Such a button is not rendered to the browser, so is there any way a malicious user would be able to trigger the action defined by the invisible button? e.g. with a JavaScript call to WebForm_DoPostBackWithOptions? Would ASP.NET accept a POST that appeared to be triggered by this button, even though it wasn't rendered?
Short answer yes.
It is always up to you (the developer) to ensure data received from user input (in this case a post) is valid. Having said that the asp.net framework will do a lot of verification for you, such as "suspicious looking post values".
It is possible to construct a post to a web endpoint, even if the page you display does not have a submit button.
Edit
This would be an example of security through obscurity and is generally not a best practice. Asp.Net "submit" buttons modify a hidden form field called __EVENTTARGET. The asp.net handlers will inspect this field when determining a button click "event". This value can be spoofed if the attacker knew the name of the event target.
Hiding/showing UI elements are good for improving the user experience, but you should always validate (on the server) user input before performing any business actions.
I don't believe it would, if it's not rendered it shouldn't accept the postback. .net uses hidden fields on the page to know which controls were on the page and can verify that during postback, so it knows what triggered the post back. if the control was not there to begin with it shouldn't accept it.
Yes, this is definitely possible. ASP.NET accepts all POST values for controls defined on the page, visible or not. Beware too of i.e. textfields that are set to "read-only". Don't use readonlyControl.Text after the post, and trust that it has the same value as it had the last time you set it.
Take a look at what is posted when you perform a submit with ASP.NET with i.e. Chrome Developer tools, Fiddler, etc, and you should be able to figure out how to add your own value to an "invisible" text field.
Related
This is a clarification question: I'm studying for MCTS 70-515 and in my training kit it states that Hidden Fields: "users can view or modify data stored in hidden fields"
Now I'm aware that users can view the source of the page and then that would display the hidden field data. But I'm curious as to the modification part. How would a user modify a hidden field data and how would it affect the site? Even if they modify the data via View Source they can't save the page and then post the data back to the server.
What am I missing that the author is assuming I know?
OK well all the answers said the same thing (at this time). I guess if the author would of said "savvy" user then that might of tipped me off. I guess I've always assumed that users wouldn't know of Firebug or any other tool that can do manipulation after the page has been displayed to the user.
Thank you for all your answers. I appreciate it!
The hidden field is just a key-value-pair represented as a key-value-pair when serialized and sent to the server, just like any other form element. There are a number of ways to modify hidden fields - one is to use FireBug or some other "developer console" in the browser, another is to manually write the request and send it to the server.
In addition to using a debugging tool such as Firebug, the user could change the value of a hidden field indirectly though other interactions (with JavaScript) making the change for them. Normally, the user would be unaware of the technical detail of what they are doing (they neither know about, nor care about the fact a hidden field got changed)
Other tools, such as Fiddler, may intercept the web request and change the value of the hidden (or any) field as it is being transfered to the server on a postback.
It is possible to change the value of a hidden field on the server during a postback, as you know, or on the client using JavaScript.
Example using jQuery: http://jsfiddle.net/JGsQ5/
Once the page has been loaded by the browser, it is stored in the DOM (http://en.wikipedia.org/wiki/Document_Object_Model) which is what JavaScript manipulates and is used by the browser to build a HTTP request which is sent back to the server as a postback.
Easy, open up a program like FireBug and change the element value. Remember, markup is client side, so the server is trusting the client to send back the right data -- however, this is easily circumvented.
It is best to store data that is essential to the security of your application in session's, whereas the data remains on the server side and is tied to the client. ASP.NET can make up of hashes to prevent the unauthorized modification of fields, amoung other things.
I have a webservice that I need called, the result of which determines whether or not the user is allowed to submit the form.
Since this call is from javascript and not from code behind is it not reliable? Is there any way the user can get around the check -- by either going in with firebug and enabling the submit button, somehow making the method give a different result than was actually returned by the webservice, any other ways of being able to get around it?
