The first is the one that has become most common in recent times -- mass mailing. An e-mail message will show up in a mailbox; usually with a garbled subject line and no text. There will be an attachment which looks like a .WAV file, but in fact links to an executable called "readme.exe". Nimda will attempt to exploit the usual Outlook vulnerabilities. These entail instances where the default options allow the software to launch attached files without user interaction. (Always a bad idea.) Thus, even previewing the message can trigger the worm.
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Assuming you have been updating Outlook with the security patches and checking the settings, this should not be a problem so long as your users have been advised about all kinds of attachments and are conscientious.
As with Melissa, the worm, once run, can send messages to every sender currently in your In Box and/or address book, as well as reply to the sender of the virused message itself.
window.open("readme.eml", null, "resizable=no,top=6000,left=6000")
The third delivery method is Web-based. Like the Code Red Worm of a few months ago, Nimda sends its code over Port 80 as a HTTP request, attempts to copy itself to unpatched Microsoft IIS web servers, which in turn will allow the worm to run on those machines.
The consequences of being struck by Nimda are nasty and annoying, but not particularly destructive. Outside of the obvious fact that your security has been compromised, the greatest annoyances will be the potential flooding of your e-mail gateways and the DoS. Once the virus has been eradicated, these will go away. If your intranet or Internet pages use standard page formats, they may be compromised with the code mentioned above.
An Ounce of Prevention
As is always the case, taking precautions before trouble strikes is always your best protection. Make sure that you are using the latest patches and updates for your servers, gateways, e-mail clients, and security-related software. Use a firewall and/or filters to assess incoming mail attachments. Be sure to educate your users about attachments and executables, and don't forget to mention that many files can be suspect no matter what their filename extension may be or what icon is displayed.
Don't allow software defaults to rule your decisions. *Never* allow e-mail clients or browsers launch executables automatically. Be certain you know which computers on your network have Network Share turned on, or any kind of peer-to-peer capabilities enabled.
Here are some patches and software you need to be aware of:
- MS patch to prevent Internet Explorer from automatically running e-mail attachments:
- MS patch for IIS Patch Available for "Web Server Folder Traversal" vulnerability
- MicroWorld Software Services Pvt. Ltd. has released a free tool to remove
the Nimda Worm from PCs
Jim Freund is the Managing Editor of CrossNodes, where this article first appeared.