RE: Relative Security Provided by Cached Domain Credentials?
From: Sarbjit Singh Gill (ssgill_at_gilltechnologies.com)
Date: 05/28/04
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To: "'Langston, Fred'" <flangston@verisign.com>, "'Kim Oppalfens'" <Kim.Oppalfens@azlan.be>, "'Kevan Smith'" <Kevan.Smith@tideworks.com>, "'Nicolas RUFF (lists)'" <ruff.lists@edelweb.fr>, <focus-ms@securityfocus.com> Date: Sat, 29 May 2004 03:29:10 +0800
So when a user logs on the w2k terminal using a smartcard + pin no (rather
then the usual A;t-Ctrl-Del), does the private certificate from the
smartcard get copied into the profile data on the disk ? If it does then EFS
works with smartcards, cause when EFS is decrypting, it looks at the use
profile currently logged on for the private certificate. Please advice.
/Gill
-----Original Message-----
From: Langston, Fred [mailto:flangston@verisign.com]
Sent: Wednesday, May 26, 2004 5:51 AM
To: 'Kim Oppalfens'; Kevan Smith; Nicolas RUFF (lists);
focus-ms@securityfocus.com
Subject: RE: Relative Security Provided by Cached Domain Credentials?
Here's a reference for EFS and smart cards:
http://www.petri.co.il/how_does_efs_work.htm
This is the relevant text:
EFS's key-storage mechanism is based on W2K's CryptoAPI architecture, which
stores users' public and private keys separately from the randomly generated
FEK. This setup lets users store their private keys on secure devices (e.g.,
NTFS volumes, smart cards). Smart cards, which require smart card readers on
computers, are credit-card sized devices that let users log on to W2K with a
PIN. Smart cards make personal information (e.g., account numbers,
passwords, digital certificates) portable.
Fred Langston, CISSP
Principal Consultant
VeriSign, Inc.
W: 206.903.8147 x223 M: 425.765.3330
FLangston@VeriSign.com
-----Original Message-----
From: Kim Oppalfens [mailto:Kim.Oppalfens@azlan.be]
Sent: Tuesday, May 25, 2004 6:56 AM
To: Kevan Smith; Nicolas RUFF (lists); focus-ms@securityfocus.com
Subject: RE: Relative Security Provided by Cached Domain Credentials?
I have seen mentioned the use of smartcards for efs certificates in this
thread a couple of times.
Although it would be nice in theory it was my understanding that this cannot
be used at present because not thought about in the efs API, so during
decreption or encryption for that matter only the personal certificate store
is checked for a key, not any smartcard related stuff.
At least that is what I understood about efs and smartcards.
Has any of you actually tested the smartcard solution, or it this how you
would theoratically handle it?
Kim Oppalfens
-----Original Message-----
From: Kevan Smith [mailto:Kevan.Smith@tideworks.com]
Sent: dinsdag 11 mei 2004 22:47
To: Nicolas RUFF (lists); focus-ms@securityfocus.com
Subject: RE: Relative Security Provided by Cached Domain Credentials?
We're testing the completeness of my understanding of Win2K3 PKIs, so feel
free to correct me, but as I understand it the situation isn't quite as dire
as Nicolas makes it sound.
True, EFS certificates (indeed, all user certificates) are stored either in
the users profile (locally on the client computer) or on a smartcard,
depending your implementation. With certs stored in your user profile, the
private key portion of the cert is stored locally on the client computer,
and possibly archived on the issuing CA *. These certs are NOT available
(to anyone) from other computers unless the user first exports/imports
his/her certificate to all his/her workstations.
So, even if an attacker cracks a user's password, he/she will still need the
certificate to access the EFS encrypted files, which requires that they
launch the attack while logged on locally to the victim's computer(s).
Granted, it may be possible to drop a remote control backdoor on the box to
log on undetected, or do any number of other nasty things to achieve the
same effect, but it certainly raises the bar.
While AEFSDR (or similar tools) looks like a handy addition to any
administrators grab-bag, it doesn't lift any hurdles facing a hacker.
If the EFS encrypted data is important enough (or the user hops workstations
enough), you can remove the user's profile from the picture by requiring
smart cards for EFS. The key pairs are stored on the card rather than the
computer, allowing users to roam, and forcing the attacker to acquire both
the username/password to logon to the computer, and the smart card/PIN to
access the files.
Cheers,
Kevan S.
* Private/Public key pairs may be archived to the CA under the following
circumstances:
- V2 certificate (V1 certificates cannot be archived)
- Issued FROM a Windows 2003 Enterprise or DataCenter edition CA
(not Standard Ed)
- Issued TO a user (as opposed to a computer)
- Issued FOR encryption/authentication purposes (not signing/
non-repudiation).
- Client certificate must be stored in the user profile. You cannot
archive certificates issued to a smart card.
-----Original Message-----
From: Nicolas RUFF (lists) [mailto:ruff.lists@edelweb.fr]
Sent: Tuesday, May 11, 2004 11:02 AM
To: focus-ms@securityfocus.com
Subject: Re: Relative Security Provided by Cached Domain Credentials?
> triple DES from memory
>
>>On a related note to part of the discussion in the 'Restricting change
>>of local admin' thread, does anyone know of a non-brute force way to
>>break the encryption on cached domain credentials? Local accounts are
>>easily modified or reset, but I'm not aware of any similar exploits
>>for cached domain credentials. Given that EFS' effectiveness to
>>secure laptop-stored data in a domain environment lives and dies by
>>the security of the cached credentials, I'm curious to know just *how
>>much* more secure they are.
Hi,
About EFS :
-----------
- EFS encryption is 3DES (unless you have a restricted export version of
Windows), with a random FEK (File Encryption Key) for each file.
- FEK is encrypted with RSA, using the EFS User Certificate (Public Key).
- Eventually, the user Private Key is encrypted with his Windows Password.
So if you know the user password, you can decipher all EFS encrypted files.
See "Advanced EFS Data Recovery" tool from ElcomSoft :
http://www.elcomsoft.com/aefsdr.html
About Cached Logons :
---------------------
Cached logons are stored in LSA Secrets and NL$ hidden keys. Basically, it
is a salted hash :
NTLMHash( username + NTLMHash(password) ) so you have to bruteforce. The
salt key is the username, so if you have N accounts to crack, it takes N
times the time to crack one account.
Since this attack is very time-consuming and has little chance to succeed if
user password > 6 chars, there is no public exploit available. Hint : get an
IDA Pro license if you want to know more :-)
-nicolas-
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- Previous message: Jake Frost: "USB Drive Privileges"
- In reply to: Langston, Fred: "RE: Relative Security Provided by Cached Domain Credentials?"
- Next in thread: Kevan Smith: "RE: Relative Security Provided by Cached Domain Credentials?"
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