General considerations
A completely secure system is a virtual impossibility, so
an approach often used in the security profession is one of
balancing risk and usability. If every variable submitted
by a user required two forms of biometric validation (such
as a retinal scan and a fingerprint), you would have an extremely
high level of accountability. It would also take half an hour
to fill out a fairly complex form, which would tend to encourage
users to find ways of bypassing the security.
The best security is often inobtrusive enough to suit the
requirements without the user being prevented from accomplishing
their work, or over-burdening the code author with excessive
complexity. Indeed, some security attacks are merely exploits
of this kind of overly built security, which tends to erode
over time.
A phrase worth remembering: A system is only as good as
the weakest link in a chain. If all transactions are heavily
logged based on time, location, transaction type, etc. but
the user is only verified based on a single cookie, the
validity of tying the users to the transaction log is severely
weakened.
When testing, keep in mind that you will not be able to
test all possibilities for even the simplest of pages. The
input you may expect will be completely unrelated to the
input given by a disgruntled employee, a cracker with months
of time on their hands, or a housecat walking across the
keyboard. This is why it's best to look at the code from
a logical perspective, to discern where unexpected data
can be introduced, and then follow how it is modified, reduced,
or amplified.
The Internet is filled with people trying to make a name
for themselves by breaking your code, crashing your site,
posting inappropriate content, and otherwise making your
day interesting. It doesn't matter if you have a small or
large site, you are a target by simply being online, by
having a server that can be connected to. Many cracking
programs do not discern by size, they simply trawl massive
IP blocks looking for victims. Try not to become one.
