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5/6/2014
04:40 PM
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Deactivated User Accounts Die Hard

New research finds deleted Windows accounts stick around for up to 10 hours and are open to abuse.

Deleted, expired, and locked-out Windows user accounts actually stay alive -- and vulnerable to abuse -- for up to 10 hours after they’ve been disabled, leaving the door open for malicious insider and targeted attacks, according to new research.

The issue is based on design weaknesses in the Kerberos protocol, as well as weaknesses in how Windows handles user account revocation, says Idan Plotnik, CEO of Aorato, which published its findings today.

Kerberos -- the authentication method used in Windows and Active Directory -- provides single sign-on for a corporate network and uses an organizational "ticket" for subsequent user access. As such, disabling a fired or other end-user account doesn't stop that user from temporarily accessing data and applications in the network. And attackers targeting an organization could use those invisibly active credentials to hack further into the targeted network, according to the research.

"This exposes the corporation to attacks. And traditional security measures don't have proper visibility of those attacks," Plotnik says of most logging and SIEM products.

Backdoor malware can track changes to the Windows Active Directory by querying it, he says. "Malware can sit there for six months and then see that Kelly is not in Active Directory anymore. It can then trigger to start using [that user] account and access resources. Everyone has access to Active Directory, but no one pays attention to it."

Aorato, which sells a directory services application firewall, says there are ways to track any abuse of disabled Windows user accounts, such as tying the ticket with the user account; tracking any changes in the state of user accounts and its activities; and terminating any disable user account requests to access a network resource.

The full report from Aorato is available here.

Kelly Jackson Higgins is the Executive Editor of Dark Reading. She is an award-winning veteran technology and business journalist with more than two decades of experience in reporting and editing for various publications, including Network Computing, Secure Enterprise ... View Full Bio

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mboller
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mboller,
User Rank: Apprentice
5/8/2014 | 3:32:15 PM
Re: malicious insider
The user credentials doesn't help. They have to be logged-in already and use the associated TGT. Say the account is disabled while the user is still logged in, that user would have the remaining lifetime of the TGT to wreak havoc. However a password protected screen-saver that has kicked in is enough to stop this attack for a normal user not able to extract the TGT (and if you have that level of access why bother?).

A semi-decent off-boarding process will help mitigate this as well.

For an adversary or pen-tester it wouldn't be the best loot because of its characteristics.

Other issues keep me awake at night, not this :) but an interesting study,
theb0x
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theb0x,
User Rank: Ninja
5/7/2014 | 4:34:34 PM
Re: clarification
This can be accomplished by Kerberos GPO. Modify the value "Maximum lifetime for user ticket" and "Maximum lifetime for service ticket".

Both have a default value of 10 hours.

You could also interupt all active OWA sessions with iisreset. Keep in mind this will kill everyones session to webmail, sharepoint, and any outlook client configured as HTTPS/RPC.
Robert McDougal
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Robert McDougal,
User Rank: Ninja
5/7/2014 | 3:21:21 PM
Re: Disabled users.
You are exactly correct!  It is best practice to move users to a disabled OU, however, many IT shops do not follow best practices.  I have seen shops that did not even disable their old users.  These shops had accounts which had not logged in over 2 years that were still active.
theb0x
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theb0x,
User Rank: Ninja
5/7/2014 | 2:44:00 PM
Disabled users.
I always thought is was common practice in AD to have a Disabled Users OU.
SgS125
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SgS125,
User Rank: Ninja
5/7/2014 | 1:57:52 PM
clarification
" and terminating any disable user account requests to access a network resource."

 

Most likely you would want to terminate any disabled user account requests, rather than stop disabling user accounts.  Most SIEM products will allow you to see into whatever detail you wish.  It just requires you to setup the appropriate filters and alerts.

This particular issue would be nice to have a pre built filter for since we know that it exists and can be a problem for us.

Isn't there a way to force the tokens to expire?
Kelly Jackson Higgins
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Kelly Jackson Higgins,
User Rank: Strategist
5/7/2014 | 12:40:48 PM
Re: malicious insider
I suppose if an advanced attacker was already inside and able to monitor some of this, they could take advantage as well. But that one seems to pose a lot more work than most attackers are willing to take on.
Randy Naramore
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Randy Naramore,
User Rank: Ninja
5/7/2014 | 12:38:51 PM
Re: malicious insider
I agree with you Kelly, insiders would be the only ones to be able to benefit from old or disabled accounts. 
Kelly Jackson Higgins
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Kelly Jackson Higgins,
User Rank: Strategist
5/7/2014 | 11:37:19 AM
malicious insider
I would think the insider threat is the biggest risk here--someone gets fired, and tries (successfully) their user credentials in order to take proprietary company information, or just to wreak havoc in some way.
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