Re: Canonicalization

[Eoin <eoinkeary () gmail com>]

Hi,
White lists are and will be always more effective than blacklists.
Its like known good Vs Known Bad.
The list of known Good is smaller and easier to manage. The list of
known bad is a function of the technology being used and may expand or
change over time, Say it changes were made to the back end.

-ek

On 21/04/06, Rossen Raykov <rossen () webhydra com> wrote:
> Andrew,
>
> Is that "simplest form" achievable? One can perform many and different
> encodings making the task of decoding them very difficult and resource
> consuming. Usually it is cheaper and safeties to do semantic checkup and
> treat the input as erroneous if it does not confirm to the expected
> input format.
>
> For example if you are expecting number anything different than a number
> is error. If you expect alphanumeric – verify if the input is composed
> only by alphas and numbers...
>
> Regards,
> Rossen Raykov
>
> --
> Slug - The Slow Web Processes Manager
> http://slug.webhydra.com/
>
>
> Andrew van der Stock wrote:
>
> > Susan,
> >
> > I am the lead OWASP Guide author so I hope I can answer your query.
> >
> > The basics of this sentence is the fact that there are many ways to
> > encode text in web apps, and if you're going to make decisions about
> > that text, or accept it for persistent storage, or re-display it,
> > it's vital that you make it "canonical" or as simple as possible
> > before you act on it.
> >
> > For example, if you get:
> >
> > select%20*%20from%20...
> >
> > from the user and you write code to tokenize input based upon spaces,
> > it will not see any spaces.
> >
> > So you must decode the that string properly (so it becomes "select *
> > from ...") and then you can process it "safely".
> >
> > Be aware of double and n-deep encodings - they can occur, and
> > obviously there are many encodings you've never seen or considered.
> > That's why I strongly advocate positive validation.
> >
> > ie (in C# and .NET, but applicable to most languages):
> >
> > Hashtable clean = new Hashtable();
> >
> > // Ensure that if the statement fails for any reason,
> > // the collection has a safe value for our field
> > clean.Add("field", "");
> >
> > // is the data a single word no more than 20 characters long, using
> > only a-z and 0 to 9?
> > if ( Regex.isMatch(Request.Form["field"], "^[a-z0-9]{1,20}$",
> > RegexOptions.IgnoreCase) )
> > {
> >     // it's safe to take the value of the string as there's most
> > likely  no nasties
> >     clean["field"] = Request.Form["field"].ToString();
> >
> >     // now ensure that the business rules make sense
> >     processFieldBusinessRules(clean["field"]);
> > }
> > else
> > {
> >     throw ...
> > }
> >
> > // Now it's moderately safe to use or store the data in clean[]
> >
> > ...
> >
> > thanks,
> > Andrew
> >
> > On 11/04/2006, at 11:12 PM, susam_pal () yahoo co in wrote:
> > I found the following paragraph in owasp.org. Can someone please
> > elaborate on this?
> >
> > Parameters must be converted to the simplest form before they are
> > validated, otherwise, malicious input can be masked and it can slip
> > past filters. The process of simplifying these encodings is called
> > "canonicalization."
>
>
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--
Eoin Keary cissp

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