Ransomware is a type of malware which restricts access to the computer system that it infects, and demands a ransom paid to the creator(s) of the malware in order for the restriction to be removed. Some forms of ransomware encrypt files on the system's hard drive (cryptoviral extortion, a threat originally envisioned by Adam Young and Moti Yung), while some may simply lock the system and display messages intended to coax the user into paying.
While initially popular in Russia, the use of ransomware scams has grown internationally; in June 2013, security software vendor McAfee released data showing that it had collected over 250,000 unique samples of ransomware in the first quarter of 2013—more than double the number it had obtained in the first quarter of 2012.
Ransomware typically propagates as a trojan like a conventional computer worm, entering a system through, for example, a downloaded file or a vulnerability in a network service. The program will then run a payload: such as one that will begin to encrypt personal files on the hard drive. More sophisticated ransomware may hybrid-encrypt the victim's plaintext with a random symmetric key and a fixed public key. The malware author is the only party that knows the needed private decryption key. Some ransomware payloads do not use encryption. In these cases, the payload is simply an application designed to restrict interaction with the system, typically by setting the Windows Shell to itself, or even modifying the master boot record and/or partition table (which prevents the operating system from booting at all until it is repaired).
Ransomware payloads utilize elements of scareware to extort money from the system's user. The payload may, for example, display notices purportedly issued by companies or law enforcement agencies which falsely claim that the system had been used for illegal activities, or contains illegal content such as pornography and pirated software or media. Some ransomware payloads imitate Windows XP's product activation notices, falsely claiming that their computer's Windows installation is counterfeit or requires re-activation. These tactics coax the user into paying the malware's author to remove the ransomware, either by supplying a program which can decrypt the files, or by sending an unlock code that undoes the changes the payload has made. These payments are often delivered using either a wire transfer, premium-rate text messages, through an online payment voucher service such as Ukash or Paysafecard, or most recently, the digital currency Bitcoin.
The first known ransomware was the 1989 "AIDS" trojan (also known as "PC Cyborg") written by Joseph Popp, which triggered a payload claiming that the user's license to use a certain piece of software had expired, encrypted file names on the hard drive, and required the user to pay US$189 to "PC Cyborg Corporation" in order to unlock the system. Popp was declared mentally unfit to stand trial for his actions, but he promised to donate the profits from the malware to fund AIDS research. The notion of using public key cryptography for such attacks was introduced in 1996 by Adam L. Young and Moti Yung. The two believed that the AIDS trojan was ineffective due to its use of symmetric cryptography, and presented a proof-of-concept cryptovirus for the Macintosh SE/30 using RSA and TEA. Young and Yung referred to this attack as being "cryptoviral extortion", an overt attack that is part of a larger class of attacks in a field called cryptovirology, which encompasses both overt and covert attacks.
Examples of extortionate ransomware became prominent in May 2005. By mid-2006, worms such as Gpcode, TROJ.RANSOM.A, Archiveus, Krotten, Cryzip, and MayArchive began utilizing more sophisticated RSA encryption schemes, with ever-increasing key-sizes. Gpcode.AG, which was detected in June 2006, was encrypted with a 660-bit RSA public key. In June 2008, a variant known as Gpcode.AK was detected. Using a 1024-bit RSA key, it was believed to be large enough to be computationally infeasible to break without a concerted distributed effort.
Encrypting ransomware returned to prominence in late 2013 the propagation of CryptoLocker—using the Bitcoin digital currency platform to collect ransom money. In December 2013, ZDNet estimated that between October 15 and December 18, the operators of CryptoLocker had procured about US$27 million from infected users. The popularity of CryptoLocker also inspired further imitators, such as CryptoLocker 2.0, CryptoDefense (which, notably contained a major design flaw that allowed the private key to be stored on the infected system in a user-retrievable location, due to its use of Windows' built-in encryption APIs) and the August 2014 discovery of a ransomware worm specifically targeting network-attached storage devices produced by Synology.
In August 2010, Russian authorities arrested ten individuals connected to a ransomware worm known as WinLock. Unlike the previous Gpcode worms, WinLock did not use encryption. Instead, WinLock trivially restricted access to the system by displaying pornographic images, and asked users to send a premium-rate SMS (costing around US$10) to receive a code that could be used to unlock their machines. The scam hit numerous users across Russia and neighboring countries—reportedly earning the group over US$16 million.
