Conficker: What It Is, How to Stop It and Why You May Already Be Protected

Conficker is a work of malware that, in the form of multiple variants, has been worming its way through unpatched Windows desktop and server machines for the past four months.

Conficker has garnered mainstream attention of late due to an April 1 trigger that researchers have identified in the most recent variant of the worm. On this date, it appears that Conficker-infected machines will change the way that they "phone home" to fetch new code and instructions from whoever holds the worm's reins.

In October 2008 Microsoft released a fix for the vulnerability that Conficker exploits, in a patch that Microsoft deemed critical enough to release outside of its typical Patch Tuesday schedule. Still, enough Windows machines have remained unpatched for Conficker to spread to what security researchers estimate to be millions of machines.

Presumably, the goal of Conficker's controllers involves the creation of a botnet that would carry out illegal machine-based activities by proxy, but there's no telling exactly what the worm's makers have in mind.

The prescription for Conficker prevention is prompt system patching (particularly when Microsoft singles out a fix for out-of-band distribution), combined with client firewall and anti-virus software for blocking the worm's activities and detecting and eliminating the malware where it surfaces.

In addition, members of the security community have prepared a set of freely available tools to aid in Conficker detection and removal for infected systems on your network.

More broadly, Conficker calls attention to the problems inherent in deploying client systems that offer up network-facing services to anonymous nodes, and highlights the importance of watching more closely the privileges granted to the system-level applications that run on mainstream operating systems.

Moreover, because Windows Vista and Windows Server 2008 machines have proved to be significantly less vulnerable to Conficker than systems running Windows 2000, XP and Server 2003, the worm also highlights the very real consequences of stepping off the so-called operating system upgrade treadmill. For all its hardware refresh requirements, potentially unwanted feature adjustments and software incompatibility wrinkles, Vista includes security enhancements that blunted the effect of Conficker on unpatched systems.

It's up to companies to consider whether to interpret all of this as a call to approach Windows upgrades—and their associated costs—with greater alacrity, or to step up evaluation of OS alternatives, such as Linux, with less upgrade friction and a better defined road map around trusted OS technologies.

How Does Conficker Work?

Conficker's primary means of propagation involves exploiting a buffer overflow vulnerability in Windows' Server system service, which is responsible for, among other things, enabling the sharing of local resources, such as disks and printers, with other machines on a network.

Conficker exploits this vulnerability to execute code on Windows systems, without requiring a system's user to open any file or visit any particular Web site—and without regard to whether a user is running with administrative or limited privileges.

Windows 2000, XP and Server 2003 are particularly vulnerable to Conficker because the affected Server service on these systems is configured to permit access from anonymous users. In October 2008, Microsoft provided information on removing the ACL (access control list) entry that permits this anonymous access, but since the ACL involved is hard-coded into the Windows DLL, this access modification would have had to be made after every boot.

With Windows Vista, Windows Server 2008 and the development builds of Windows 7, the vulnerable service limits access to authenticated users by default, but enabling the no-password file-sharing option on these systems would restore anonymous access—and vulnerability to Conficker.

Unpatched Windows XP SP2, Vista and Server 2008 machines shipped out of the box with Windows' firewall enabled to block the vulnerable RPC (remote procedure call) interface, but the common firewall exception that enables file and print sharing opened the door to Conficker. Even with a firewall exception, however, Vista and Server 2008 machines would allow access to the vulnerable service only from other machines in the same network zone. For instance, sharing a resource on a private network would not permit access to Conficker-infected nodes.

Firewall and service authentication requirements aside, Windows Vista and Server 2008 worked to mitigate Conficker infection with Address Space Layout Randomization, which, combined with the Data Execution Protection functionality introduced in XP SP2, makes it significantly more difficult to exploit buffer overflow vulnerabilities such as the one targeted by Conficker.