First I got an exception : System.Web.HttpRequestValidationException (0x80004005): A potentially dangerous Request.Form value....
from searching the web I found that adding to my web.config :
<system.web>
<httpRuntime requestValidationMode="2.0" />
</system.web>
should solve the problem, but I get an error : requestValidationMode is not allowed.
I am using framework 3.5, maybe thats the reason... can someone help ?
In 3.5, you don't need the requestvalidationmode. To suppress asp.net validation on postback, just set the #Page parameter validaterequest="false" in the .aspx.
You need this if, for example, you are handling submitted HTML. You also then need to sanitize your input carefully.
Related
It looks like there is a bug in customErrors default redirect in web.config. In my web.config file I have the following customErrors setting
<customErrors defaultRedirect="~/generalerror.html?" mode="On" />
As far as I know this should send all errors to the custom generalerror.html page. It seems to work for some invalid URLS like
http://website.com/?x="<p>"
http://website.com/"<p>"
BUT it is not working when “&” is used in the URL and there is no “?” and there is an HTML tag. So this
http://website.com/&x="<p>"
totally ignores customErrors and you are given the default yellow Runtime Error instead of being sent to the custom generalerror.html page. How do I get this URL to also be redirected to the custom error page ?
If I turn mode="Off" in the web.config I get the following error
A potentially dangerous Request.RawUrl value was detected from the client (="/&x="<p>"").
Since you are passing HTML tags in the URL, it could be an indicative of cross-site scripting attack. Not all HTML tags are dangerous, but when HTML characters are followed by certain characters like '&' in your case, asp.net considers it as a cross-site scripting attack and doesn't allow it by default.
You should consider encoding the URL to get around this. And it is always a best practice. Here is a good explanation about XSS. And here is a link that explains in detail how to get around this issue.
To change this behavior, you can set request validation to false in web.config.
<configuration>
<system.web>
<pages validateRequest="false" />
</system.web>
</configuration>
But in this case, requests need to be validated in the pages.
Breaking changes were made to ASP.NET request validation in .NET 4.0 and this entry is required to revert the behavior to .NET 2.0 where invalid URLs will redirect to custom error page.
<httpRuntime requestValidationMode="2.0" />
Is there a difference between requestValidationMode="4.5" and requestValidationMode="2.0"? I have a .net 4.5 application, there is a control which I don't want to validate, as users can enter html tags in:
<asp:TextBox ID="txtTitle" runat="server" ValidateRequestMode="Disabled" />
in my web.config i have:
<compilation debug="true" strict="false" explicit="true" targetFramework="4.5">...</compilation>
<httpRuntime targetFramework="4.5" requestValidationMode="2.0" />
initially I have put requestValidationMode="4.5" but that didn't work, I would still get the error about the tags - "A potentially dangerous Request.Form value was detected from the client ..." as soon as would submit the form. However if I set it to requestValidationMode="2.0" it works, i'm able to hit the PageLoad and encode the value from that field.
Yes there is a difference between the two. Anything requestValidationMode specified as 4.0 or above will use the 4.0 way and any requestValidationMode specified as below 4.0 will use the 2.0 way. Below is a description of the two:
http://msdn.microsoft.com/en-us/library/system.web.configuration.httpruntimesection.requestvalidationmode.aspx
4.0 (the default). The HttpRequest object internally sets a flag that indicates that request validation should be triggered whenever any HTTP request data is accessed. This guarantees that the request validation is triggered before data such as cookies and URLs are accessed during the request. The request validation settings of the pages element (if any) in the configuration file or of the # Page directive in an individual page are ignored.
2.0. Request validation is enabled only for pages, not for all HTTP requests. In addition, the request validation settings of the pages element (if any) in the configuration file or of the # Page directive in an individual page are used to determine which page requests to validate.
As a note: There are other solutions, since you are using asp.net 4.5 you may want to look it to validating on a per control level, that way you can leave the requestValidationMode property in the web.config at 4.5 and only change it on controls that need it.
http://msdn.microsoft.com/en-us/library/system.web.ui.control.validaterequestmode.aspx
I agree with Chris_dotnet's answer.
However, I would like to add a small side note:
In your web.config file, enclose the requestValidationMode="2.0" tag under the location tag so you only allow a specific page to have this "waiver" to skip the validation.
