Exploit-DB #46533
Description: Microsoft Windows - '.reg' File / Dialog Box Message Spoofing
Type: dos
Platform: windows
Date: 2019-03-13
Author: hyp3rlinx

Exploit:
[+] Credits: John Page (aka hyp3rlinx)		
[+] Website: hyp3rlinx.altervista.org
[+] Source:  http://hyp3rlinx.altervista.org/advisories/MICROSOFT-WINDOWS-.REG-FILE-DIALOG-BOX-MESSAGE-SPOOFING.txt
[+] ISR: ApparitionSec          
 

[Vendor]
www.microsoft.com


[Product]
A file with the .reg file extension is a Registration file used by the Windows registry. These files can contain hives, keys, and values.
.reg files can be created from scratch in a text editor or can be produced by the Windows registry when backing up parts of the registry.


[Vulnerability Type]
Windows .Reg File Dialog Box Message Spoofing


[CVE Reference]
N/A


[Security Issue]
The Windows registry editor allows specially crafted .reg filenames to spoof the default registry dialog warning box presented to an end user.
This can potentially trick unsavvy users into choosing the wrong selection shown on the dialog box. Furthermore, we can deny the registry editor
its ability to show the default secondary status dialog box (Win 10), thereby hiding the fact that our attack was successful.

Normally when a user opens a .reg file UAC will launch (if user is run as Admin) if targeting a non privleged user we can still hijack HKCU reg settings
without having to deal with UAC. After they will get the registry security warning dialog box asking them if they "trust the source" and
"Are you sure you want to continue?" etc and will also have a choice of either 'Yes' or 'No' to select from.

However, we can inject our own messages thru the filename to direct the user to wrongly click "Yes", as the expected "Are you sure you want to continue?"
dialog box message is under our control. The registry dialog echoes back the filename plus any text we add and allows us to terminate part of its
default security warning message. We achieve this using % encoded characters in the filename like %n or %r and %0.

Example, the "do not add it to the registry" and "Are you sure you want to continue?" default warning messages can be done away with using %0.

This spoofing flaw lets us spoof the "Are you sure you want to continue?" warning message to instead read "Click Yes" or whatever else we like.
Potentially making a user think they are cancelling the registry import as the security warning dialog box is now lying to them.

Denial of secondary registry editor status dialog box (hiding successful attacks) in Windows 10:
------------------------------------------------------------------------------------------------
Typically, upon a successful import the registry editor pops up another dialog box with a status message telling us
"the keys and values contained in <REGFILE> have been successfully added to the registry".

We can obstruct that behavior to deny this secondary registry editor dialog from appearing by tacking on a (null) right before the
end of our filename using %1 or %25 like: "Microsoft-Security-Update-v1.2-Windows-10.r%e%g%r%nC%l%i%c%k%b%Y%e%s%b%b%b%1%0.reg"

If don't want to use (null) use %3 but it will display a asian char instead but still prevents the secondary registry dialog box you.
You will have to manually refresh the registry written to in order to see the values stored when using these dialog denial of service methods.

Note: Denial of the secondary dialog box seems to only work on Windows 10.

Behaviors I discovered playing with registry filenames that affect the dialog box, depending on Windows OS version you will get different results.

% - can be used for obfuscation e.g. %h%a%t%e = hate
%b will create white-space
%n makes a newline
%r makes a newline
%1 creates (null) - important as we prevent the second registry dialog from appearing after a successful import!
%0 Important terminates string
%25 (Windows 10) creates (null) - Important as we prevent the second registry dialog from appearing after a successful import!
%3 - Important as we prevent the second registry dialog from appearing after a successful import! (but shows asian char)
%5 (Windows 10) duplicates the default registry dialog box message by "n" amount of times per amount of %5 injected into the filename 
%25 (Windows 7) duplicates the default registry dialog box message by "n" amount of times per amount of %25 injected into the filename 
%2525 prevents registry editor from opening
%169 will show our junky filename in the dialog box (we don't want that)
%3, %197, %17 and some others change the default language shown in the registry dialog box to asian characters etc

Each injected character can be separated by a percent "%" sign without messing up our spoofed message, we can leverage this to obfuscate the end of the filename.
We then use %0 to terminate the message string so that the second .reg extension and default registry messages are not displayed in the registry dialog box.

The filename "Microsoft-Security-Update-v1.2-Windows-10.r%e%g%r%nC%l%i%c%k%b%Y%e%s%b%b%b%1%0.reg" will show as "Microsoft-Security-Update-v1.2-Windows-10.reg"
in the registry dialog box, along with our spoofed user directions.
 
While this spoofing vulnerability requires user interaction and bypassing Windows UAC (if targeting Admin) prompt to succeed, the fact the we can prevent secondary
registry dialogs and modify registry messages displayed to the user makes it a viable attack vector. If we are successful in our attack we can achieve a persistent
RCE backdoor all while the user thinks they have aborted the import. Moreover, targeting a non privileged user allows us to hijack programs and not worry about UAC.


[POC Video URL]
https://vimeo.com/322684636


[Exploit/POC]
Persistent Remote Code Execution Backdoor: 

This will add entry to "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\iexplore.exe"
for a persistent rundll32 payload targeting MSIE that references a JScript XML based file on our remote server.

1) Create a Windows .REG Registry