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Dumaru, W32.Dumaru@mm


This mass-mailer worm was discovered on 19th of August, 2003. Dumaru is a file infector and a mass-mailer worm which tries to disguise itself as a security patch coming from Microsoft. The worm drops an IRC-controlled backdoor component to the infected system.


Based on the settings of your F-Secure security product, it will either move the file to the quarantine where it cannot spread or cause harm, or remove it.

A False Positive is when a file is incorrectly detected as harmful, usually because its code or behavior resembles known harmful programs. A False Positive will usually be fixed in a subsequent database update without any action needed on your part. If you wish, you may also:

  • Check for the latest database updates

    First check if your F-Secure security program is using the latest updates, then try scanning the file again.

  • Submit a sample

    After checking, if you still believe the file is incorrectly detected, you can submit a sample of it for re-analysis.

    Note: If the file was moved to quarantine, you need to collect the file from quarantine before you can submit it.

  • Exclude a file from further scanning

    If you are certain that the file is safe and want to continue using it, you can exclude it from further scanning by the F-Secure security product.

    Note: You need administrative rights to change the settings.

Technical Details

Dumaru is packed with an unmodified version of UPX. The unpacked size of the worm is 20480 bytes.

When first run the worm infects the system by placing several of its copies in the system.

One copy goes to the System Directory as 'load32.exe' which is added to the registry as:

  • 'HKLM\Software\Microsoft\Windows\CurrentVersion\Run\load32'

Another copy of the worm is placed to the Windows Directory using the file name 'dllreg.exe' and added to 'win.ini' as follows:


Third one is copied to System Directory as 'vxdmgr32.exe' which is registered to 'system.ini':

Shell=explorer vxdmgr32.exe

The backdoor is dropped to the Windows directory as 'windrv.exe'and started. This file is detected by F-Secure Anti-Virus as Backdoor.Small.d.

Email propagation

Dumaru uses its own SMTP engine to send emails with infected attachments. The worm searches for email addresses on all drives recursively in files with the following extensions:


Using its SMTP engine Dumaru sends infected emails to the addresses it collected. The infected emails have the following appearance:

From: "Microsoft" []
Subject: Use this patch immediately !
 Dear friend , use this Internet Explorer patch now!
There are dangerous virus in the Internet now!
More than 500.000 already infected!
 Attachment: patch.exe

The email addresses the worm collects are written to a file called 'winload.log' in the Windows Directory.

File infection

If the infected system is installed on NT Filesystem Dumaru tries to infect EXE files with a companion method using the streams feature of NTFS. The original file content is copied to 'filename.exe:STR' stream and the file 'filename.exe' is overwritten with a copy of the virus. When 'filename.exe' is invoked the worm executes 'filename.exe:STR' instead.