How to handle possible (but undesired) HTML input?
Hi
I have a number of pages where it is valid for the user to enter HTML. On these pages, I have turned off RequestValidation ("ValidateRequest = false" in the page directive) so that the HttpRequestValidationException that gets thrown if HTML is included in the Form, doesn't get thrown.
This is fine.
However, on some of those pages, there are fields where I don't want
it to be possible for the user to enter HTML. There are also other
pages where I don't want the user to enter HTML, but I *don't* want
the exception to get thrown - I want to handle it nicely, preferably
with a nice validation message.
This exception gets thrown by System.Web.HttpRequest.ValidateInput,
and its message is "A potentially dangerous Request.Form value was
detected from the client [name of control][value causing problem]".
This method validates the querystring, form, and cookies. Thus, it
gets thrown well before execution starts on any of my own code -
before Page.OnInit, that is.
As far as I can see, there are a couple of ways I can proceed:
* I can turn off Request Validation, and Server.HtmlEncode all the
values in the Form that are not allowed to be HTML. This will
translate "<" to "<" and so on - this is problematic because I use
fixed-length varchars in my database (it's a DB-driven site), and .NET
currently truncates them silently. So I someone typed "<b>Headline
for fantastic article</b>" in a field that allowed 40 characters, it
would get stored as "&<lt;b>Headline for fantastic article" -
without the trailing "</b>". I would have to change my text
length validation to validate the Encoded strings (which would mean
encoding them client-side, or doing without client-side validation),
or I would have to make all my DB fields four times as big as they
needed to be. Neither solution is very attractive to me.
* I can turn off Request Validation, and instead call the method that
throws it inside of a Custom Validator that returns invalid if it
catches the HttpRequestValidationException. I'm worried about how to
repopulate the fields after a postback if they potentiall contain HTML
- what would it do to my page layout? Also, this would only tell me
that there is HTML in the Form, not which field it's in - ideally
(since we're doing this *properly*! :) ), I want to be able to
return a validation-error for each field that contains HTML (where
it's not allowed). This is because once the method finds a Form
variable that has HTML, it throws the exception; so it'd only be able
to point at the first field that has HTML, not all of them. Unless
it's possible to validate individual Form variables with the method -
I don't know yet.
* I can turn off Request Validation, and write (or more likely find) a
Regular Expression to validate each text input for HTML (probably just
matched pairs of angle brackets with text inside, not a check against
a list of W3C tags), and add a client-side validator to all of my text
inputs. This strikes me as maybe the simplest, but relies on finding
such an Expression, and also that the multiple calls to it are not too
heavy. It also means that if the user has Javascript turned off, my
Form will have potential HTML in it, which it will write back to the
fields from their ViewState on PostBack. However, the error message
from the serverside Condition for the RegexValidator should still be
displayed in such a case - I'm just worried about what the page might
look like, past the field with the HTML in.
For reference, the site uses Javascript, and I'm in the happy position
of being able to make "turn Javascript on" a requirement of using it -
it's primarily for corporate intranet use, where the environment is
pretty controlled.
I'm not entirely certain how to proceed - does anyone have any
comments or suggestions? Thanks in advance.
Regards
Peter Mounce
Software Developer, X-RM Ltd
PS I've bought and am using the excellent "Professional Validation and More" package by Peter Blum - this is why some of the validation controls I've named are not the same as the .NET basic ones.
|
