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AOL4Free Trojan Horse

AOL4Free Trojan Horse

16 April 1997
13 February 2007

This trojan horse program should not be confused with the AOL4Free virus hoax message, which was distributed under the same name in the same timeframe (March 1997).

Likelihood: Common
Region Reported: America Online e-mail
Characteristics: Trojan Horse
Target Platform: DOS, Windows 3.1, Windows 95

The AOL4Free trojan horse program was first reported as being distributed through America Online e-mail in early March 1997.

Attached to the e-mail message is the archive file named AOL4FREE.COM, which is actually converted from a batch file using the DOS utility BAT2EXEC version 1.5. This utility is commonly used for converting large batch files to enhance speed. This trojan horse first searches for the DOS program DELTREE.EXE in various directories, and then uses DELTREE.EXE to delete all files from your C drive.

After deleting your files, it produces the DOS error message "Bad Command or file name" and continuously displays an obscene message. AOL4FREE can't delete your files if it is unable to find DELTREE.EXE, but the obscene message will always display. This works on both DOS and Windows 95 environments as long as DELTREE.EXE is present and accessible.

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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.