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Re: Microsoft NT "un-removable user" Vulnerability.
From: John Lange <lists () DARKCORE NET>
Date: Wed, 6 Sep 2000 10:00:39 -0500

After your email, I retested this in NT4 SP6a and it is still true.

Using a custom user managment tool, I added the user "testuser;" (no
quotes). I then tried to remove the user using the Microsoft Provided "User
Manager". It reports an error "The user name or group name parameter is
invalid".

I then tried "net user testuser; /delete" and it also refused saying "The
syntax of this command is..."

If I'm using the command incorrectly, please let me know. I'm not sure how
to escape characters in NT (I also tried "net user testuser\; /delete" and
various other forms but none worked.

John Lange

-----Original Message-----
From: Bugtraq List [mailto:BUGTRAQ () SECURITYFOCUS COM]On Behalf Of Steve
Sent: Tuesday, September 05, 2000 9:08 PM
To: BUGTRAQ () SECURITYFOCUS COM
Subject: Re: Microsoft NT "un-removable user" Vulnerability.


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Microsoft NT “un-removable user” Vulnerability.

Vulnerable: Tested on NT4 SP4. All version of NT are thought to be
vulnerable.

Could not reproduce at SP6a.


A vulnerability exists in the Microsoft Windows NT operating
system in which
a userid can be added which conations special characters which
are normally
not allowed. These special userids can not be removed using the
normal user
management interface as supplied from Microsoft.

What custom User Manager are you using.  The normal interface
provided does not allow this, nor do a few of the more popular
add-ins to NT.


The problem exists because the integrity checking on the userIDs
occurs at the GUI level and not on the system itself.

I agree that this *COULD* be an issue but not a likely one.

Exploit:
A malicious user can create a user with special characters (e.g.
 “testuser;”) using a custom user management interface which does
not perform validation checks on the userid. It then can not be
removed using the standard WindowsNT user management interface.

Yes, but a simple net user ~testuser;") /delete will take care of
this.

This could be a significant problem if the user was maliciously
added into the Administrators group. A system scrub or another
custom user management interface would be the only way to remove
the user.

Incorrect.  Net user /delete works just fine.

Credit: This vulnerability was discovered by Jeff Also while
developing a web based User Management interface.

Reported:
---------
I advised Microsoft Security about this on 28 Aug 2000. They
responded that
since they had no custom user management tools, they could not test
this vulnerability.

I fail to see how this is a vulnerability.  If a malicious user
already has "rooted" the NT box, why would he be stupid enough to add
a user to the system that would be obviously out of place.  Why would
he not add a machine name ID, or even a generic username and hope the
sys-admin doesn't notice.  Seeing the existance of ~testuser;") would
be a dead give-away that something is going on.

Regards;


Steve Manzuik
Moderator - Win2K Security Advice

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