Threat Explorer

The Threat Explorer is a comprehensive resource consumers can turn to for daily, accurate, up-to-date information on the latest threats, risks and vulnerabilities.



13 February 2007
Also Known As:
Dis, Free Love, One_half, One Half.3544

One_Half is a dangerous virus. It slowly encrypts any hard drive that it infects. The encrypted portion of the drive is available to the user as long as the virus remains in memory. When about half the drive has been encrypted, the virus displays the message:

Dis is one half.
Press any key to continue...

Once the virus is removed, the encrypted data is lost unless the antivirus program is capable of un-encrypting it.

Antivirus Protection Dates

  • Initial Rapid Release version 15 December 2000
  • Latest Rapid Release version 20 August 2008 revision 017
  • Initial Daily Certified version 15 December 2000
  • Latest Daily Certified version 20 August 2008 revision 016
  • Initial Weekly Certified release date pending
Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

One_Half is an advanced multipartite virus. It infects the master boot record (MBR) on the first physical hard disk (drive 80h, the C: drive) and .EXE and .COM files. One_Half uses stealth techniques to hide the MBR infection and polymorphic techniques to make file detection and removal difficult to impossible. When the virus is in memory, a clean copy of the MBR is displayed and the infection size is hidden when files are displayed.

The MBR infection routine is rather generic. After initial infection, One_Half uses the last eight sectors of side 0, track 0 to store its additional infection code and a clean copy of the MBR and partition table. A major concern is that One_Half slowly encrypts any hard drive that it infects. When an infected hard drive is cold booted, One_Half encrypts two cylinders at the end of the hard drive with an XOR routine and a random key (it does not encrypt the diagnostic cylinder). Each successive cold boot from the hard drive results in two more cylinders becoming encrypted . These encrypted cylinders are available to the user only as long as One_Half remains in memory. When it has encrypted approximately one-half of the hard drive, One_Half displays the following message:

Dis is one half.
Press any key to continue...

One_Half poses a significant problem for anti-virus programs that use generic repair or inoculation techniques. Although a generic repair successfully removes One_Half from an infected MBR, all data in the encrypted areas is lost. For example, running FDISK/MBR removes the virus, but all data in the encrypted area of the drive is lost.

One_Half only infect files with a .COM or .EXE extension. During the file infection routine, One_Half first scans the filename for text strings relating to anti-virus software. If it finds SCAN, CLEAN, FINDVIRU, GUARD, NOD, VSAFE, or MSAV in the filename, it does not infect. If infection is successful, One_Half inserts portions of itself into random points within the host file and appends the bulk of the encrypted infectious code.

One_Half appears to be generally compatible with most versions of DOS and Windows 3.1. Some Windows configurations do not load when One_Half is memory.


Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

  • Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
  • Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
  • Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
  • Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
  • Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
  • Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
  • If a threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
  • Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services.
  • Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
  • Isolate compromised computers quickly to prevent threats from spreading further. Perform a forensic analysis and restore the computers using trusted media.
  • Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.
  • If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
  • For further information on the terms used in this document, please refer to the Security Response glossary.