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Copyright: Dr. Nikolai Bezroukov 1994-2013. Unpublished notes. Version 0.80.October, 2013
Contents : Foreword : Ch01 : Ch02 : Ch03 : Ch04 : Ch05 : Ch06 : Ch07 : Ch08 : Ch09 : Ch10 : Ch11 : Ch12 : Ch13
Chapter 5: Macro Viruses
Introduction *
How the Virus Operates *
How to Detect the Virus *
How to Disinfect Infected Files *
Disinfection, using antivirus software *
Manual disinfection *
Disinfection using Microsoft Excel XLSCAN tool *
Laroux is a macro virus that infects Microsoft Excel spreadsheets. It contains no payload and just replicates, so the result of infection is lost time and, in case of panic, possibly some data.
Laroux infects Excel versions 5 (included in Office 4.3), version 6 (Excel95 from Office 95) including localized (German, French, etc.) versions. Version 8 (Excel 97 from Office 97) has a built-in tool to detect macros and will warn users. During conversion to VBA5, Laroux will be detected and disabled. As such, Excel 97 is to a certain extent protected from Laroux (it was especially written for older versions of Excel). At the same time, various versions of Laroux that are manually converted to VBA5 can spread in the Excel 97 environment. LAROUX is platform dependent and because of the way it searches for PERSONAL.XLS (PERSONAL..XLS is a DOS filename), it does not function properly on Macintosh platforms.
Attention: If user has PERSONAL.XLS file in the Excel XLSTART directory, then Laroux virus will not infect this file. So creation of the PERSONAL.XLS is a very effective measure for preventing spread of Laroux.A. The simplest way to create it is to install Microsoft XLSCAN add-on available from Microsoft site and from NETWARE95_US server (in the directory DOS\UTILS\BBS\ANTIVIR\EXCEL\ )
lso if Excel is infected with Laroux, after disinfection Excel became immune to further infections as Laroux.A creates the PERSONAL.XLS file in the Excel XLSTART directory that contain the virus body and check for the presents of this for on further infections. When a user opens an infected attachment the virus checks for the presence of this file and if it is present does not infect Excel environment. As after the first disinfection the file PERSONAL.XLS is always present in the XLSTART directory Excel became immune to further infections.
The virus exists in several modifications: Laroux, Laroux.B, Laroux.E, etc. Only Laroux B has the same effects as Laroux, but can also overwrite macros in Personal.XLS.
Excel macro viruses are quite close relatives of MS Word macro viruses, and the knowledge of how to fight MS Word macro viruses can be easily transferred to this new type of virus. For example, LAROUX consist of two macros, namely auto_open and check_files. They are visible in the Tools|Macro menu and can be deleted manually (See below).
The infected spreadsheets are usually spread via e-mail attachments.
Laroux virus checks the XLSTART directory to see if PERSONAL.XLS is present. If it is there, the virus won't infect it. If it is not there, the virus creates it and copies its Macros to the hidden sheet called "laroux". Macros from PERSONAL.XLS are loaded when Excel is started much like templates in the STARTUP directory in MS Word. This means that if PERSONAL.XLS is infected, virus macros will be loaded each time Excel is started.
PERSONAL.XLS is the default filename for any macros recorded under Microsoft Excel and is similar to NORMAL.DOT in MS Word. One can (and probably should, as it will protect him/her from the virus) have PERSONAL.XLS even he/she is not infected by this virus.
The startup path is by default set as \MSOFFICE\EXCEL\XLSTART, but it can be changed from Microsoft Excel's Tools|Options|General|Alternate Startup File menu option.
1. Start Microsoft Excel.
2. Click Macro on the Tools menu.
3. Infection is likely if the following macro names are listed:
Auto_Open
Check_files
PERSONAL.XLS!auto_open
PERSONAL.XLS!check_files
4 .If you have any infected workbooks open in the background, you may also see the following names listed:
<bookname>!auto_open
<bookname>!check_files
(where <bookname> is the name of the opened workbook).
5. The virus also creates the personal.xls file in the C:\MSOFFICE\EXCEL directory if one does not exist. Infected PERSONAL.XLS includes a hidden worksheet named "laroux." To make it visible, use Windows|Hide, File|UnHide.
There are several methods for Laroux disinfection:
F-macro 2.12b or later will disinfect infected spreadsheets from the virus. Earlier versions of F-macro are not recommended.
To manually disinfect Excel Macro/Laroux:
1. Start Microsoft Excel.
2. Click Macro on the Tools menu.
3. Delete any of the following macro names that appear in your workbook:
Auto_Open
Check_files
PERSONAL.XLS!auto_open
PERSONAL.XLS!check_files
4. Click Exit on the File menu and click "Yes" to save all changes. Excel is now clean. Note: If the user does not have any macros stored in PERSONAL.XLS, he/she can simply delete PERSONAL.XLS using file manager or file explorer to make Excel clean.
5. Continue to open all infected workbooks one by one, starting with the workbooks from the "recently used list" (at the bottom of the File menu). Press and hold the shift key while you open them to bypass any automacros.
6. For each workbook, click Macro on the Tools menu. If Auto_open and Check_files macros exist, delete these macros.
7. Click Save on the File menu to save disinfected file.
Microsoft Virus Tool is now available for Microsoft Excel 5, 6(Excel 95) and 7(Excel 97). This tool is called XLSCAN.XLA (XLSCAN97 for Excel 97) It is available from: http://www.microsoft.com/excel/productinfo/vbavirus/add_in.htm and from NETWARE95_US. server (in the directory DOS\UTILS\BBS\ANTIVIR\EXCEL\ ). Documentation on how to install it is in the file XLSCAN.HTM.
The latest one is XLSCAN97, designed for Excel 97 is also available.
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