Researchers release CryptoLocker decryption tool

Posted by   Virus Bulletin on   Aug 6, 2014

Tool uses private keys found in database of victims.

Please note: this blog post was written in August 2014 and refers to a particular kind of encryption-ransomware that was active until June 2014. The tools mentioned are unlikely to work to decrypt newer versions of ransomware, including those branded as 'CrytpoLocker'.

The CryptoLocker ransomware is one of the nastiest pieces of malware to have targeted Internet users in recent years. The malware uses strong file encryption (more particularly, AES encryption with a key that has been encrypted using an RSA-2048 private key) to deny the user access to their files unless they pay a ransom of around US$300.

At a time when we often seem to be learning about accidental or intentional vulnerabilities in encryption, the fact that CryptoLocker didn't contain any such flaws (unlike some other ransomware) has been the cause of much frustration, and the only way to recover encrypted files (until now) has been to use a backup, or to pay the ransom.

However, there is good news for victims of CryptoLocker. Researchers at FireEye and Fox-IT have worked together to develop a free tool that will decrypt affected files. The tool requires users to upload a single encrypted file to a website, after which it is able to decrypt all other files found on the PC.

Rather than crack the encryption, the researchers made use of a database of the RSA private keys that was obtained during the recent takedown of the GameOver Zeus botnet - which was used to distribute the ransomware.

While this is certainly good news for those who have had their files encrypted with this ransomware, it is important to note that the encryption itself still hasn't been cracked. Moreover, following the 'success' of CryptoLocker, many copycats have sprung up (even including one targeting NAS devices). The multi-layered approach we advised back in November thus remains as important as ever.

More details on CryptoLocker can be found on the Fox-IT blog here and on FireEye's blog here.

Posted on 06 August 2014 by Martijn Grooten

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