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 store user input and then sanitize it whenever it is accessed/used, or do you sanitize the input immediately and then store this "cleaned" version? Maybe there are also some other approaches I haven't though of in addition to these. I am leaning more towards the first method, because any data that came from user input must still be approached cautiously, where the "cleaned" data might still unknowingly or accidentally be dangerous. Either way, what method do people think is best, and for what reasons?
Unfortunately, almost no one of the participants ever clearly understands 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.
It depends on what kind of sanitizing you are doing.
For protecting against SQL injection, don't do anything to the data itself. Just use prepared statements, and that way, you don't have to worry about messing with the data that the user entered, and having it negatively affect your logic. You have to sanitize a little bit, to ensure that numbers are numbers, and dates are dates, since everything is a string as it comes from the request, but don't try to do any checking to do things like block keywords or anything.
For protecting against XSS attacks, it would probably be easier to fix the data before it's stored. However, as others mentioned, sometimes it's nice to have a pristine copy of exactly what the user entered, because once you change it, it's lost forever. It's almost too bad there's not a fool proof way to ensure you application only puts out sanitized HTML the way you can ensure you don't get caught by SQL injection by using prepared queries.
I sanitize my user data much like Radu...
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.
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.
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.
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.
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.
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)
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.
Early is good, definitely before you try to parse it. Anything you're going to output later, or especially pass to other components (i.e., shell, SQL, etc) must be sanitized.
But don't go overboard - for instance, passwords are hashed before you store them (right?). Hash functions can accept arbitrary binary data. And you'll never print out a password (right?). So don't parse passwords - and don't sanitize them.
Also, make sure that you're doing the sanitizing from a trusted process - JavaScript/anything client-side is worse than useless security/integrity-wise. (It might provide a better user experience to fail early, though - just do it both places.)
My opinion is to sanitize user input as soon as posible client side and server side, i'm doing it like this
(client side), allow the user to
enter just specific keys in the field.
(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.
(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.
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.
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.
Only do modifying SQL actions, such as, INSERT, UPDATE, DELETE through POST. Never GET.
Escape everything.
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.
Use filters. Clean up unwanted characters.
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.
Assume all users are malicious.
Sanitize all input as soon as possible.
Full stop.
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.
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.
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.
I've been freelance working on the development of a web app for a company, and I realised that in any of the textboxes you can just type html tags or lines of javascript, which is obviously very problematic as I don't want the users to be able to do things that mess how it looks or functions. Is there a way of making sure html/javascript can't be written into text boxes?
The best approach is to assume that all data being POSTed, or sent via the URI to the server is malicious, until you check explicitly that it is not (Perl actually has a taint mode to enforce this), and validate the data received is valid for the data type you're expecting. You shouldn't rely on validation (only) on the client, as a malicious user may craft special requests without actually using your front end.
Despite the fact that I dont have a lot of info for the problem I will give a try, so be nice to me!! (please provide some more info)
Html or Javascript they have some common expressions, you can exclude those from the textfields by writing a custom javascript validator.
You should validate any user input (textboxes, etc.). This means in example that if you are asking for a number, then you check that the user input is a number, and reject anything else.
You can't (and you should not) "forbid to write HTML/JavaScript", you must "check that the input is valid against what you are expecting".
You should validate the input as soon as you want to use it. If you have some sort of input, keypress, keyup or similar event handler, you should validate the data before using it.
Also you should not inject user data as HTML. In example, don't use element.innerHTML = data; but instead use element.textContent = data; so the data are not parsed as HTML but just injected as text. (if you are using jQuery, use $(...).text(data) instead of $(...).html(data).
This seems like a silly question, but recently I've been more and more in need of a quite common feature, that I can't find anywhere: I need to do some manipulation with the form data before sending it, but I don't want to show this to the user - I don't want to set the text of the field on the screen, but rather access the place where the values are stored and change one of them right before submit. Normally I'd leave the text field without name, and create a hidden field that would be sent. Upon submit, I copy the text of the nameless field, using it's id, parse it, and then set that to the hidden field, that is finally post. But that seems quite a bit of effort to do something so simple. There are plenty of reasons why one would do such a thing: remove masks, encrypt a password, change a date from local native format to database... All this things can be done server side, of course, but most of them would be way better client side: you'd need to send less data over network, it'd greatly simplify posterior server-side validation... Anyway, is there a simpler way to do it?
I'm initiating my first start-up. I can't stand attempting to read captchas when signing up for websites, don't want my users to. I looked for alternatives, and I found the checkbox captcha. How is this done, using JavaScript to load a checkbox, and validate it with the same code as would normally be used to make a sign up form?
Thanks.
I looked at the example linked from the article you posted. At first glance, it seems like this can be easily bypassed.
The checkbox captcha works on the basis that spam-bots don't parse or use JavaScript code embedded in webpages. Because of this, they will not find the captcha checkbox element within the form it is searching, and therefore will not send a post value for the checkbox along with the form, and on the server side, you would reject the form if the checkbox value wasn't sent.
The problem with this is:
The checkbox name is always the same (gasp_checkbox)
A bot could easily be "trained" to detect this javascript on your page and act accordingly
Even if you output a random name and value that must be used for the checkbox, it could still be detected
The outcome of those 3 problems means that this is much easier to break than image captchas or other methods. All a bot has to do when they submit your form is add: gasp_checkbox=on to their HTTP request.
That said, if you implement this for yourself on your own site, it is unlikely that any bots will able to get past it because its use is not widespread.
You could make it more secure by doing the following:
Generate unique name/value pairs for the checkbox on the server side, and output those values in obfuscated javascript to the client
Serve the script away from your form, preferably in an external javascript file that is generated by a script.
Verify that the values sent for the checkbox match a pair that was previously generated, and not used before.
If you do those things, I think you could have an effective checkbox captcha. If someone does catch on to it on your site, it may still be trivial to defeat, even with the above safeguards in place, but it may take a while, and still be effective for you most of the time.
Despite my paranoia I've never really gotten around to understanding web security more, so my lack of knowledge is causing me a bit of confusion for this.
Example: Let's say you have 2 text boxes, both are for user input.
The user types in whatever they want into those two text boxes and clicks a button, the button then uses a bit of JavaScript and concatenates whatever is in those two text boxes and displays it out in a div.
My question is, in this case, since it's using JavaScript client side, do you need to really sanitize user input?
What if it outputted to a text box instead of a div? Or as an alert?
I understand that when it comes to forms/PHP you always want to sanitize input, but I'm not really familiar with JavaScript security precautions.
It's my understanding that since this is client-side, and no data is being saved by the server, that whatever the user does (tries to throw in some malicious code or whatnot) won't affect anyone but that user, correct?
No this is not a security issue. The reason why is because an attacker has to force a victim's the browser into making this action in order for it to be XSS.
However, if you grab input from something like document.location and then print it to the page using document.write() then this is DOM based XSS. But this is very a uncommon form of XSS.
You don't have to sanitize anything that is not going to the server.
If people want to do something to their instance of your page, the only one they can hurt is themselves. Look at everything you can do with an extension like GreaseMonkey ... we're talking a lot more than just concatenating strings and displaying them.