<location path="YourPage.aspx">
<system.web>
<httpRuntime requestValidationMode="2.0"/>
</system.web>
</location>
I have a .NET 4 WCF service running as an IIS website with ASP.NET compatibility mode. Once of the service methods accepts a string parameter and when there is a value which contains a '&' the ASP.NET pipeline raises a validation exception. I've assigned the following config settings:
<system.web>
<httpRuntime requestValidationMode="2.0"/>
<pages validateRequest="false"/>
</system.web>
And the error persists. The error occurs regardless of whether the input is encoded. I found a potential solution here which suggests providing a custom implementation of System.Web.Util.RequestValidator, however I was wondering whether there are alternatives that can be done with configuration settings only.
EDIT:
I've also found this, however the proposed solution does not fix the problem.
I've found the solution. I set the following configuration setting:
<system.web>
<httpRuntime requestPathInvalidCharacters=""/>
</system.web>
By default, and ampersand is an invalid path character.
Some links that may help:
summary: MS is stating that you should use %26 if the ampersand appears in the query string.
http://connect.microsoft.com/wcf/feedback/details/527185/wcf-rest-uritemplate-does-not-recognise-or-amp-ampersand-as-same-character
summary: poster is talking about encoding the & in the body of the WCF message using &
XmlReader chopping off whitespace after ampersand entity?
So, if it's in the query string encode it with %26. If it's in the body of the message encode as an html entity: &
I've seen this error before but cannot seem to get around it. In this case, I have an ASHX page spitting out a simple HTML form with a textbox into which XML may be posted. When I try to read the form, I receive the "A potentially dangerous Request.Form value...".
Since it's a generic handler the "ValidateRequest" attribute isn't available. However I already had this defined in web.config:
<location path="xml/MyGenericHandler.ashx">
<system.web>
<pages validateRequest="false" />
</system.web>
</location>
This snippet predates a move from .NET 3.5 to 4.0 so I'm guessing that's where the breakage originated.
Any idea how to get around this error for ASHX pages?
The 3.5-4.0 change that clipped you was some stepped up runtime security features for ASP.NET 4.0. The quick fix is to apply the following attribute:
<httpRuntime requestValidationMode="2.0" />
Unfortunately, that opens all pages up to 2.0 request validation, so I'd only do this if you've got a relatively small attack surface.
While not a direct answer to your question, I would say to read this previous post. it does give you a way to ensure that the error is not thrown. It's a risky way in one sense, because it means turning off a basic protection. However, the answer is well-reasoned, and the it clearly states that you should only implement it when you're absolutely sure you're encoding all output.
A potentially dangerous Request.Form value was detected from the client
As a side note, I would also recommend using the Microsoft Anti-Xss Library rather than the built in Server.HtmlEncode functions.
However, if you can modify the ashx, a simpler solution would be to just modify the error code and add an "if" statement to not log errors if the error message contains the string you want to filter.
You'd better disable validation for you handler page only:
<location path="MyGenericHandler.ashx">
<system.web>
<!-- requestValidationMode is to avoid HTML-validation of data posted to the handler -->
<httpRuntime requestValidationMode="2.0"/>
</system.web>
</location>
Or use this property from within your handler to avoid triggering the exception:
context.Request.Unvalidated.Form
I am using freetextbox and have added to the web.config of my app but I still get the following error when submitting text with html:
A potentially dangerous Request.Form value was detected from the client (ctl00_MainContent_FreeTextBox1="
I know this is not the preferred way to set up an app but why am I getting these errors even though I have turned off request validation in my app?
The short answer is you shouldn't be getting such an error if you turned off Request Validation.
Did you do one of these two things correctly?
Disable on the page by inserting this at the top of the ASPX
Add the below section to your web.config.
<configuration>
<system.web>
<pages validateRequest="false" />
</system.web>
</configuration>
If that doesn't work then check the machine.config and see if the pages validaterequest value is set to true in there as that would override the web.config.
I had the same problem, and it was actually my fault. Maybe you have done the same mistake: I placed <httpRuntime requestValidationMode="2.0"/> inside
<configuration><location><system.web> instead of <configuration><system.web>.
Ensure that you haven't enabled request validation for this page. I would keep validation running for your site - but turn it off on pages where you need this control.
Be sure to sanitize anything that gets posted and be prudent about security.