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Michael Horowitz's picture
Michael Horowitz

Defensive Computing

Removing malware is best done from the outside

A few days ago, at WashingtonPost.com Brian Krebs blogged about businesses that had money stolen from them courtesy of malware on their computers.

One of the companies was Slack Auto Parts in Gainesville, Ga., which lost nearly $75,000 when "cyber intruders used malware planted on the controller's Windows PC .. [to] ... break into the company's bank accounts, create new user accounts at the bank, and then wire payments to money mules around the country."

What makes this particulary interesting is that after the fact, the anti-virus software used by the company (which Krebs did not identify) failed to find any malware. So too a "hired cyber security expert" gave the infected machine a clean bill of health. It wasn't until the company sought a second opinion was the keystroke logging "Clampi" Trojan horse program detected.

The failure of an anti-malware application to detect a particular piece of malware is not news. Many malicious programs do a great job of hiding themselves.

What to do?

Scan a suspect system from the outside, without the suspect operating system running.

This insures that the malicious software does not get a chance to defend itself.
 
My preferred software to enable scanning from the outside is the free Ultimate Boot CD for Windows (UBCD4WIN). It includes a handful of free anti-malware programs that can run from the CD and even self-update themselves before scanning. Among the free software included with UBCD4WIN is Avira's AntiVir, SUPERAntiSpyware, McAfee's Stinger, Spybot Search and Destroy and AVG free. A full list is available here.

In addition to running software off the CD, UBCD4WIN can also share the infected C disk over a network, allowing it to be safely scanned by your favorite anti-malware program residing on another (presumably clean) computer.
 
Scanning from the outside is a great first crack at detection, but, by itself, it's not sufficient. You still need to scan for malware from inside the infected operating system after scanning from the outside. The main reason: the registry.
 
Interestingly, both MalwareBytes and SUPERAntiSpyware are working on mounting the infected registry even when they scan a system from the outside. This should be a big step forward in malware detection and removal.
 
For more, see a series of articles I wrote on this subject for eSecurity Planet: 

 

What People Are Saying

I read the third post, but i

I read the third post, but i don't understand...dose the infected machine with UBCD4WIN have to be on a network (i.e. home network or business network) or can it connect with the clean computer through remote desktop connections or remote assisence connections and do all of the steps?

explaining UBCD4WIN

The infected machine, running UBCD4WIN does not need to be on any network. If you are happy with the anti-malware software on the UBCD4WIN CD, then just connect the machine to the Internet so the anti-malware software can self-update and run the software from the CD.

To use other anti-malware software to scan the infected machine over a LAN, I documented sharing the C disk of the infected computer using normal ordinary shared folders.

There is some remote control software included with UBCD4WIN, but I don't see a need for it.

scanning from outside doesn't work with FDE

Scanning from outside doesn't work when a drive has been enciphered with a Full Disk Encryption product.

I don't know about other products

but Check Point Endpoint Security Full Disk Encryption offers the possibility to create a bootable CD or USB (with WinPE) that will allow you to access the disk but I haven't heard too many success stories.

Excellent point

That's an excellent point and is another argument against FDE. To me, FDE looks great on paper and is an easy sell to non-techies, but I think sensitive files are better off being stored on an encrypted external hard drive. I wrote about the
Lenovo ThinkPad USB Secure Hard Drive
recently and, not to shill for Lenovo (I have not used this particular drive) but this is seems a better approach.

Another advantage to the encrypted external hard disk is that its fast and easy to protect files, just pull out the USB wire.

Another disadvantage to FDE is how it affects disk image backups.

What about pagefile?

When using an encrypted external hard disk, isn't there still the possibility that your sensitive data could be in plain text within your pagefile, which would be on the internel, non-encrypted hard disk? My understanding is that any swapping your system does to the unencrypted disk would remain in plain text until it's written over.

pagefile and encrypton

That's a great point. I suppose, that yes, after using an application to process a sensitive file that parts of the file could exist on the paging file even after closing the application. I'm no expert in exactly how Windows treats the pagefile though. But, its not just Windows, every OS has a page/swap file.

Windows can run without a pagefile, assuming, of course, that you have enough ram. I'm not sure if OSX and Linux can run without a paging file.

Linux & pagefile

I was trying to leave Linux out of this ;) but since you mention it -- Linux *can* be configured without a swap partition, but it is not *usually* done. An exception is on netbooks with small HD's (like SSD's). But you're right, all OS's generally have a plain-text page/swap file/partition unless you specifically configure them not to.

I know there is a setting to make Windows delete the pagefile on shutdown -- but we all know that "deleting" something doesn't actually remove the bits from the platter. I wonder if it is possible to *only* encrypt the pagefile on a Windows system (though there will be a performance hit). I bet a skilled user could put a static pagefile on a separate partition and encrypt that partition only.

encrypting the paging file

Yes one way to encrypt the paging file would be to store it in its own encrypted partition. Whether FDE products encrypt the FULL hard disk or individual partitions, I don't know. To so many people, the hard drive is a single partition so the terms are used interchangeably.

I wonder if Windows' own internal encryption (EFS) can be used on the paging file?