[unisog] Windows Encrypted File System (EFS)

Stejerean, Cosmin cosmin at cti.depaul.edu
Thu Feb 9 21:55:32 GMT 2006

EFS is meant to encrypt files while on the drive (so if the laptop is
stolen nobody will be able to get the files) and while it is far from
perfect it would have helped in the recent cases in the news when
laptops of bank employees were stolen.

EFS will not encrypt files once the files on USB drives, FTP, email etc
(the name is Encrypted File System). If you need to send encrypted files
you can look at PGP or other solutions for encrypting files.

I see EFS as an addition to ACLs, since ACLs may fail and reveal
sensitive information. EFS ensures that in the event ACLs do fail the
data remains useless. However without a proper implementation EFS can
bite (recovery agents, password changes, etc).

For a free solution that works cross-platform as well I recommend
TrueCrypt. It can create encrypted volumes within files or encrypt
entire volumes and you can choose to only mount them when necessary.


Cosmin Stejerean

-----Original Message-----
From: unisog-bounces at lists.sans.org
[mailto:unisog-bounces at lists.sans.org] On Behalf Of Clyde Hoadley
Sent: Thursday, February 09, 2006 3:19 PM
To: UNIversity Security Operations Group
Subject: [unisog] Windows Encrypted File System (EFS)

I acknowledge that there are a lot of things I just don't know very
much about.  Windows Encrypted File System (EFS) is a good example
of something I know very little about.

Do I understand the following correctly?

If a Windows user copies a file from an EFS folder onto a USB drive,
the copy on the USB drive is not encrypted.  Is that true?

If a Windows user attaches a file from an EFS folder to an Email
message, the attachment is not encrypted?  The same is true
if they were to FTP the file.  Is that true?

There is malware (virus, worms, Trojans) that run in the context
of the logged on user.  Some of these are known to transmit
random files off of the victims computer to places unknown.
Wouldn't such malware, running in the context of the logged on user,
have access to EFS files?  If it should randomly select an EFS file
for transmittal (via Email, IM, ftp, etc...) the file be transmitted
unencrypted?  Is that true?

Laptop is joined to the domain, and while at work, the user
logges into the domain and works on some EFS files (on the laptop),
then they log off and take the laptop home.  The user will not be
able to access their EFS files on the laptop using a local account.
Is that true?

EFS is intended to protect files stored on the hard drive.  It does
not protect files that are being shipped off-site.  Is that true?

My gut feeling is that EFS, while good, it isn't a complete
encryption solution.  Some users may need additional encryption
solutions.  I also get the feeling that EFS could create more
problems that it solves.  What is your opinion?

Clyde Hoadley, CISSP, GSEC
Security & Disaster Recovery Coordinator
Department of Information Technology
Metropolitan State College of Denver
<hoadleyc at mscd.edu>
(303) 556-5074 (office)
(720) 232-4737 (personal cell)

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