Do I need to validate against malicious POST backs using .net form components?

Ok, I'm moving from classic asp to .net 3.5 (I know big jump)

In classic ASP you always needed to verify POSTs weren't malicious, however I'm wondering how much checking I need to do with .net.

ie. I have a textbox called textbox1.  I've set textbox1.maxlength=20.  I then want to insert textbox1.text into a DB column varchar(20), I feel as if I do not need to verify the length as was needed in classic ASP, because I'm guessing textbox.text won't return a value longer then textbox1.maxlength would allow.

One step further, take dropdownlist1, bind it to a datasource.  When I insert dropdownlist1.selectedvalue into a DB, do I need to check that .selectedvalue exists in my datasource, or can I assume that when binded .selected value will only return items from the value field of the datasource?

Thanks for your help 

1/18/2008 8:39:24 PM 93655 articles. 5 followers. Follow

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Here is a good example of how .net protects against this for you.  Good luck with your upgrade and let me know if you get stuck on anything.


Remember to mark any post that is helpful as an answer and change posts status to Resolved to help others in the future.


1/18/2008 9:32:34 PM

Just on a general note, it's a good idea to ALWAYS treat input as bad input.  Check out this sobering article from ScottGu:

Please click Mark as Answer for helpful posts!

"Give a man a fish, and he eats for a day. Give him a fishing pole, he will eat for life."

--I'm still a script kiddie at heart.
1/18/2008 9:33:25 PM


 Yes, you are right the textbox.maxlength will not allow anything longer than what's specified to go through -- except in the case of multi-line textboxes.  I think what you really mean by asking about malicious posts is a bit more.  Even if the textbox length is within limits (20 in this case), this can become a peep-hole into your table structure and perhaps the base for a successful SQL injection attack on your site.  I don't know whether you have Validation controls in classic asp -- and these are very cool when it comes to effectively blocking any damaging content by permitting only the characters that a user may input.  Then , there are new features which can be used to make 'session hijacking' virtually impossible, especially by enabling ViewStateUserKey.  Hence, only legitimate users of your site make page requests -- and the question of whether the submitted value exists in your datasource, disappears. 

As someone said, however, security is just a matter of making the bar higher.  Not impossible, but higher.  Hence, it will be prudent to keep your existing checks as they are -- just to be ready when some really nasty stuff comes along.

Somewhere, something incredible is waiting to be known -- Carl Sagan
1/18/2008 9:37:39 PM

The TextBox's MaxLength property does NOT prevent larger postbacks on a single line (<input type='text'>) textbox. It prevent typing in a value too large. A user (hacker) can set the value property of the <input> tag to anything they want and it will be sent back. The server side TextBox control never verifies the length to my knowledge, so you should act defensively.

Since the ASP days, hackers have become more educated on how to launch an "input attack" (SQL Injection, cross site scripting, etc). Microsoft has worked on the problem, especially with its validateRequest="true" property in the <@Page > tag. Yet, that can block good text. (Suppose you want the user to be able to type in actual HTML. It would be blocked by the validateRequest property.)

If you have to turn off validateRequest, you are responsible for the security issues. There are many as hackers attack through hidden fields, querystring parameters, and cookies, all of which are checked by validateRequest. I have implemented a commercial solution in my Peter's Data Entry Suite. Even if you don't want to buy software for this, download the trial version and use its "Input Security User's Guide" to educate you on the issues.

--- Peter Blum
Creator of Peter's Data Entry Suite (formerly Professional Validation And More and Peter's Date Package) and Peter's Polling Package
1/19/2008 6:51:26 PM

Peter, I realize that the data being posted to the server could be invalid, and too long.  However, if i'm sending textbox1.text to my DB and not request.form("textbox") shouldn't there be no way for textbox1.text to be set to a string longer then textbox1.maxlength?  I would hope the textbox class prevents that. 

1/22/2008 2:33:25 PM

To my knowledge, the TextBox control does not trim the value from Request.Form when you retrieve it in the Text property.

--- Peter Blum
Creator of Peter's Data Entry Suite (formerly Professional Validation And More and Peter's Date Package) and Peter's Polling Package
1/23/2008 5:55:38 PM

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