6/16/2008
11:14 PM
George V. Hulme
George V. Hulme
Commentary

Antivirus Firm: Use File Recovery Tools To Defeat Ransomware Attack

While Kaspersky Lab says it's currently not possible to decrypt files encrypted by the most recent variant of the so-called "ransomware" Gpcode virus, file recovery tools just might get your data back.



While Kaspersky Lab says it's currently not possible to decrypt files encrypted by the most recent variant of the so-called "ransomware" Gpcode virus, file recovery tools just might get your data back.Most readers of InformationWeek are probably aware that deleted files aren't really deleted. When you hit the delete key, the space used by the file is marked as writable, and the "deleted" file exists until either overwritten through normal system use, or forensically wiped by special software. Wiping tools overwrite files a certain number of times until file recovery is virtually impossible. (I know: there are theoretical ways to potentially recover files using electronic wave-algorithm voodoo and an electron microscope. But unless you are NASA or the NSA, you don't have these resources, or the skills.)

This means that those infected by Gpcode, which we covered last week, and have their files encrypted and are then asked to pay a ransom for the decryption key, can try to use file recovery software in an attempt to get their data back.

The most recent update of the Kaspersky Gpcode.ak advisory suggests using a program, PhotoRec, to "recover your original files which were deleted by Gpcode after the virus created an encrypted version of the files."

I've no experience with PhotoRec, and I imagine many file recovery tools could get the job done. The trick is to write as little as possible to the hard-drive where the deleted files reside.

It doesn't appear that Gpcode overwrites the clear-text files before they're scrambled, not even once, after it runs its encryption scheme. While this step would render recovery software near useless, it would also be time consuming, and most users would notice the hit on their system performance during the process. Not a desirable condition for a successful attack.

Instead, the virus writers are relying on users' ignorance that they may actually be able to recover the unencrypted, and deleted versions, of the encrypted files.

If you happen to be one of the relatively few and unfortunate to have been nailed by Gpcode, using recovery software would probably be your best shot at recovering your files. Unless the Gpcode authors have some type of error in their use of the encryption (which happens to be Microsoft Enhanced Cryptographic Provider v1.0 and built into the OS) no one is cracking the RSA 1024-bit key. Not in this decade.

However, something all of us should do regularly also would be the best file-recovery solution available. At his blog, Bruce Schneier makes the great point: regular use of backup software is essential security:

The single most important thing any company or individual can do to improve security is have a good backup strategy. It's been true for decades, and it's still true today.

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