Microsoft Windows Vista/2003/XP/2000 file management security issues

2007-03-07T00:00:00
ID SECURITYVULNS:DOC:16276
Type securityvulns
Reporter Securityvulns
Modified 2007-03-07T00:00:00

Description

Title: Microsoft Windows Vista/2003/XP/2000 file management security issues Author: 3APA3A, http://securityvulns.com/ Vendor: Microsoft (and potentially another vendors) Products: Microsoft Windows Vista/2003/XP/2000, Microsoft resource kit for Windows 2000 and different utilities. Access Vector: Local Type: multiple/complex (weak design, insecure file operations, etc)

  1. Intro

This is an article I promised to publish after Windows ReadDirectoryChangesW (CVE-2007-0843) [1] issue. It should explain why you must never place secure data inside insecure directory.

It contains a set of attack scenarios to demonstrate security weakness in few very common Windows management practices. Neither of the problem explained is critical, yet combined together they should force you to review your security practices. I can't even say "vulnerabilities" because there is no something you can call "vulnerability". It's just something you believe is secure and it's not.

1.1 Problem: inability to create secured file / folder in public one. Attack: folder hijack attack

First, it's simply impossible with standard Windows interface to create something secured in insecure folder.

Scenario 1.1:

Bob wishes to create "Bob private data" folder in "Public" folder to place few private files. "Public" has at least "Write" permissions for "User" group. Bob:

 I   Creates "Bob private data" folder
 II  Sets permission for folder to only allow access to folder himself
 III Copies private files into folder

Alice wants to get access to folder Bob created. She

 Ia  Immediately  after  folder  is  created,  deletes "Bob private
     data"  folder  and creates "Bob private data" folder again (or
     simply  takes  ownership  under  "Bob  private data" folder if
     permissions allow). It makes Alice folder owner.
 IIa Immediately  after  Bob  sets permissions, she grants herself
     full control under folder. She can do it as a folder owner.
 IIIa  Reads  Bob's  private  files,  because  files permissions are
     inherited from folder

Alice can use "Spydir" (http://securityvulns.com/soft/) tool to monitor files access and automate this process. As you can see, [1] elevates this problem significantly.

This is not new attack. Unix has "umask" command to protect administrators and users. Currently, Windows has nothing similar.

CreateFile() API supports setting file ACL on file creation (just like open() allows to set mode on POSIX systems). ACL can be securely set only on newly created files. This raises a problem of secure file creation.

1.2 Problem: Inability to lock / securely change permissions of already created file Attack: pre-open file/directory attack.

There are few classes of insecure file creation attack (attempt to open existing file), exploitable under Unix with hardlinks or symlinks. It's believed Windows is not vulnerable to this attacks because

I.  There  is  no  symlinks  under Windows. Symlink attacks are not
    possible.
II. Security  information  in  NTFS  is  not  stored  as  a part of
    directory entry, it's a part of file data. Hard link attacks are
    not possible.
III. File  locks  in  Windows  are  mandatory.  It  means,  if  one
     application  locks  the file, another application can not open
     this  file, if user doesn't have backup privileges. It mitigate
     different file-based attacks.

There is at least one scenario, attacker can succeed without symbolic link: to steal data written to file created without check for file existence regardless of file locks and permissions.

Attack description: if attacker can predict filename to be written, he can create file, open it and share this file for all types of access. Because locking and permissions are only checked on file open, attacker retain access to the file even if it's locked and it's permissions are changed to deny file access to attacker.

Exploit (or useful tool): http://securityvulns.com/files/spyfile.c

Opens file, shares it for different types of access and logs changes, keeping the file open.

Compiled version is available from http://securityvulns.com/soft/

Scenario 1.2.1:

Bob is now aware about folder hijack attack. He use xcopy /O /U /S to synchronize his files to newly created folder. xcopy /O copies security information (ownership and permissions) before writing data to file.

Alice use "Spydir" to monitor newly created folders and files in Bob's directory. She use Spyfile to create spoofed files in target directory and waits for Bob to run xcopy. Now, she has full control under content of Bob's files despite the fact she has no permissions to access these files.

In a same way directory content may be monitored by pre-opening directory.

Scenario 1.2.2:

Enterprise directory structure is replicated every day to another user-writable location in order to alow users to recover suddenly deleted or modified files. xcopy or robocopy (from resource kit) is used for replication. Attacker can hijack content of newly created files in newly created folders.

Same problem may happen on archive extraction or backup restoration.

Vulnerable applications: xcopy (from all Windows versions), robocopy (Windows 2000 Resource Kit), different archivers backup restoration utilities

By default, xcopy warns user the file exists, unless /Y or /U key is specified. But I. /Y is always specified for replication II. /Y can be specified via COPYCMD environment variable. COPYCMD environment variable can be created in autoexec.bat file. At least under upgraded Windows 2000 installation autoexec.bat may be controlled by unprivileged user. III. Neither xcopy nor another utilities warn user on existing directory. Pre-open directory attack will always succeed.

As you can see, [1] again dramatically elevates this problem.

1.3 Problem: user can completely block access to the files Attack: open file deletion (including Windows file replication service DoS)

If files is deleted while it's open, it still present in file system
under  it's  old  name  until  close.  Any  operation  on  this file
(including  attributes  requests)  fails,  regardless of application
rights and permissions (including backup ones).

Exploit:  use  spyfile,  delete  file while it's spied. Now, without
closing  spyfile,  attempt  any  operation on this file (e.g. try to
find it's ownership).

Scenario 1.3.1

Now Bob found an copy application to securely copy files. It deletes
old file before creating new one. But it fails if Alice tries to spy
on  Bob  files,  because  attempt  to delete file succeeds, but file
still present and is unmanageable.

Scenario 1.3.2

Windows  file  replication  service  (FRS) is used to replicate data
between  2  public  DFS  folders  to  distribute  load.  Folder  has
permissions:
 Everyone: Add & read
 Creator Owner: Full Control
Thouse, Alice has no permissions to delete files created by Bob.

Replicated  folder  is  available as a share on 2 different servers:
\\SERVER1\Share    and    \\SERVER2\Share.    Bob    is    connected
to \\SERVER1\Share.

Alice uses "Spydir" to monitor files creation by Bob. Every time Bob
creates  new  file  on  \\SERVER1\Share, Alice use spyfile to create
file  with same name on \\SERVER2\Share. It effectively leads to FRS
collision.  While  trying  to resolve collision, FRS fails to delete
file  created  by  Alice  and  Bob file is deleted (original file is
moved to special hidden folder only accessible by administrator).

Workaround:  never  try  to  use  creator-owner based permissions in
replicated folders.

Again, [1] seriously escalates this problem.
  1. Conclusion:

It's simply impossible to securely create something in public folder. At least DoS conditions are always possible. Developers should not consider mandatory file locking as a security feature. Developers should care about secure file creation to store sensitive information. CREATE_NEW should always be used and ACL should be set with lpSecurityAttributes of CreateFile. No attempt to open existing file should be made. Never try to create secure folder in public one. If you are forced, disconnect all users before this operation. Never use replication, archive extraction or backup restore to user-accessible folder. Bob and Alice should finally marry.

  1. Vendor:

All timelines are same with [1].

[1]. Microsoft Windows ReadDirectoryChangesW information leak (CVE-2007-0843) http://security.nnov.ru/news/Microsoft/Windows/ReadDirector.html

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