SEC Consult Vulnerability Lab Security Advisory < 20191211-0 >  
title: File Extension Spoofing  
product: Windows Defender Antivirus  
vulnerable version: 4.18.1908.7-0  
fixed version: Virus Definition Update of 2019/09/30  
CVE number: -  
impact: High  
found: 2019-09-25  
by: David Haintz (Office Vienna)  
SEC Consult Vulnerability Lab  
An integrated part of SEC Consult  
Europe | Asia | North America  
Vendor description:  
"Keep your PC safe with trusted antivirus protection built-in to Windows 10.  
Windows Defender Antivirus delivers comprehensive, ongoing and real-time  
protection against software threats like viruses, malware and spyware across  
email, apps, the cloud and the web."  
Business recommendation:  
Update to the latest version of the Windows Defender Antivirus definitions.  
Vulnerability overview/description:  
The vulnerability is based on the file extension spoofing method using the RTL  
unicode character to display a spoofed file extension. This method uses the LTR  
unicode character, that instructs the following text to be shown in left-to-right  
order. Lets assume [LTR] is the LTR unicode character, an attacker can use this  
unicode character to fool a user into believing that a file has a different extension.  
For example an attacker may name an executable file (.exe) 'spoofed-[LTR]gpj.exe',  
which would be displayed as 'spoofed-exe.jpg' on an LTR-based system. The most important  
point here is to have the extension you want to be shown in reverse order, since it will  
be shown right-to-left.  
Combined with the right file icon, an attacker can imitate an arbitrary file extension.  
Same goes for other extensions too, like 'xlsx' for a Microsoft Excel Sheet. During testing  
it happened that 'xlsx' was typed in the wrong order ('xslx' instead of 'xlsx' since reverse  
order) and Windows Defender Antivirus removed the test file while we tried to execute it.  
As a result, two files were created, with the exact same executable but with different fake  
1. spoofed-[RTL]xslx.exe (displayed as 'spoofed-exe.xlsx')  
2. spoofed-[RTL]xlsx.exe (displayed as 'spoofed-exe.xslx')  
The second one was deleted, while the first one could be executed without any problem.  
Therefore, other extensions related to Microsoft Office were tested as well, but it seems  
only the xlsx extension had a detection for it.  
While the security issue of spoofing the file extension by using the RTL unicode character  
(on RTL systems it is the same just with LTR) is widely known, it seems to be unknown that  
Microsoft already started to add detection mechanisms for this issue. But since it is not  
implemented for all extensions and it seems to be implemented in the wrong order, this  
feature is mostly unknown.  
Proof of concept:  
For the proof of concept a file has to be renamed in Unicode mode using the Unicode  
character '202E' ('\u202E' in C), which stands for RTL. The sample code is written in  
C/C++ and uses the unicode API of Windows. A Python PoC has been made as well.  
#include <Windows.h>  
int main(int argc, char** argv)  
wchar_t opath[] = L"test.exe";  
wchar_t npath_ok[] = L"spoofed-\u202Exslx.exe"; // String for filename 'spoofed-exe.xlsx'  
wchar_t npath_wrong[] = L"spoofed-\u202Exlsx.exe"; // String for filename 'spoofed-exe.xslx'  
// Copy 'test.exe' to file shown as 'spoofed-exe.xlsx'  
CopyFileW(opath, npath_ok, false);  
// Copy 'test.exe' to file shown as 'spoofed-exe.xslx'  
CopyFileW(opath, npath_wrong, false);  
from shutil import copyfile  
opath = "test.exe"  
npath_ok = "spoofed-\u202Exslx.exe" # String for filename 'spoofed-exe.xlsx'  
npath_wrong = "spoofed-\u202Exlsx.exe" # String for filename 'spoofed-exe.xslx'  
# Copy 'test.exe' to file shown as 'spoofed-exe.xlsx'  
copyfile(opath, npath_ok)  
# Copy 'test.exe' to file shown as 'spoofed-exe.xslx'  
copyfile(opath, npath_wrong)  
There will be two new files after the execution (as long as 'test.exe' exists) and the file  
shown as 'spoofed-exe.xslx' will be deleted while trying to execute (or earlier) as shown  
in figure 1.  
[ win-defender-ext-spoofing1.png ]  
Figure 1: File gets deleted by Windows Defender Antivirus.  
But the file shown as 'spoofed-exe.xlsx' will be executed without any problem.  
[ win-defender-ext-spoofing2.png ]  
Figure2: Test file is executed.  
Vulnerable / tested versions:  
Windows Defender Antivirus has been tested in its latest version 4.18.1908.7-0, updated at 25th  
of September 2019.  
Vendor contact timeline:  
2019-09-26: Providing vendor the advisory through  
2019-10-01: Microsoft answered that this is no vulnerability, but the virus definition  
database will be updated  
2019-12-11: Public release of security advisory  
The update of the virus definition database of the 30th of September provides a fix.  
There is no workaround available.  
Advisory URL:  
SEC Consult Vulnerability Lab  
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EOF David Haintz / @2019