One_Half

SUMMARY

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.

Threat Assessment

Wild

• Wild Level: Low
• Number of Infections: 0 - 49
• Number of Sites: 0 - 2
• Geographical Distribution: Low
• Threat Containment: Easy
• Removal: Easy

Damage

• Damage Level: Low
• Modifies Files: .EXE files, .COM files

Distribution

• Distribution Level: Low
• Target of Infection: DOS

TECHNICAL DETAILS

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.

Recommendations

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

• Turn off and remove unneeded services. By default, many operating systems install auxiliary services that are not critical, such as an FTP server, telnet, and a Web server. These services are avenues of attack. If they are removed, blended threats have less avenues of attack and you have fewer services to maintain through patch updates.
• If a blended 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 (for example, all Windows-based computers should have the current Service Pack installed.). Additionally, please apply any security updates that are mentioned in this writeup, in trusted Security Bulletins, or on vendor Web sites.
• 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.
• Configure your email server to block or remove email that contains file attachments that are commonly used to spread viruses, such as .vbs, .bat, .exe, .pif and .scr files.
• Isolate infected computers quickly to prevent further compromising your organization. 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.