mailing list archives
Re: Microsoft Windows Vista/2003/XP/2000 file management security issues
From: 3APA3A <3APA3A () SECURITY NNOV RU>
Date: Fri, 9 Mar 2007 15:40:04 +0300
Dear M. Burnett,
--Friday, March 9, 2007, 7:12:31 AM, you wrote to 3APA3A () SECURITY NNOV RU:
MB> 3APA3A, I just wanted to say that is very clever research you have done.
MB> It's true that this does require some re-thinking of security practices, but
MB> I don't think it's accurate to say it's impossible to secure a private
MB> folder in a public one--I believe there is a way to do it securely.
Of cause, there is always a way to solve one specific problem. A
solution for problem #1 has problem #2. Solution for problem #2 has
MB> There are three basic attacks you described:
MB> 1. Attacker deletes a users' new folder then immediately re-creates it,
MB> establishing ownership of the folder
MB> 2. Attacker predicts filename, creates the file, then keeps it open for all
MB> access, retaining rights on the file
MB> 3. Attacker creates and deletes a file but keeps it open, denying any other
MB> access to that file.
4. By forcing irresolvable replication collision on DFS replicated
folder in conjunction with  attacker can delete newly created
files (and probably folders, I did no test) created by another user
even in case of permissions like this:
Users: Add & read
Creator Owner: full control (or something, doesn't matter).
MB> The last two attacks could be prevented by creating new files in a private
MB> folder that prevents others from creating files. However, item 1 could
MB> compromise the security of that folder when it is initially created. To
MB> prevent that situation you would need to make sure that, within a public
MB> directory, only the CREATOR OWNER can delete a folder, which I believe is
MB> the default setting.
MB> But, as you noticed, others can still delete that folder. There is a quirky
MB> thing in NTFS that allows users to delete subfolders and files without even
MB> having delete permissions on those files (see
MB> http://xato.net/bl/2007/01/04/pointless-permissions/). I think that if you
MB> set permissions on a folder that prevented users from deleting children, and
MB> only allowed CREATOR OWNER to delete new folders, when a user creates a new
MB> folder it will be secure, therefore protecting you from 2 and 3.
The problem with replicated folder has no relation to NTFS permissions.
It's not replication service itself, not user who deletes the file of
folder, because of collision. Attacker only forces collision situation.
This can not be fixed by NTFS permissions. "Add" NTFS permission is
only required to remove somebody's newly created file.
For replication service attacks is:
1. Victim creates Folder
2. Attacker creates Folder with same name on different replication mirror
and locks it.
3. Replication service detects collision and removes Folder
4. Attacker creates Folder again
5. Folder is replicated. Attacker is now folder owner.
MB> I haven't fully tested this to verify it, but I believe this would prevent
MB> all the scenarios you described, although a user could still prevent the
MB> initial folder creation if they could predict the filename.
As you can see, there is at least one situation where your assumption
is wrong, a case of DFS replication.
Of cause, it still can be solved somehow, but who can guarantee it's
impossible to find problem #5 in solution for problem #4?
MB> Nevertheless, these are still important issues that illustrate some of the
MB> confusion that the NTFS quirkiness leads to.
MB> Mark Burnett
MB> -----Original Message-----
MB> From: 3APA3A [mailto:3APA3A () SECURITY NNOV RU]
MB> Sent: Thursday, March 08, 2007 12:59 PM
MB> Subject: Microsoft Windows Vista/2003/XP/2000 file management security
MB> To: bugtraq () securityfocus com; full-disclosure () lists grok org uk
MB> This is an article I promised to publish after Windows
MB> ReadDirectoryChangesW (CVE-2007-0843)  issue. It should explain why
MB> you must never place secure data inside insecure directory.
MB> Title: Microsoft Windows Vista/2003/XP/2000 file management security issues
MB> Author: 3APA3A, http://securityvulns.com/
MB> Vendor: Microsoft (and potentially another vendors)
MB> Products: Microsoft Windows Vista/2003/XP/2000, Microsoft resource kit
MB> for Windows 2000 and different utilities.
