SECURITY.NNOV: file locking and security

Type securityvulns
Reporter Securityvulns
Modified 2001-12-05T00:00:00


Hello bugtraq,

Topic : File locking and security Author : 3APA3A <> Affected software : Windows NT 4.0, Windows 2000 and may be another systems Exploitable : Yes Remotely exploitable : No Category : Design flow


Application can lock the file after file description is open by application (or in open() call itself). Usually there are 2 modes for locking - SHARE 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.


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.


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.


For unprivileged user

  1. It's possible to stop security policies by locking security policies 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... ...


It's not a bug to be patched. You can always find a user who locked a file.


Microsoft was contacted on September, 7 2001. Last reply on this issue was on October, 13.

-=-=- "Microsoft Security Response Center" <> -=-=-

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 that can be done in this situation.


You can use attached locktest.c (for compiled version see to test file locking issues under Windows.

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