What Was Wiped? Part 1

What was Wiped - by David Cowen - Hacking Exposed Computer Forensics Blog

Hello again Reader,

I've actually put an appointment on my calendar now to remind me to blog, let's see if reminders will ensure regular posts.This is a short beginning for part 1 to insure I meet these weekly updates.

Many times when you are working an investigation the question of spoliation will come up. In the most obvious scenarios of spoliation a suspect will use a tool that will to some extent wipe out his tracks. These tools come in three flavors:

1. Whole disk wipers: It's fairly obvious when this happens, though some suspects may tell you it's just encrypted. If they say that ask them what program they used to encrypt it and to please hand over the key.

2. File/directory wipers: If someone were to run a program such as bcwipe or eraser to delete files or directories the first thing these programs do is rename the file to prevent you from recovering what file was deleted. So if your suspect wiped 1,000 files you would find 1,000 randomly named files all seeming modified within seconds of each other on the disk from a different date. After renaming the file, it sets the time and after overwriting the contents of the file it sets the size to 0.

Here is a ftk imager view of a directory named temp with some random new files made:

What was Wiped - by David Cowen - Hacking Exposed Computer Forensics Blog

Here is the same directory in ftk imager a second after wiping:

What was Wiped - by David Cowen - Hacking Exposed Computer Forensics Blog

"How long these file stick around seems to vary by the file system. In older cases I found them months after the fact but on my Windows 7 system that I'm running ftk imager doing a view of my local physical drive some random files disappear in a couple seconds, which accounts for why we don't see 7 random files. " *This isn't exactly true, please see the update below* This wipe was done using bcwipe, the behavior of what wipers leave behind and how it runs on each OS and file system sounds like a good post for me to work on.

In part 2 we will go into system cleaners like CCleaner and some research into what they leave behind.


Looks like my disappearing wiped files are not a product of a different version of windows or the file system, it was the windows write cache. I made a couple of new files before and just wiped them immediately after, looks like they didn't actually get committed to the disk before I wiped them and thus would not be around afterwords.

To test this I downloaded a random set of source code from sourceforge, extracted it to a directory and then rebooted to make sure everything was flushed.

After rebooting I wiped seven files from a directory in the source tree and got seven wiped entries as expected:

What was Wiped - by David Cowen - Hacking Exposed Computer Forensics Blog

As you can see, seven randomly named files all again with the date of 4/30/1986 and the time 11:43am. I guess this goes back to my last post, if something seems wrong double check your assumptions.

When I wipe the entire directory tree it then appears as an orphaned directory with all of the directory names and file names changed again to random letters with the same date as we saw before, except for the directories which remain the correct date (these times are in UTC so the date appears as 1/20/11):

What was Wiped - by David Cowen - Hacking Exposed Computer Forensics Blog

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1 comment :

  1. Yeah, I've been searching for a decent tool that would wipe the FAT/MFT with zeros and NOT result in showing random filenames, regardless of the way I would wipe the data.

    Also, I propose a "not-so-pseudo" random method where the resulting file doesn't look flat like crypto in a histogram after the fact. Something like "fake jpeg" or "almost ascii" where the resulting output wouldn't stand out during analysis. (neither zeros, or even chaos)

    My complaints to the Eraser folks have fallen on deaf ears. I think I'll go back to BCWipe. (Though Eraser does permit using benign files as the filler for wiping, so you can manually populate the wiped areas with likely-looking data, just the same thing over and over.)