Email-Worm.Win32.Cholera

Parent class: VirWare

Viruses and worms are malicious programs that self-replicate on computers or via computer networks without the user being aware; each subsequent copy of such malicious programs is also able to self-replicate. Malicious programs which spread via networks or infect remote machines when commanded to do so by the “owner” (e.g. Backdoors) or programs that create multiple copies that are unable to self-replicate are not part of the Viruses and Worms subclass. The main characteristic used to determine whether or not a program is classified as a separate behaviour within the Viruses and Worms subclass is how the program propagates (i.e. how the malicious program spreads copies of itself via local or network resources.) Most known worms are spread as files sent as email attachments, via a link to a web or FTP resource, via a link sent in an ICQ or IRC message, via P2P file sharing networks etc. Some worms spread as network packets; these directly penetrate the computer memory, and the worm code is then activated. Worms use the following techniques to penetrate remote computers and launch copies of themselves: social engineering (for example, an email message suggesting the user opens an attached file), exploiting network configuration errors (such as copying to a fully accessible disk), and exploiting loopholes in operating system and application security. Viruses can be divided in accordance with the method used to infect a computer:

• file viruses
• boot sector viruses
• macro viruses
• script viruses

Any program within this subclass can have additional Trojan functions. It should also be noted that many worms use more than one method in order to spread copies via networks.

Class: Email-Worm

Email-Worms spread via email. The worm sends a copy of itself as an attachment to an email message or a link to its file on a network resource (e.g. a URL to an infected file on a compromised website or a hacker-owned website). In the first case, the worm code activates when the infected attachment is opened (launched). In the second case, the code is activated when the link to the infected file is opened. In both case, the result is the same: the worm code is activated. Email-Worms use a range of methods to send infected emails. The most common are: using a direct connection to a SMTP server using the email directory built into the worm’s code using MS Outlook services using Windows MAPI functions. Email-Worms use a number of different sources to find email addresses to which infected emails will be sent: the address book in MS Outlook a WAB address database .txt files stored on the hard drive: the worm can identify which strings in text files are email addresses emails in the inbox (some Email-Worms even “reply” to emails found in the inbox) Many Email-Worms use more than one of the sources listed above. There are also other sources of email addresses, such as address books associated with web-based email services.

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Platform: Win32

Win32 is an API on Windows NT-based operating systems (Windows XP, Windows 7, etc.) that supports execution of 32-bit applications. One of the most widespread programming platforms in the world.

Description

Technical Details

This is a virus-worm that spreads via the Internet and local network. It appears as a "SETUP.EXE" file attached to an e-mail message that has the "Ok..." subject and the message body contains just a "smile":

:-)

This attached file itself is a Microsoft C++ executable file about 40Kb in length. The majority of the file's code is occupied by C++ run-time libraries and data, and just about 7Kb of its code is "pure" worm code.

The worm got its name because of the text string in its code:

CH0LERA - Bacterium BioCoded by GriYo / 29A

This string, as well as other of the worm's data, are encrypted in the worm's body.

Installing into the system

When the worm is executed for the first time (being run from an infected attachment), it gets its module name and installs itself to the Windows directory with the RPCSRV.EXE name. To force Windows to run this file upon the next reboot, the worm writes an additional "Run=" instruction to the WIN.INI file in the Windows directory (under Win9x), or modifies a corresponding key in the system registry (under WinNT).

To locate the Windows directory, the worm does not call corresponding Windows functions, but scans all available local drives, looks for subdirectory names: WINDOWS, WIN95, WIN98, WIN, WINNT, and then looks for a WIN.INI file in the directory. If such a file is located, the worm installs itself into the directory.

As a result, the worm may create several of its copies on the same computer, and infect all Windows installations on it. In case a multiboot loader is installed, and there are several different Windows versions installed, this trick allows the virus to activate upon any Windows copy start-up.

To hide its activity, the worm displays the fake message:

Further spreading

Upon the next Windows start-up, the worm copy is activated by the Run command in the WIN.INI file. It takes control, registers itself in the Windows memory as a hidden application (invisible service) that also allows the worm to stay active whenever a user logs off. The worm then runs two more routines in addition to the installation one. The first of these new routines spreads the worm through the local network, and the second one sends infected e-mail messages. The installation routine is also active, and the worm is able to infect a new Windows copy if it appears on the computer. All the routines are run as main-process threads, so they do their work in parallel.

The first of the new routines spreads the worm copy through the network. It enumerates all network drives, scans them for Windows directories, copies the worm's RPCSRV.EXE file to there and registers it in the WIN.INI file in the same remote directory. As a result, upon the next rebooting the worm on a remote computer will be activated and spread itself further.

The second routine sends infected messages to the Internet addresses. To send its copy, the worm uses SMTP protocol and sends itself by direct connection, and as a result, the worm spreading does not depend on the type of e-mail application that is used in the system.

Once per six seconds, this routine enumerates all active program windows and looks for Internet applications: Outlook, Cuteftp, Internet Explo, Telnet, Mirc. If any of these applications is active, it means that the computer is connected to the Internet (this is necessary because of the direct SMTP connection used by worm).

The worm then gets SMTP server address and user e-mail addresses from the system registry keys, builds a new message, attaches its copy with the SETUP.EXE name and sends it.

The Internet addresses to where the worm sends its copies are collected from disk files in the Windows directory and subdirectories. The worm scans all files there, searches for files with extensions .HTM, .TXT, .EML, .DBX, .MBX, .NCH, .IDX, and then scans these files and gets email-address-like strings from there. Upon each sending, the worm sends itself to not more than ten addresses.

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