In 2011, a ransomware worm imitating the Windows Product Activation notice surfaced that informed users that a system's Windows installation would have to be re-activated due to "[being a] victim of fraud". An online activation option was offered (like the actual Windows activation process), but was unavailable, requiring the user to call one of six international numbers to input a 6-digit code. While the malware claimed that this call would be free, it was routed through a rogue operator in a country with high international phone rates, who placed the call on hold, causing the user to incur large long distance charges.
In February 2013, a ransomware worm based on the Stamp.EK exploit kit surfaced; the malware was distributed via sites hosted on the project hosting services SourceForge and GitHub that claimed to offer "fake nude pics" of celebrities. In July 2013, an OS X-specific ransomware worm surfaced, which displays a web page that accuses the user of downloading pornography. Unlike its Windows-based counterparts, it does not block the entire computer, but simply exploits the behavior of the web browser itself to frustrate attempts to close the page through normal means.
In July 2013, a 21-year-old male from Virginia turned himself in to police after receiving ransomware containing a fake FBI warning accusing him of possessing child pornography. After an investigation, it was discovered that the user's computer, coincidentally, did contain pornographic photographs of underaged females, with whom he had conducted inappropriate communications. He was charged with child sexual abuse and possession of child pornography.
In 2012, a major ransomware worm known as Reveton began to spread. Based on the Citadel trojan (which itself, is based on the Zeus trojan), its payload displays a warning purportedly from a law enforcement agency (leading to its nickname as the "police trojan"), claiming that the computer had been used for illegal activities, such as downloading pirated software or child pornography. The warning informs the user that to unlock their system, they would have to pay a fine using a voucher from an anonymous prepaid cash service such as Ukash or Paysafecard. To increase the illusion that the computer is being tracked by law enforcement, the screen also displays the computer's IP address, while some versions display footage from a computer's webcam to give the illusion that the user is being purportedly recorded as well.
Reveton initially began spreading in various European countries in early 2012. Variants were localized with templates branded with the logos of different law enforcement organizations based on the user's country; for example, variants used in the United Kingdom contained the branding of organizations such as the Metropolitan Police Service, the collection society PRS for Music (which specifically accused the user of illegally downloading music), and the Police National E-Crime Unit. In a statement warning the public about the malware, the Metropolitan Police clarified that they would never lock a computer in such a way as part of an investigation. When authorities do have suspicion that someone is downloading or uploading child pornography, they will not give the suspect a heads up, as that would give them time to flee or dispose of evidence.
In May 2012, Trend Micro threat researchers discovered templates for variations for the United States and Canada, suggesting that its authors may have been planning to target users in North America. By August 2012, a new variant of Reveton began to spread in the United States, claiming to require the payment of a $200 fine to the FBI using a MoneyPak card.
Encrypting ransomware reappeared in September 2013 with a worm known as "CryptoLocker", which generated a 2048-bit RSA key pair—uploaded in turn to a command-and-control server, and used to encrypt files using a whitelist of specific file extensions. The malware threatened to delete the private key if a payment of Bitcoin or a pre-paid cash voucher was not made within 3 days of the infection. Due to the extremely large key size it uses, analysts and those affected by the worm considered CryptoLocker to be extremely difficult to repair. Even after the deadline passed, the private key could still be obtained using an online tool, but the price would increase to 10 BTC—approximately US$2300 as of November 2013.
CryptoLocker was isolated by the seizure of the Gameover ZeuS botnet, officially announced by the U.S. Department of Justice on June 2, 2014. The Department of Justice also publicly issued an indictment against the Russian hacker Evgeniy Bogachev for his alleged involvement in the botnet. 
As with other forms of malware, security software might not detect a ransomware payload, or, especially in the case of encrypting payloads, only after encryption is underway or complete, particularly if a new version unknown to the protective software is distributed. If an attack is suspected or detected in its early stages, it takes some time for encryption to take place; immediate removal of the malware (a relatively simple process) before it has completed would limit its damage to data. Security experts have suggested precautionary measures for dealing with ransomware, such as using software or other security policies to block known payloads from launching, along with "offline" backups of data stored in locations inaccessible to the malware.
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