Security Experts:

Critroni: First File-Encrypting Ransomware to Use Tor

A new file-encrypting piece of ransomware advertised on underground forums since mid-June is being increasingly used by cybercriminals, a security expert reported last week.

The threat, dubbed "CTB-Locker" and detected as Critroni.A by Microsoft, was initially used against Russian-speaking users, but according to French researcher known as Kafeine, an English version has also been launched recently. The name CTB, which stems from Curve/Tor/Bitcoin, describes some of the key advantages of using this piece of ransomware.

The malware developers claim that the elliptic curve cryptography that's used to encrypt victims' files makes it impossible to decrypt them without paying the ransom. The Tor anonymity network is utilized to hide the malware's command and control (C&C) servers in order to make operations more difficult to disrupt and to protect the identity of the owner, the developers of Critroni said.

According to ThreatPost, this is the first crypto ransomware that uses Tor to protect C&C servers, a technique usually seen in banking Trojans. Furthermore, unlike other threats that rely on the anonymity network, the Tor components are embedded in the malware's body to make it more efficient and to help it avoid detection, said Kaspersky Senior Malware Analyst Fedor Sinitsyn.

Kaspersky Lab has analyzed the threat, which it has dubbed "Onion Ransomware," and plans on publishing a detailed report on it in the upcoming days.

The cybercriminals claim they're creation uses Bitcoin for ransom payments because the "purse is impossible to block and remove," and since the loot isn't stored on the server, it isn't lost even if something happens to the server.  The malware authors advise customers to demand 0.5 Bitcoin ($310) from victims in the US, Canada and Europe, and 0.25 Bitcoin ($155) for other regions. However, they point out that customers can set any ransom fee they want.

Critroni is currently being sold for $3,000, which includes free support. However, extended support costs an extra $300 per month.

Kafeine, who has spotted multiple instances of the ransomware in the wild, says the threat is sometimes distributed as a second stage payload by the Angler exploit kit.

Users whose computers are infected with Critroni are given 72 hours to pay the ransom. After the time runs out, victims are informed that the "locker" will remove itself from the system and that their files could be lost forever.

CryptoLocker, the most successful file-encrypting ransomware, was disrupted by law enforcement authorities at the beginning of summer. However, the rise of threats like Critroni demonstrates that cybercriminals are not ready to give up on such operations, most likely because they can be very profitable. As far as CryptoLocker is concerned, communications between infected devices and the botnet were cut off as a result of the recent operation, but researchers say its infrastructure is currently used for other threats.

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Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.