Scalper affects systems running FreeBSD running the vulnerable version of Apache web server.
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Scalper is a worm that propagates from a FreeBSD system to another via a security vulnerability in Apache web server, known as chunked encoding vulnerability.
At the time of writing, F-Secure have not received any direct reports about infected systems from the field.
If the worm gains access to the server, it creates a temporary file "/tmp/.uua", which is an uuencoded worm. This file is decoded to "/tmp/.a" and executed. The uuencoded file is removed.
At this point the worm sets up a backdoor to UDP port 2001 and starts scanning predefined set of Class-A networks. If the worm finds a web server, it checks if the server is running Apache, and if so, it will attempt to infect it. While the exploit code that Scalper uses will only infect systems running FreeBSD, these attempts will be visible in Apache servers running on other platforms as well.
The backdoor component of the worm allows a remote control of the worm, sending of email, uploading of files and executing of arbitary programs. The execution of programs happens with the same user privilege as the Apache server. The backdoor can also perform different kind of denial of service attacks against arbitary hosts.
The worm does not modify the system configuration, and it is visible in the system process list as a process ".a".
Scalper can be removed from the system by deleting file "/tmp/.a" and terminating the worm process with command "killall -9 .a".
The vulnerability used by the worm is fixed in Apache server versions 1.3.26 and 2.0.39. Further information is available from:
Apache Sofware Foundation: https://httpd.apache.org/info/security_bulletin_20020620.txt