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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.
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)
Original advisory: http://securityvulns.com/advisories/winfiles.asp
Securityvulns.com news: http://security.nnov.ru/news/Microsoft/Windows/files.html
0. Intro
This article 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.
Different situations are possible, where autoexec.bat is writable by
attacker, if:
- Default Windows 2000 permissions are used or applied with domain
policy [2].
- One can try to re-create autoexec.bat using POSIX subsystem
III. Neither xcopy nor other 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.
2. 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.
3. 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
[2]. Windows 2000 system partition weak default permissions
http://securityvulns.ru/news2205.html
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