Tuesday, October 29, 2019

How to fix PDFs ending in .EXE

Malware writers can trick you in 2 ways into thinking your file is a "PDF looking" file. 

Firstly, maliciously constructed “.exe” can be built to display an PDF icon, so it looks like PDF default reader will open this file. If the filename is really long then, you can't see the extension (see image below). 

2ndly and may not be so obvious, malicious PDF filename is constructed as with a right-to-left override character is such a way that the file ends ".pdf" extension, but really is an ".exe".  


So in example below, the 2nd file looks like a ".txt" file, but is really a ".docx" file (the 1st file). The 1st file has been cleansed of the RTL Unicode character, and ends in ".docx". 

The PDF file is actually an ".exe" file, but looks like it will open with default PDF reader. 


Download RTLExample.7z ( it includes the above files with PDF ".exe" example. The files contain no viruses or malware. The PDF is safe ".exe", and just opens this page in Chrome). 

Here's the same files as viewed from the command (cmd.exe) line. The box character represents the RTL character.


Note: Detection of malicious file is never done by a filename alone, so a good antivirus will flag the contents of this file, for known signatures. BUT you can remove the annoying RTL character with the free tool below! 

How is RTLO being abused by malware writers?

In apps that support Unicode like Window Explorer, the right-to-left override malware method uses  a RTL Unicode character, that will reverse the order of the characters that follow it. It's used mainly for Middle Eastern/Asian languages that you read right-to-left.

RTLO can be used to spoof fake extensions. To do this we need a hidden RTL Unicode character in the file name.

What is “Right-to-Left Override” RTLO?
The RTLO method is used to hide the true type of a file, so it might trick you into open text file (.txt) which really is a Word file (.docx) with malicious malware. More recently this file could hide a .wav file. Audio files such .wav file are being embedded with malware, is on the forefront of malware maliciousness. Read about that on my post here.
The method exploits a feature built into Windows Explorer. Since Microsoft Windows does a great job of supporting different languages from around the world, some of those languages that are written from right-to-left (RTL). 
Let’s say you want to use a right-to-left written language, like Hebrew or Arabic, on a site combined with a left-to-right written language like English or French. In this case, you would want bidirectional script support.
Bidirectional script support is the capability of a computer system to correctly display bi-directional text. In HTML we can use Unicode right-to-left marks and left-to-right marks to override the HTML bidirectional algorithm when it produces undesirable results:
left-to-right mark: ‎ (U+200E) Unicode character
right-to-left mark: ‏ (U+200F) Unicode character
How do you fix files that have the RTLO or other bad characters ? 

Here's a tool I built to clean up Right-to-Left Mark (and many others) and Unicode Control Characters from your files. It's super fast, small and written in native C++.

Download touchRTL.7z (you need https://www.7-zip.org/ to unpack) 

To get touchPRO.7z, use contact form, Pro has flags to remove Unicode spaces and punctuations (math symbols, currency, open closing braces, and accent marks).  

Just run this command and it will recursively rename filenames to remove those characters under the specified directory name. If directory name, contains spaces you need quotes.


touchRTL -v -l -R "directory name"

where


Usage: touchRTL [-acdhlmprRtuvx] [-r REFFILE | -t TIME | -d DATETIME] FILE...

A FILE argument that does not exist is created empty, unless -c or --no-create
is supplied.

  -a, --access-time        change only the file access time
  -c, --no-create          do not create any new files - If the file exists, touch will update the access time, else will do nothing.
  -l, --RTL                remove Unicode control & format characters (esp. infamous right-to-left) from filename (ditto -u)
  -m, --modif-time         change only the file modification time
  -p, --pause-exit         pause on exit (non-GNU extra)
  -R, --recursive          recursively touch files in specified directory and all subdirectories (non-GNU extra)
  -u, --uni-cntrl-chars    remove Unicode control & format characters (esp. infamous right-to-left) from filename (ditto -l)
  -v, --verbose            output the result of every file processed (non-GNU extra)
  -x, --creation-time      change only the file creation time (non-GNU extra)
  -r, --reference REFFILE  use this file's times instead of current time

  -t, --time TIMESTAMP     use [[CC]YY]MMDDhhmm[.ss] instead of current time
                           Where
                            CC: First two digit of the year
                            YY: Last two digits of the year
                            MM: Month (two-digit numeric month)
                            DD: Day (two-digit numeric day i.e. day of month)
                            hh: Hour
                            mm: Minutes
                            ss: Seconds
                           [] indicates that field is optional

  -d, --date DATETIME      use YYYY-MM-DDThh:mm:ss[.ms] instead of current time (non-GNU, does not parse string)
                           accepted "2033-04-01T07:07:07", "2033-04-01 07:07:07", "2033-04-01 07:07:07.1200"

  -h, --help               Display this help and exit.
  --version                Display version information and exit.

Note: -d and -t options accept different time-date formats.

Copyright © 2019 Metadata Consulting <metadataconsult@gmail.com> - https://metadataconsulting.ca/
Open source by Stephane Duguay <s@binarez.com> - https://www.binarez.com/touch_dot_exe/

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