MB> Access Vector: Local
MB> Type: multiple/complex (weak design, insecure file operations, etc)
MB> Original advisory: http://securityvulns.com/advisories/winfiles.asp
MB> Securityvulns.com news:
MB> 0. Intro
MB> This article contains a set of attack scenarios to demonstrate security
MB> weakness in few very common Windows management practices. Neither of the
MB> problem explained is critical, yet combined together they should force
MB> you to review your security practices. I can't even say
MB> "vulnerabilities" because there is no something you can call
MB> "vulnerability". It's just something you believe is secure and it's not.
MB> 1.1 Problem: inability to create secured file / folder in public one.
MB> Attack: folder hijack attack
MB> First, it's simply impossible with standard Windows interface to create
MB> something secured in insecure folder.
MB> Scenario 1.1:
MB> Bob wishes to create "Bob private data" folder in "Public" folder to
MB> place few private files. "Public" has at least "Write" permissions for
MB> "User" group. Bob:
MB> I Creates "Bob private data" folder
MB> II Sets permission for folder to only allow access to folder himself
MB> III Copies private files into folder
MB> Alice wants to get access to folder Bob created. She
MB> Ia Immediately after folder is created, deletes "Bob private
MB> data" folder and creates "Bob private data" folder again (or
MB> simply takes ownership under "Bob private data" folder if
MB> permissions allow). It makes Alice folder owner.
MB> IIa Immediately after Bob sets permissions, she grants herself
MB> full control under folder. She can do it as a folder owner.
MB> IIIa Reads Bob's private files, because files permissions are
MB> inherited from folder
MB> Alice can use "Spydir" (http://securityvulns.com/soft/) tool to
MB> monitor files access and automate this process. As you can see, 
MB> elevates this problem significantly.
MB> This is not new attack. Unix has "umask" command to protect
MB> administrators and users. Currently, Windows has nothing similar.
MB> CreateFile() API supports setting file ACL on file creation (just like
MB> open() allows to set mode on POSIX systems). ACL can be securely set
MB> only on newly created files. This raises a problem of secure file
MB> 1.2 Problem: Inability to lock / securely change permissions of already
MB> created file
MB> Attack: pre-open file/directory attack.
MB> There are few classes of insecure file creation attack (attempt to
MB> open existing file), exploitable under Unix with hardlinks or
MB> symlinks. It's believed Windows is not vulnerable to this attacks
MB> I. There is no symlinks under Windows. Symlink attacks are not
MB> II. Security information in NTFS is not stored as a part of
MB> directory entry, it's a part of file data. Hard link attacks are
MB> not possible.
MB> III. File locks in Windows are mandatory. It means, if one
MB> application locks the file, another application can not open
MB> this file, if user doesn't have backup privileges. It mitigate
MB> different file-based attacks.
MB> There is at least one scenario, attacker can succeed without symbolic
MB> link: to steal data written to file created without check for file
MB> existence regardless of file locks and permissions.
MB> Attack description: if attacker can predict filename to be written, he
MB> can create file, open it and share this file for all types of access.
MB> Because locking and permissions are only checked on file open,
MB> attacker retain access to the file even if it's locked and it's
MB> permissions are changed to deny file access to attacker.
MB> Exploit (or useful tool): http://securityvulns.com/files/spyfile.c
MB> Opens file, shares it for different types of access and logs changes,
MB> keeping the file open.
MB> Compiled version is available from http://securityvulns.com/soft/
MB> Scenario 1.2.1:
MB> Bob is now aware about folder hijack attack. He use xcopy /O /U /S to
MB> synchronize his files to newly created folder. xcopy /O copies
MB> security information (ownership and permissions) before writing data
MB> to file.
MB> Alice use "Spydir" to monitor newly created folders and files in
MB> Bob's directory. She use Spyfile to create spoofed files in target
MB> directory and waits for Bob to run xcopy. Now, she has full control
MB> under content of Bob's files despite the fact she has no permissions
MB> to access these files.
MB> In a same way directory content may be monitored by pre-opening
MB> Scenario 1.2.2:
MB> Enterprise directory structure is replicated every day to another
MB> user-writable location in order to alow users to recover suddenly
MB> deleted or modified files. xcopy or robocopy (from resource kit) is
MB> used for replication. Attacker can hijack content of newly created
MB> files in newly created folders.
