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."
-------------------------------------------------------------------------
This List Sponsored by: SPI Dynamics
ALERT: "How A Hacker Launches A Web Application Attack!"
Step-by-Step - SPI Dynamics White Paper
Learn how to defend against Web Application Attacks with real-world
examples of recent hacking methods such as: SQL Injection, Cross Site
Scripting and Parameter Manipulation
https://download.spidynamics.com/1/ad/web.asp?Campaign_ID=701300000003gRl
--------------------------------------------------------------------------
--
Eoin Keary cissp
-------------------------------------------------------------------------
This List Sponsored by: SPI Dynamics
ALERT: "How A Hacker Launches A Web Application Attack!"
Step-by-Step - SPI Dynamics White Paper
Learn how to defend against Web Application Attacks with real-world
examples of recent hacking methods such as: SQL Injection, Cross Site
Scripting and Parameter Manipulation
https://download.spidynamics.com/1/ad/web.asp?Campaign_ID=701300000003gRl
--------------------------------------------------------------------------
