[unisog] Windows Encrypted File System (EFS)

Daniel Feenberg feenberg at nber.org
Thu Feb 9 21:44:28 GMT 2006




On Thu, 9 Feb 2006, Clyde Hoadley wrote:

> 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?

I may be completely off-base, but I thought the point of encrypted file 
systems was so that portable hard drives could be sent off-site with 
reduced security concerns, rather than as an improvement on the "read 
permission" bit of the ACL. As you point out, there doesn't seem to be 
much substance to using it to enhance security on a mounted drive.

The off-site application isn't nothing, however, given the publicity
given to lost tapes recently.

Daniel Feenberg
feenberg isat nber dotte org
>
> -- 
> Clyde Hoadley, CISSP, GSEC
> Security & Disaster Recovery Coordinator
> Department of Information Technology
> Metropolitan State College of Denver
> http://www.mscd.edu/~infotech/security/
> <hoadleyc at mscd.edu>
> (303) 556-5074 (office)
> (720) 232-4737 (personal cell)
>
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