MB> Same problem may happen on archive extraction or backup restoration.
MB> Vulnerable applications:
MB> xcopy (from all Windows versions),
MB> robocopy (Windows 2000 Resource Kit),
MB> different archivers
MB> backup restoration utilities
MB> By default, xcopy warns user the file exists, unless /Y or /U key is
MB> specified. But
MB> I. /Y is always specified for replication
MB> II. /Y can be specified via COPYCMD environment variable. COPYCMD
MB> environment variable can be created in autoexec.bat file.
MB> Different situations are possible, where autoexec.bat is writable by
MB> attacker, if:
MB> - Default Windows 2000 permissions are used or applied with domain
MB> policy .
MB> - One can try to re-create autoexec.bat using POSIX subsystem
MB> III. Neither xcopy nor other utilities warn user on existing
MB> directory. Pre-open directory attack will always succeed.
MB> As you can see,  again dramatically elevates this problem.
MB> 1.3 Problem: user can completely block access to the files
MB> Attack: open file deletion
MB> (including Windows file replication service DoS)
MB> If files is deleted while it's open, it still present in file system
MB> under it's old name until close. Any operation on this file
MB> (including attributes requests) fails, regardless of application
MB> rights and permissions (including backup ones).
MB> Exploit: use spyfile, delete file while it's spied. Now, without
MB> closing spyfile, attempt any operation on this file (e.g. try to
MB> find it's ownership).
MB> Scenario 1.3.1
MB> Now Bob found an copy application to securely copy files. It deletes
MB> old file before creating new one. But it fails if Alice tries to spy
MB> on Bob files, because attempt to delete file succeeds, but file
MB> still present and is unmanageable.
MB> Scenario 1.3.2
MB> Windows file replication service (FRS) is used to replicate data
MB> between 2 public DFS folders to distribute load. Folder has
MB> Everyone: Add & read
MB> Creator Owner: Full Control
MB> Thouse, Alice has no permissions to delete files created by Bob.
MB> Replicated folder is available as a share on 2 different servers:
MB> \\SERVER1\Share and \\SERVER2\Share. Bob is connected
MB> to \\SERVER1\Share.
MB> Alice uses "Spydir" to monitor files creation by Bob. Every time Bob
MB> creates new file on \\SERVER1\Share, Alice use spyfile to create
MB> file with same name on \\SERVER2\Share. It effectively leads to FRS
MB> collision. While trying to resolve collision, FRS fails to delete
MB> file created by Alice and Bob file is deleted (original file is
MB> moved to special hidden folder only accessible by administrator).
MB> Workaround: never try to use creator-owner based permissions in
MB> replicated folders.
MB> Again,  seriously escalates this problem.
MB> 2. Conclusion:
MB> It's simply impossible to securely create something in public folder.
MB> At least DoS conditions are always possible.
MB> Developers should not consider mandatory file locking as a security
MB> Developers should care about secure file creation to store sensitive
MB> information. CREATE_NEW should always be used and ACL should be set
MB> with lpSecurityAttributes of CreateFile. No attempt to open existing
MB> file should be made.
MB> Never try to create secure folder in public one. If you are forced,
MB> disconnect all users before this operation.
MB> Never use replication, archive extraction or backup restore to
MB> user-accessible folder.
MB> Bob and Alice should finally marry.
MB> 3. Vendor:
MB> All timelines are same with .
MB> . Microsoft Windows ReadDirectoryChangesW information leak
MB> . Windows 2000 system partition weak default permissions
Почтенные ископаемые! Жду от вас дальнейших писем. (Твен)
Full-Disclosure - We believe in it.
Hosted and sponsored by Secunia - http://secunia.com/
Re: Microsoft Windows Vista/2003/XP/2000 file management security issues Brent Stackhouse (Mar 10)
Re: Microsoft Windows Vista/2003/XP/2000 file management security issues M. Burnett (Mar 09)
Re: Microsoft Windows Vista/2003/XP/2000 file management security issues KJKHyperion (Mar 09)
- Re: Microsoft Windows Vista/2003/XP/2000 file management security issues 3APA3A (Mar 09)