Basically is there any way to call a webservice from javascript and have it's result determine whether or not a form can be submitted, and actually prevent the user from submitting the form at all? -- whether or not they have firebug, etc...
No, not possible.
Just to name a few possible reasons:
what if javascript is disabled?
what if the user submits the raw POST (using libcurl, for example)?
what if the browser, that the user is using interprets javascript in a way, different from your expectations (think, portable devices)?
Javascript validation is there for your users' convenience only and should never ever be used as a means of providing security.
You can never prevent the user from making an HTTP request that mimics submission of the form. While disabling the form via Javascript prevents submission for 95% of the users who both have Javascript enabled and don't want to circumvent your access control, anyone who understands HTTP can make the call and you are correct in showing that anyone with Firebug can do it in a matter of seconds.
Javascript isn't reliable for preventing anything. It shouldn't be seen as a security-wall, as it's too easily disassembled with things like firebug, iedevelopertoolbar, and many other browser toys.
Even if you could prevent them from submitting your form on your page, nothing stops them from creating a brand new form, on their own page, and point it toward the action of your form. Thus they're removing themselves from your "secure" environment, and instead chosing to play in their own.
Your suspicion is correct; the user can easily get around any possible Javascript validation.
You will need to use server-side code.
No, it is not reliable. Try disabling Javascript in your browser to see for yourself how easily you can get around it.
The user could simply disable javascript in their browser, or use something like NoScript. The best you could do is to try setting the form action itself in the return from the Ajax request, that way the form, as loaded, won't submit (except to itself). This will probably stop casual users but would be no impediment to a slightly more determined (or just bored and tech savvy) user. You will need to check on the server side whatever you do.
In general, no. You can make the form hard to submit without going through Javascript. Make the submit button not an actual submit button (<input type="submit">), but a pushbutton (<input type="button">) that submits the form in its onClick handler.
As everyone else said, no you can't do it. The only real solution is to have the web service return some dynamic value which the Javascript inserts in a hidden form input. Then whatever server-side code processes the form submission should reject the request if that value is not present.
I have a custom made ASP control which is based on a select (drop down menu). If I disable my control by adding the word "disabled" in its html, I start getting null pointer errors when processing the form data.
I figure, either the browser doesn't post back disabled form items or ASP.NET ignores them when processing the form data. I'm not sure which one it is. I'm trying to understand where I'm loosing data.
Thanks for your help.
PS. I realize that there are better way to create and disable controls than manually editing html but there's a context here that doesn't allow me to do otherwise.
Yes setting control's Enable = false is prevents control's value to be added posted data collection.
you can use readonly attribute instead.
here in MSDN it says :
The Text value of a TextBox control
with the ReadOnly property set to true
is sent to the server when a postback
occurs, but the server does no
processing for a read-only text box.
This prevents a malicious user from
changing a Text value that is
read-only. The value of the Text
property is preserved in the view
state between postbacks unless
modified by server-side code.
Also here is the Microsoft's reply to a bug report related to topic.
but if you use in classical way like that it will work :
txt2.Attributes.Add("readonly", "readonly");
It will prevent the control from posting back but remember this web paradigm is a client/server technology. A person could modify the client data (HTML and / or Javascript) and force a postback no matter what you send him.
Therefore don't rely on this for security sensitive operations such as money manipulation and so on.
Always do a check on the server-side too for sensitive operations.
I have a requirement where I ask user for confirmation and also display messages.
The programmers used for this were from Windows forms background. Hence have used the MsgBox in every nook and corner. Even in business logic part they have used the Messageboxes which requires Yes/No style confirmation from user.
When we tested the site from the remote machine we found that it gives error of using DefaultDesktopOnly/ServiceNotification. But when tested we found that this is totally different from what we were looking for.
Now my requirement is a confirmation box is shown from the code like Delete record" yes no and based on the reply we take the action.
This is to be done using updatepanel.
As you use this code in several places, I suggest you make a custom control, that takes your message and displays and Update panel with message and yes/no buttons.
