Data
Execution Prevention
Published
By Marc Liron - Microsoft MVP
Introduction To
Using Data Execution Prevention In Windows
XP SP2
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Data Execution Prevention (DEP) helps prevent damage from viruses
and other security threats that attack your computer by running (sometimes
called executing)
malicious code from memory locations that only Windows and other
programs should use. This type of threat causes damage by taking
over one or more memory locations in use by a program. Then it
spreads and harms other programs, files, and even your e-mail
contacts.
This type of "attack" is very common and that is why
Microsoft have introduced DEP into Service Pack 2 for Windows XP.
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Unlike a firewall or
antivirus program though, Data Execution Prevention DEP does NOT help
prevent harmful programs from being installed on your computer.
Instead, it monitors your programs to determine if they use system
memory SAFELY. To do this, DEP software works alone or with
compatible processors to mark some memory locations as
"non-executable". If a program tries to run code—malicious or
not—from a protected location, DEP closes the program and notifies
you with an warning message.
Software or Hardware
DEP?
There are two types of
Data Execution Prevention "Software Enforced" and "Hardware
Enforced".
Most users reading this
article are going to be using Software Enforced data
execution prevention - which is available to any machine running
Windows XP Service Pack 2.
Hardware enforced
DEP is only available (as of writing this article) to users who have
machines based on the AMD Operton 32 or 64 bit processor, or the AMD
Athlon 64 processor with NX (no execute page protection), or the
Intel Itanium processor with XD (execute disable bit).
Where
Can I Find DEP On My PC?
1. On the My
Computer icon right click with your mouse and choose Properties. (My
Computer can be found on the desktop and/or the Start menu depending
on how you have your Windows XP setup.)
2. A small
window will have appeared called: System Properties. Look for
the "tab" along the top called Advanced, now click it.
3. Now look
for the section called Performance and click the button
called Settings.
4. You will
now see a new window appear called Performance Options, click
the tab along the top called Data Execution Prevention and
you should get the same window view as the image below.

Fig 1.1 - The DEP tag of the Performance Options
Window.
By default Data
Execution Prevention is turned on for Windows programs and services
only and for most users this is fine.
However, you can gain
more protection by selecting the second option and DEP will now work
for ALL programs and services on your computer. BUT this is
know to generate some error messages as legitimate software can
conflict with DEP.
If this happens, you
get a warning dialog box on your screen informing you of an error
and the software you are trying to use will not start.
If this was a nasty
piece of software lurking on your computer then DEP will have done
its job. BUT if it happens to you when trying to work with some
software you fully trust, this can be a problem, but fortunately
there is a solution...
...you will need to
tell Data Execution Prevention to ignore the software you want to
use!
(Note: The DEP
error message should always tell you which software caused the
problem and who the publisher is, if you do NOT recognise the
information provided then suspect fowl play and seek the advice of a
more experienced user/technician....)
Enabling this option
can ALSO cause a conflict with some Hardware drivers and
present you with a warning dialog saying something along the lines
of: (ATTEMPTED_EXECUTE_OF_NONEXECUTE_MEMORY).
If that happens you
will need to try and track down updated Windows XP drivers for the
Hardware involved - preferably marked "Digitally Signed"
for Windows XP
To find out how to tell
Data Execution Prevention to ignore the software you want to use
read this page:
http://www.updatexp.com/dep-exceptions.html
To find out how to
update a hardware driver read this page:
http://www.microsoft.com/windowsxp/using/helpandsupport/learnmore/russel_02october14.mspx
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Kind Regards

Marc Liron -
Bio
Microsoft MVP
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