Coordinated botnet disruptions have increased in pace and popularity over the last few years as more private companies work with international law enforcement agencies to combat malware infections on a grand scale. Operation Tovar, announced on June 2 2014, is the latest to make headlines. The target of the investigation, Evgeniy Mikhailovich Bogachev, was indicted by the Department of Justice and is wanted by the FBI for his role as alleged leader of the Gameover ZeuS and CryptoLocker botnets. Four other defendants were indicted using their pseudonyms. Though Bogachev’s current activities aren’t known, the Operation Tovar task force has maintained control of the botnet infrastructure and remediation efforts are ongoing.
While new malware strains are released with increasing frequency, it’s easy to forget why Gameover and CryptoLocker are worthwhile targets for takedown operations. Both offered more advanced features than their peers and typified the increasingly sophisticated cybercriminal enterprises behind botnets.
Since the ZeuS source code was released in 2011, several new variants have appeared in the wild. Citadel, KINS, ICE IX, and Gameover have all improved upon the basic ZeuS model by introducing new features, using better encryption, and modifying command and control (C2) communication methods.
Gameover uses a peer-to-peer (P2P) system for C2 communication. Though other P2P botnets such as Kelihos exist, Gameover is notable for its use of proxy nodes to introduce complexity into the standard P2P infrastructure. These proxy nodes are specific machines designated as relay points through which the botnet operators send commands and receive stolen information. This minimizes the number of systems that actually communicate with C2 servers. C2 commands are signed using RSA-2048 and encrypted with RC4 making it very difficult to tamper with the botnet.
Additionally, Gameover maintains a failsafe mechanism: a domain generation algorithm (DGA) that produces 1,000 domains each week. This feature enables the operators to maintain control of their botnet even if the P2P infrastructure is compromised. The DGA produces long, nonsensical strings at one of six top-level domains: .com, .net, .org, .biz, .info, and .ru that can be registered and used to send commands to the botnet.
ZeuS and all its variants are information-stealing trojans. We refer to them as banking trojans because that’s where they excel and Gameover is no exception. Gameover is able to trick the user into handing over personal information and can even defeat two-factor authentication. It accomplishes this by injecting custom code into the browser when a victim visits certain websites. Gameover’s arsenal of bank account takeover tools includes 1,500 web injections that were custom-made to target the websites of more than 700 financial institutions worldwide.
In addition to its exceptional abilities as a banking trojan, Gameover is capable of a wider variety of data theft activities. An Operation Tovar task force member, speaking to Brian Krebs on the condition of anonymity, said they have evidence of additional harvested data and that Gameover targeted proprietary information.
Not content with merely engaging in widespread banking credential and information theft, the Gameover criminal operators decided to maximize returns by infecting systems with CryptoLocker. It is a type of ransomware that encrypts the files on infected machines and then demands a ransom of hundreds of dollars in order to receive a decryption key. Typically, victims were given 72 hours to pay the ransom in bitcoins or risk losing their data.
Unwilling to miss out on any opportunity to generate revenue, the criminal operators set up a website to assist victims in paying the ransom in bitcoins. Through this website, victims could complete the transaction and track the status of their “order” – the ransom payment in exchange for the decryption key. Some victims, unwilling or unable to pay the ransom, missed the 72-hour deadline only to see the ransom demand increase fivefold.
Law enforcement officials discouraged people from paying the ransom since it would fund a criminal organization, but without back ups many victims had little choice but to pay. A US police department paid $750 for two Bitcoins as ransom after CryptoLocker was installed on a system used for police reports and booking photos. CryptoLocker encrypts files using asymmetric encryption, making use of a public and a private key. Without the private key, located on the criminals’ servers, infected files probably cannot be decrypted.
Operation Tovar’s investigation began with a server in the UK. A trail of wire transfers, money mules, criminal servers, and at least one confidential source led investigators to Bogachev. He is a Russian citizen wanted on charges of conspiracy to participate in racketeering activity, bank fraud, conspiracy to violate the Computer Fraud and Abuse Act, conspiracy to violate the Identity Theft and Assumption Deterrence Act, aggravated identity theft, conspiracy, computer fraud, wire fraud, and money laundering. The FBI estimates the financial toll of Gameover at over $100 million and another estimate is that more than $27 million in ransom payments were made in the first two months of CryptoLocker’s distribution.
Obtaining an indictment against a Russian national who will likely never be extradited to the United States isn’t sufficient to put an end to a criminal organization. In 2011, Russian citizen Aleksandr Andreevich Panin was indicted in the US on 23 counts related to the development and distribution of SpyEye but was not arrested until 2013 when he flew through Hartsfield-Jackson Atlanta International Airport. The Russian government, in a travel warning to its citizens, specifically mentions Panin and recommends that Russians facing legal action in the US should refrain from travelling internationally.
Drawing on the technical expertise of its members, the Operation Tovar task force was able to exploit flaws in the design of Gameover’s P2P network to manipulate the peer list and redirect traffic to nodes under its control. The specific technical details have not been released to the public in order to prevent the criminals from regaining control.
Gameover’s failsafe mechanism, the DGA that was supposed to have allowed the criminals to maintain control in the event of a P2P disruption, was reverse engineered by task force members. The FBI then obtained a restraining order to redirect any attempts to register those domains to a government-run server. Furthermore, US service providers are required to block connections to the Russian .ru domains generated by the DGA since the US has no jurisdiction to prevent their registration.
CryptoLocker also used a DGA for determining C2 locations. The algorithm was reverse engineered and the C2 servers were identified and seized by the Operation Tovar task force. Due to the use of an asymmetric key algorithm, CryptoLocker victims whose files remain encrypted currently have no avenue of remediation.
Operation Tovar’s success can be measured by two factors: (1) Have the criminals regained control of their botnets and (2) Is the malware being removed from infected machines? While we can’t say for certain that the people responsible for Gameover and CryptoLocker have ceased all criminal activity, they have not regained control of the network disrupted by Operation Tovar. Based on this fact alone, the task force should be commended. Successful botnet disruptions are very challenging. Attempted takeovers over Kelihos have been undone after only two weeks.
The remediation of infected machines is an even more difficult task. US-CERT has published a list of recommended actions and resources, and a number of the private companies involved in Operation Tovar have released scanning tools. The onus remains on individuals and organizations to use these resources to determine if they are infected and take the appropriate steps to remediate the problem. Statistics published by The Shadowserver Foundation show the number of machines infected with Gameover has remained essentially flat since the takeover. There are simply not enough people taking advantage of the resources available to remediate their systems.
The task force has taken control of the C2 network and now some people may believe that the malware is neutered and no further action is required. It is important to remember that any malware is unauthorized code running on a computer. The integrity of the system is still compromised, regardless of who is in control of the botnet.