Internally set some value for yes, no, cancel... so that you get something just like MessageBox.
Update panel or not, you'll have to attach some javascript that would call confirm() javascript function. Based on it's result you cancel javascript default link/button behaviour...
This will give you something to scratch your head for a start:
http://www.dotnetfunda.com/tutorials/ajax/updatepanel.aspx
That's not really a question, but a requirement.
Anyway ... MessageBox is a Windows function, it is not an HTML or browser function. Now you can mimic it in one of two ways, via a javascript confirm function or via Yes/No buttons and the appropriate event.
Given that your requirements are for something that works in an update panel I'd guess that wiring up javascript events manually for this isn't going to be something you are comfortable with, so I'd suggest an asp:Panel inside the UpdatePanel which has yes and no buttons, with server side events bound to them. In the UpdatePanel logic show this when you want confirmation and hide everything else, the act accordingly.
If you want to do server-side confirms, you'll get into more complicated code generation. First of all, you'll have two views. The first one has a link/button delete but will actually be just a postback to the second view that will display confirmation form with yes/no. In this form, your yes button will actually be your delete action...
But I'd still chose a hybrid (especially if this is a grid we're talking about) of javascript and serverside (since alert() and confirm() are evil from user experience perspective):
you have a linkbutton delete
when user clicks on it, you replace this control with a div, that displays two linkbuttons yes/no
send a postback with one of the two
Addendum
No linkbutton could be just dummy, to hide this confirmation and display delete again - so it means there won't be any server round trip
you could even create a usercontrol that mimics this sophisticated delete link behaviour to make it reusable application wide.
Here's the situation: I have a label's text set, immediately followed by a response.redirect() call as follows (this is just an example, but I believe it describes my situation accurately):
aspx:
<asp:Label runat="server" Text="default text" />
Code-behind (code called on an onclick event):
Label.Text = "foo";
Response.Redirect("Default.aspx");
When the page renders, the label says "default text". What do I need to do differently? My understanding was that such changes would be done automatically behind the scenes, but apparently, not in this case. Thanks.
For a little extra background, the code-behind snippet is called inside a method that's invoked upon an onclick event. There is more to it, but I only included that which is of interest to this issue.
A Response.Redirect call will ask the user's browser to load the page specified in the URL you give it. Because this is a new request for your page the page utilises the text which is contained in your markup (as I assume that the label text is being set inside a button handler or similar).
If you remove the Response.Redirect call your page should work as advertised.
After a redirect you will loose any state information associated to your controls. If you simply want the page to refresh, remove the redirect. After the code has finished executing, the page will refresh and any state will be kept.
Behind the scenes, this works because ASP.NET writes the state information to a hidden input field on the page. When you click a button, the form is posted and ASP.NET deciphers the viewstate. Your code runs, modifying the state, and after that the state is again written to the hidden field and the cycle continues, until you change the page without a POST. This can happen when clicking an hyperlink to another page, or via Response.Redirect(), which instructs the browser to follow the specified url.
ASP and ASP.Net are inherently stateless unless state is explicitly specified. Normally between PostBacks information like the value of a label is contained in the viewstate, but if you change pages that viewstate is lost because it is was being stored in a hidden field on the page.
If you want to maintain the value of the label between calls you need to use one of the state mechanisms (e.g. Session, Preferences) or communication systems (Request (GET, POST)).
Additionally you may be looking for Server.Transfer which will change who is processing the page behind the scenes. Response.Redirect is designed to ditch your current context in most cases.
To persist state, use Server.Transfer instead of Response.Redirect.
So, if I may answer my own question (according to the FAQ, that's encouraged), the short answer is, you don't persist view state through redirects. View state is for postbacks, not redirects.
Bonus: Everything you ever wanted to know about View State in ASP.NET, with pictures!
For what it's worth (and hopefully it's worth something), Chapter 6 of Pro ASP.NET 3.5 in C# 2008, Second Edition is a terrific resource on the subject. The whole book has been great so far.