Summary

Fever is an Internet worm that spreads itself as an attachment to email messages. The worm works under Win32 systems only. The worm itself is a PE executable about 8 kb long and is compressed with Petite file compressor.

When the worm attachment is opened, it activates and first checks if the system is already infected. If not, it tries to copy itself to Windows System directory with a random name (the name is generated from a victim computer's name and looks like this '*32.EXE', for example 'BM32.EXE').

On Windown 9x systems the worm adds a startup key for its file to the following key in the Registry:

 HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\RunServices

On Windows NT systems the worm uses Service Control Manager to register itself as a service.

As a result of the above manipulations, Windows will launch the worm's file automatically at every startup.

The worm uses Windows debugging capabilities to capture WSOCK32.DLL loading and modifies the beginning of 'send' function so that it will give control to the worm's code when called. This way the worm can intercept and analyse the outgoing traffic. It captures SMTP commands to get the e-mail addresses. Whenever an email is sent, the worm remembers the sender's and recepient's e-mail addresses and sends a copy of itself to the recepient of the previous e-mail.

The infected e-mail message sent by Fever worm looks like that:

 From: sender@address
 To: recepient@address
 Subject: pic.gif

 [empty body]

The worm's file with .SCR extension is attached to every message.

The worm contains the following text strings:

 '[ Yellow Fever BioCoded by GriYo / 29A ]'

 '[ Disclaimer: This software has been designed for research purposes only.
 The author is not responsible for any problems caused due to improper or
 illegal usage of it ]'

[Analysis: Gergely Erdelyi, F-Secure Corp.; May 2001]