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Ransomware is a type of malware that restricts access to a computer system that it infects in some way, and demands that the user pay a ransom to the operators of the malware to remove the restriction.

Some forms of ransomware systematically encrypt files on the system's hard drive (cryptoviral extortion, a threat originally envisioned by Adam Young and Moti Yung) using a large key that may be technologically infeasible to breach without paying the ransom, while some may simply lock the system and display messages intended to coax the user into paying. Ransomware typically propagates as a trojan, whose payload is disguised as a seemingly legitimate file.

While initially popular in Russia, the use of ransomware scams has grown internationally;[1][2][3] 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.[4]

Wide-ranging attacks involving encryption-based ransomware began to increase through trojans such as CryptoLocker, which had procured an estimated US$3 million before it was taken down by authorities,[5] and Cryptowall, which has procured an estimated $15 million as of June 2015.[6]


Ransomware typically propagates as a trojan, entering a system through, for example, a downloaded file or a vulnerability in a network service. The program then runs a payload.

Payloads vary, with the simplest simply displaying a fake warning notice. Such scareware-based ransomware display notices that imitate those issued by companies or law enforcement agencies and falsely claim that the system has been used for illegal activities, or contains illegal content such as pornography and pirated software or media.[7][8] Some ransomware payloads imitate product activation notices, falsely claiming that a computer's installation is counterfeit or requires re-activation.[9]

A second type of payload is in the form of an application designed to lock or restrict the system until payment is made, typically by setting the Windows Shell to itself,[10] or even modifying the master boot record and/or partition table to prevent the operating system from booting until it is repaired.[11]

The most sophisticated payloads encrypt files,[12][13][14] with many using strong encryption to encrypt the victim's files in such a way that only the malware author has the needed decryption key.

Payment is generally always the goal, and the victim is coerced into paying to remove the ransomware, either by supplying a program that can decrypt the files, or by sending an unlock code that undoes the payload's changes. A key element in making ransomware work for the attacker is a convenient untraceable payment system. A range of such payment methods have been used, including: wire transfer, premium-rate text messages,[15] online payment voucher service such as Ukash or Paysafecard,[1][16][17] - and most recently, the digital currency Bitcoin.[18][19]


Encrypting ransomware[edit]

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" for the means 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.[20] 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.[12]

Examples of extortionate ransomware became prominent in May 2005.[21] By mid-2006, trojans 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.[22] In June 2008, a variant known as Gpcode.AK was detected. Using a 1024-bit RSA key, it was believed large enough to be computationally infeasible to break without a concerted distributed effort.[23][24][25][26]

Encrypting ransomware returned to prominence in late 2013 with the propagation of CryptoLocker—using the Bitcoin digital currency platform to collect ransom money. In December 2013, ZDNet estimated based on Bitcoin transaction information that between 15 October and 18 December, the operators of CryptoLocker had procured about US$27 million from infected users.[27] In turn, CryptoLocker would influence a range of copycats that began to spread in the months following, including CryptoLocker 2.0, CryptoDefense (which initially contained a major design flaw that stored the private key on the infected system in a user-retrievable location, due to its use of Windows' built-in encryption APIs),[19][28][29][30] and the August 2014 discovery of a trojan specifically targeting network-attached storage devices produced by Synology.[31]

Non-encrypting ransomware[edit]

In August 2010, Russian authorities arrested ten individuals connected to a ransomware trojan known as WinLock. Unlike the previous Gpcode trojan, 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.[8][32]

In 2011, a ransomware trojan surfaced that imitated the Windows Product Activation notice, and informed users that a system's Windows installation had 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 international long distance charges.[9]

In February 2013, a ransomware trojan 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.[33] In July 2013, an OS X-specific ransomware trojan 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.[34]

