Form

Form is a virus with viral code that is two sectors in length. Form reserves 2K of memory by modifying the available memory word at 40:13. Thus, on a 640K machine, MEM would report 638K and CHKDSK would report 653,312 bytes of free memory. On the hard drive, the second virus sector and the original boot sector are stored on the last two sectors of the infected drive. The virus does not protect these sectors, so they can be overwritten by data at a later time, making the drive hang during the boot sequence. The drive, however, will still be accessible. On a floppy disk, the virus stores its second sector and the original boot in an unused cluster and marks the cluster as bad in the FAT to protect it. CHKDSK reports 1024 bytes (1K) in bad sectors on an infected floppy disk. Upon booting from an infected disk, the virus checks the date using interrupt 1Ah function 4. The virus checks the value in the dl register for 18. This return is in binary-coded decimal, not hexadecimal , so the virus is testing for the 18th of the month, not the 24th. Some sources mistakenly have the 24th as the trigger date. On the 18th of every month, the virus produces a clicking sound whenever a key is pressed. The second virus sector contains the text:

The FORM-Virus sends greetings to everyone whos reading this text. FORM doesnt destroy data! Dont panic!

It also includes a short, obscene message to someone named Corinne. The message is not displayed, but can be seen by examining the next to last sector on the hard drive partition or the first sector in the bad cluster on a floppy disk with a disk editor. While the Form virus contains no intentionally damaging code, two bugs in the virus may cause problems. First, the virus does not retry disk reads and hangs the system on a failed disk read. Second, the unprotected second sector and original DOS boot sector on the hard disk may be overwritten if the drive fills completely, rendering the drive unbootable.

Recommendations

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.