RE: How to handle "special characters"

[<appsec () technicalinfo net>]

One of the many problems in dealing with "special characters" within web applications is not so much the individual 
character, but at what stage (or host-service) within the application is the character interpreted into something 
meaningful.

The type, and level, of character filtering required by an application is totally dependant upon the layers and 
sub-services within the application.  Thus, characters are often dangerous at only 'one' layer, while being 'required' 
or irrelevant for other layers.

It is not possible to provide answers in the form of a list.  Unfortunately, to deal with the problem sufficiently, a 
fair understanding of the types of threats and how different characters effect different application layers is 
required.  And from there, relating these threats to your own layered application configuration.

Even then you may get caught out - especially when application layers repeat data processing code.  The classic example 
is double-decode (or even triple-decode) of unicode characters such as % -- for example the character sequence %%25

I would recommend that anyone interested in understanding the impact of different characters review the following three 
papers - especially the multiple examples - to have a clearer idea:

http://www.technicalinfo.net/papers/URLEmbeddedAttacks.html
http://www.technicalinfo.net/papers/CSS.html
http://www.nextgenss.com/papers/advanced_sql_injection.pdf 

Cheers,

Gunter

-----Original Message-----
From: Tony Langley [mailto:tonyl () s2s ltd uk]
Sent: Wednesday, December 10, 2003 11:56 AM
To: 'Sekurity Wizard'
Cc: webappsec () securityfocus com
Subject: RE: How to handle "special characters"


I hope you get a good clear set of answers, or at least some links for
further reference. It would be extremely useful to have a definitive
single point of reference for this.

Please let us know of anything useful which isn't copied to the mailing
list?

It would certainly be useful to know:

1) Which chars are always safe (if there are any).
2) Which chars are always dangerous.
3) Those which are sometimes one or the other.

Thanks...

Tony Langley.

Systems Architect
S2S Limited
-----------------------
 Tel: +44 8703 504 525
 Fax: +44 8703 504 526
-----------------------
http://www.s2s.ltd.uk

-----Original Message-----
From: Sekurity Wizard [mailto:s.wizard () boundariez com] 
Sent: 10 December 2003 13:34
To: webappsec () securityfocus com
Subject: How to handle "special characters"


Greetings,
  I had a developer pose an interesting question today, and I wasn't
100% sure what the answer was - so I figured I'd turn to the community
for advice.

  There are certain characters which pose threats at different levels of
the application tier model.  Some at the client, some at the web server,
and others in the database.  Characters such as the &, |, ', ", and -
can be associated with database hacks, for the most part.  If a
requirement is there to absolutely keep these characters in, for
example, interface with a back-end legacy database, whats the best way
to handle their existance?  As a developer, what are the necessary and
proper steps to take to avoid SQL Injection, command execution or other
attacks?

Just looking for some good best-practices..
  s.Wizard