This ransomware is particularly nasty because infected users are in danger of losing their personal files forever.

Spread through email attachments, this ransomware has been seen targeting companies through phishing attacks.

Cryptolocker will encrypt users’ files using asymetric encryption, which requires both a public and private key.

The public key is used to encrypt and verify data, while private key is used for decryption, each the inverse of the other.

The bad news is decryption is impossible unless a user has the private key stored on the cybercriminals’ server.

Currently, infected users are instructed to pay $300 USD to receive this private key.

Infected users also have a time limit to send the payment. If this time elapses, the private key is destroyed, and your files may be lost forever.

Files targeted are those commonly found on most PCs today; a list of file extensions for targeted files include:
3fr, accdb, ai, arw, bay, cdr, cer, cr2, crt, crw, dbf, dcr, der, dng, doc, docm, docx, dwg, dxf, dxg, eps, erf, indd, jpe, jpg, kdc, mdb, mdf, mef, mrw, nef, nrw, odb, odm, odp, ods, odt, orf, p12, p7b, p7c, pdd, pef, pem, pfx, ppt, pptm, pptx, psd, pst, ptx, r3d, raf, raw, rtf, rw2, rwl, srf, srw, wb2, wpd, wps, xlk, xls, xlsb, xlsm, xlsx

In some cases, it may be possible to recover previous versions of the encrypted files using System Restore or other recovery software used to obtain “shadow copies” of files.

Removal:

Malwarebytes  Anti-Malware detects Cryptolocker infections using multiple names, to include Trojan.Ransom and Trojan.CriLock.XL, but it cannot recover your encrypted files due to the nature of asymmetric encryption, which requires a private key to decrypt files encrypted with the public key.

Backups.

However, a local backup may not be enough in some instances, as Cryptolocker may even go after backups located on a network drive connected to an infected PC.

Ref: https://blog.malwarebytes.com/101/2013/10/cryptolocker-ransomware-what-you-need-to-know/