In July 2013, a 21-year-old man from Virginia, whose computer coincidentally did contain pornographic photographs of underaged girls with whom he had conducted inappropriate communications, turned himself in to police after receiving and being deceived by ransomware purporting to be an FBI message accusing him of possessing child pornography. An investigation discovered the incriminating files, and the man was charged with child sexual abuse and possession of child pornography.[35]

Notable examples[edit]


A Reveton payload, fraudulently claiming that the user must pay a fine to the Metropolitan Police Service

In 2012, a major ransomware trojan 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 (a characteristic referred to as the "police trojan" or "cop trojan"), claiming that the computer has been used for illegal activities, such as downloading pirated software or child pornography.[36] 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.[1][37]

Reveton initially began spreading in various European countries in early 2012.[1] 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.[38] 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.[1][7]

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.[39] 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.[2][3][37] In February 2013, a Russian citizen was arrested in Dubai by Spanish authorities for his connection to a crime ring that had been using Reveton; ten other individuals were arrested on money laundering charges.[40]

In August 2014, Avast Software reported that it had found new variants of Reveton that also distribute password stealing malware as part of its payload.[41]


Main article: CryptoLocker

Encrypting ransomware reappeared in September 2013 with a trojan 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 trojan considered CryptoLocker extremely difficult to repair.[18][42][43][44] 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.[45][46]

CryptoLocker was isolated by the seizure of the Gameover ZeuS botnet, as 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.[47][48] It was estimated that at least US$3 million was extorted with the malware before the shutdown.[5]

CryptoLocker.F and TorrentLocker[edit]

In September 2014, a wave of ransomware trojans surfaced that first targeted users in Australia, under the names CryptoWall and CryptoLocker (which is, as with CryptoLocker 2.0, unrelated to the original CryptoLocker). The trojans spread via fraudulent e-mails claiming to be failed parcel delivery notices from Australia Post; to evade detection by automatic e-mail scanners that follow all links on a page to scan for malware, this variant was designed to require users to visit a web page and enter a CAPTCHA code before the payload is actually downloaded, preventing such automated processes from being able to scan the payload. Symantec determined that these new variants, which it identified as CryptoLocker.F, were again, unrelated to the original CryptoLocker due to differences in their operation.[49][50] A notable victim of the trojans was the Australian Broadcasting Corporation; live programming on its television news channel ABC News 24 was disrupted for half an hour and shifted to Melbourne studios due to a CryptoWall infection on computers at its Sydney studio.[51][52][53]

Another trojan in this wave, TorrentLocker, initially contained a design flaw comparable to CryptoDefense; it used the same keystream for every infected computer, making the encryption trivial to overcome. However, this flaw was later fixed.[28] By late-November 2014, it was estimated that over 9,000 users had been infected by TorrentLocker in Australia alone, trailing only Turkey with 11,700 infections.[54]


Another major ransomware trojan targeting Windows, Cryptowall, first appeared in 2014. One strain of Cryptowall was distributed as part of a malvertising campaign on the Zedo ad network in late-September 2014 that targeted several major websites; the ads redirected to rogue websites that used browser plugin exploits to download the payload. A Barracuda Networks researcher also noted that the payload was signed with a digital signature in an effort to appear trustworthy to security software.[55] The most recent version, Cryptowall 3.0, uses a payload written in JavaScript as part of an email attachment, which downloads executables disguised as JPG images. To further evade detection, the malware creates new instances of explorer.exe and svchost.exe to communicate with its servers. When encrypting files, the malware also deletes volume shadow copies, and installs spyware that steals passwords and Bitcoin wallets.[56]

The FBI reported in June 2015 that nearly 1000 victims have contacted the bureau's Internet Crime Complaint Center to report Cryptowall infections, and estimated losses of at least $18 million.[6]

Ransomware as a Service[edit]

Dark web vendors have increasingly started to offer the technology as a service. Brands include the now defunct 'Tox'[57] and 'Encryptor RaaS'.[58]


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.[59] 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.[60][61] 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.[18][62]

See also[edit]


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