Hello bugtraq,
Topic : File locking and security
Author : 3APA3A <3APA3A@security.nnov.ru>
Affected software : Windows NT 4.0, Windows 2000 and may be
another systems
Exploitable : Yes
Remotely exploitable : No
Category : Design flow
Background:
Application can lock the file after file description is open by
application (or in open() call itself). Usually there are 2 modes for
locking - SHARED and EXCLUSIVE locks. Only one application can put
EXCLUSIVE lock on file. If file is locked exclusively no lock can be put
on file by another process (we will not consider a case of parent/child
processes). The main problem of file locking is this mechanism (at least
on tested systems - *BSD, Windows NT, Linux) doesn't check any file
permission or the mode the file is open before locking. It makes it
possible for application with read-only access to the file to lock it
exclusively.
The way file locks interfere with file access depends on OS. There are 2
possible situations: moderate and non-moderate file locks. *BSD and
linux use non-moderate locking, while Windows NT locking is moderate.
What does it mean? Under Unix file locking is only checked then another
application tries to lock the file. If application doesn't use file
locking it will not be affected by file locking. Under Windows things
are different. If one application exclusively locks the file another
application can't access this file even if it doesn't tries to lock the
file. It should be treated as a design flow, because insecure in nature
mechanism of file locking interacts with secure mechanism of file
access.
Resume:
Security aware application should correctly process the situation of
locked file. Application should not rely on ability to lock (or in case
of Windows on ability to access) publicly readable files.
Problem:
Many security-critical mechanism under Windows (I am not aware about
Unix ones, but it doesn't mean that only Windows is affected) can be
DoS'ed by file locking.
Details:
For unprivileged user
1. It's possible to stop security policies and logon scripts by locking
policy files on domain controllers
2. It's possible to lock screensaver file to prevent workstation to
be locked by another user
3. It's possible to deny access to administrative utilities and/or batch
jobs from running by administrator or system
4. It's possible to deny another user's logon in many ways
5. It's possible to deny access to shared programs, documents, etc...
...
Workaround:
It's not a bug to be patched.
Vendor:
Microsoft was contacted on September, 7 2001. Last reply on this issue
was on October, 13.
-=-=- "Microsoft Security Response Center" <secure@microsoft.com> -=-=-
Wanted to get together and let you know what we've found out and the
plan moving forward. You're right that it's possible for someone to
block group policy by locking a file. We've considered quite a few
different options for preventing someone from putting a lock on the
file, but so far all of them would require fairly massive changes to the
system architecture, and we're very leery of making such drastic changes
via a patch.
I'd like to propose a different solution, and see what your reaction
would be. We currently have an auditing event that occurs when group
policy fails to be applied for any reason. The description of the error
isn't as clear as it could be, and we'd propose making the error message
much more descriptive and useful to the administrator. Also, we'd
propose that anytime group policy can't be applied, a pop-up would
appear on the client machine, describing the problem and instructing the
user to contact the system administrator. Clearly, if an attacker saw
the error message, he wouldn't call the administrator -- but one of the
other users on the system would. The administrator could then check the
error log, find out who had locked the file, and take appropriate action
against them.
-=-=-=-=-=-=-=-=-
Of cause, it's "security through obscurity", but I believe that's best
can be done in this situation.
Testing:
You can use attached locktest.c (for compiled version see
http://www.security.nnov.ru/files/locktest.exe) to test file locking
issues under Windows.
Try
locktest.exe READ NONE <filename>
(be careful - during WRITE test locktest damages the file, test it only
on specially created files)
--
http://www.security.nnov.ru
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locktest.c
