wait til his request is processed by a remote server to learn about the typo he did.
Nevertheless, as an attacker can bypass client side validation (and may even not use the browser at all), server side validation is required, and must be the real gate to protect your backend from nefarious users.
If you are doing light validation, it is best to do it on the client. It will save the network traffic which will help your server perform better. If if it complicated validation that involves pulling data from a database or something, like passwords, then it best to do it on the server where the data can be securely checked.
I have an interactive html5 app that let's people customize products on the client-side. As a user is customizing, the price updates based on individual component prices.
Once this is done, the person hits checkout, I send the customization to the server, and the server takes over the checkout process.
Obviously sending price data to the server is pointless, since anyone could spoof the POST data. So, I tried writing a server side script that will regenerate the price according to the components selected (sent to the server with the customization data), and show that on checkout instead. However, the server side calculation is proving very difficult, due to complex customizations offered; and it seems like I have to rewrite the entire client-side customization logic on the server-side (which is a lot of work).
Before I continue writing the server-side script I wanted to know if it is at all possible to send the price data to the server in a way (asymmetric key auth perhaps) that cannot be spoofed?
Never trust the information clients send you.
To answer your question, it is possible to use encryption to send data from the client to the server. However, the problem here is the data being encrypted can be modified even before the encryption. So, encryption is not really the solution. The client might modify the data before encryption and the server will not know this if there are no server-side checking.
Even with public-private key encryption, this will still be unsafe. There will only be an assurance that your data will not be tampered from the user to the server. It cannot be spoofed by third parties but your user can spoof it, thereby making the encryption pointless. So, do not trust the user to send you valid legitimate information.
You should really double check everything in the server. And you will not need encryption unless privacy is an issue (which probably is based on your description). This can easily be solved by using https.
The client-side javascript you created can serve as immediate feedback to the users, they see prices as they make changes to the forms. This is good because they do not need to wait for the server to process the information, which the server should upon submission of the form.
You should never never never trust client-side code. If you want to spare yourself the headache of rewriting your client-side code, you could just put the relevant JavaScript on the server and call it from your server code (Not that I'm suggesting you should do that). Another thing I would recommend doing is recalculating the price based on the contents of a full postback once the user is done with their customization, and then displaying the recalculated price to the user in order to allow them to confirm it. That way if any client-side shenanigans have occurred, the user will get the real price anyway, and I'm sure that if it differs even slightly from the previous price, your support people will hear all about it.
Supose I have some form with javascript client side validation and no server side validation.
If user disable javascript in his browser there will no be submit button so he can not send me any data without js enabled.
But I do not know is there any way to change my validation instructions from client browser so he could send me untrusted data and make some damage to my database.
Thanks in advance and sorry for my (possibly) obvious question!!!
No. it is not safe. Use server side validation.
For example, even without the browser, I can read your source code. Then simply use CURL to send a post request with malicious data.
Never, ever trust the client.
No, it's not safe. For example, I could just open up the JavaScript debugger in my browser and override your validation.
Anyone who is capable of disabling javascript on their own is probably also capable of making arbitrary POST requests on their own as well. Ok I might be exaggerating but it's still not safe as someone can do the POST requests without using browser, let alone your form, at all.
I would suggest use both client side and server side validation.
Benifit of client side validation is it's fast, we never hit server until user enter correct input. This saves time and avoid user flustration. This is really helpful in large forms.
However demerit is a bad guy can override the client side validation, Hence to avoid such case we should validate in serverside also before processing data.
Since you can modify the Javascript file when you are in Chrome and press F12, the developer tab, will anyone be able to modify the Javascript there and change the behavior of my site and therefore how it interacts with the server?
Reason I am asking is I want to decide whether to put a piece of code for validation (eg. check email) on the server side or the client side.
Yes, anyone can change the Javascript and HTML of your site to submit anything to your server. You can even change POST/GET requests as well (for example, this plugin is available to Firefox users: https://addons.mozilla.org/en-us/firefox/addon/tamper-data/).
Always have server side validation for any input.
You can never depend on client-side processing being secure. There are many ways to alter or bypass client-side javascript. Client-side validation is nice to be able to give the user a quick response if validation fails, but it should always be backed up by server-side validation.
You should use a double validation:
On the server side to prevent data corruption etc.
On the client side to help the user to fill quickly his form without the need to submit (using a check or cross sign when the user exits a given field
For the business sake, you should only rely on the server side to determine if an given input is valid.
a savvy individual can modify the javascript in their browser, but that only affects their browser.
So yes, someone could disable a validate mechanism implemented in JS, which is why you need to validate on the server.
Anyone can modify the code in client-side but only users with FTP or other thing permissions can modify them server-side.
You should always do validation in server side. If you want (for example make server load smaller) you can make client side validation but it is not enough, since user can easily disable JavaScript.