[NT] Windows XP Security Concerns (Fast Switch, Password Reset, Remote Desktop)

From: support@securiteam.com
Date: 12/20/01

From: support@securiteam.com
To: list@securiteam.com
Date: Thu, 20 Dec 2001 08:45:11 +0100 (CET)

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  Windows XP Security Concerns (Fast Switch, Password Reset, Remote Desktop)


Below is a description of three security problems with Windows XP
Professional that should be considered as security bugs.


Vulnerable systems:
Windows XP Professional in a Workgroup

I. Problem with account locking due to fast user switching
Fast user switching is a new Windows XP feature, which allows simultaneous
logging on of more than one user. It is based on Terminal Services
technology and runs unique user sessions that enable each user's data to
be entirely separated. Fast User Switching is enabled by default on a
stand-alone or workgroup-connected computer. It is not available in

While extensively using this new feature, a possible denial-of-service bug
has been found that locks out accounts on the machine. To recreate, do the

1. Set the account lockout threshold to 3 attempts.
2. Create 10 user accounts with user level privileges (User1 - User10).
3. Logon using User1 account.
4. Using fast user switching, logon using User2 account.
5. Use fast switching to change from User1 to User2 3 times.
6. Attempt to logon using User3 account.

At this point, every account on the machine would be locked out (except
Administrator account of course). Security Log would now show logon
failure (ID529) and account locked (ID539) entries. It was also noted that
there is no need to switch between two users. Even switching between one
user (logging on and logging off using fast user switching) results in all
accounts being locked out.

Microsoft was notified on December 5, 2001 and the following response was

From: Microsoft Security Response Center [mailto:secure@microsoft.com]

Sent: Wednesday, December 12, 2001 10:54 PM
To: Tomasz Polus
Cc: Microsoft Security Response Center
Subject: RE: Fast User Switching blocks user accounts [cb]
[...] "Fast User Switching is a feature that's designed primarily for home
users. One thing that Fast User Switching does is to check local accounts
for blank passwords to determine if a prompt should be provided for a
particular user or not. Users who have elected to maintain blank passwords
are not shown the prompt for their account when they switch accounts.
Because of this, if account lockouts are enabled in conjunction with Fast
User Switching, it is possible for this feature to inadvertently lockout
accounts. If you want to enable the account lockout feature, it is
recommended that you not use the Fast User Switching feature. I hope this
is helpful in clarifying what you are seeing. Please let us know if you
have any questions or concerns." [...]

As you can see, Microsoft admitted this to be a problem and recommended
not to use fast user switching in conjunction with Account Lockout. This
as a significant limitation on the new feature, and/or a forced
downgrading of security settings.

2. Problem with reset password disk
Windows XP introduced a new feature - "Password Reset Disk", which can be
used to recover user account and personalized computer settings if a user
forgets his password.

The problem is that in certain conditions (Minimum password age <> 0) user
may not be able to reset his password using above mentioned disk and the
only solution is the reset password feature available to the
Administrator. First, make sure the "Minimum password age" policy is set
to a value other than zero. Now, supposing the user forgets his password
before its age expires, he will not be able to reset it with the disk
until the password expires.

What's more, changing password by an Administrator using MMC or control
panel (in other words - GUI) leads to user data loss (i.e. EFS files)
because of private key loss. The only solution seems to be "net user"
command issued by an administrator.

3. Remote Desktop sends recently used username in plaintext
Remote Desktop client remembers account name that has been used recently
to establish RD session with another machine. When sniffing the network,
it was discovered that RD client has send login to the other computer in
plain text. It was further clarified that what was actually sent is not a
user account name on the destination machine, but username which has been
used recently to logon with RD client.
However, assuming that the logon is made to the same computer as recently,
RD client sends in clear text user account name present on the destination
computer. In some cases, this can pose a big security risk. For example,
if RD client is used by users connecting to a terminal server, the
attacker can sniff all the TS user accounts.


The information has been provided by <mailto:Tomasz_Polus@BSI.NET.PL>
Tomasz Polus.


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