Analyzing affiliate structures in Ransomware-as-a-Service campaigns

Episode 2: The All-Stars

This is the second installment of the McAfee Advanced Threat Research (ATR) analysis of Sodinokibi and its connections to GandGrab, the most prolific Ransomware-as-a-Service (RaaS) Campaign of 2018 and mid 2019.

GandCrab announced its retirement at the end of May. Since then, a new RaaS family called Sodinokibi, aka REvil, took its place as one of the most prolific ransomware campaigns.

In episode one of our analysis on the Sodinokibi RaaS campaign we shared our extensive malware and post-infection analysis, which included code comparisons to GandCrab and insight on exactly how massive the new Sodinokibi campaign is.

The Sodinokibi campaigns are still ongoing and differ in execution due to the different affiliates spreading the ransomware. Which begs more questions to be answered, such as how do the affiliates operate? Is the affiliate model working? What can we learn about the campaign and possible connections to GandCrab by investigating the affiliates?

It turns out, through large scale sample analysis and hardcoded value aggregation, we were able to determine which affiliates played a crucial role in the success of GandCrab’ criminal enterprise and found a lot of similarity between the RaaS enterprise of GandCrab and that of Sodinokibi.

Before we begin with the Sodinokibi analysis and comparison we will briefly explain the methodology that we used for GandCrab.

GandCrab RaaS System

GandCrab was a prime example of a Ransomware-as-a-Service. RaaS follows a structure where the developers are offering their product to affiliates, partners or advertisers who are responsible for spreading the ransomware and generating infections. The developers take a percentage of the earned income and provide the other portion to the affiliates.

Figure 1 High level overview of the GandCrab RaaS model

Operating a RaaS model can be lucrative for both parties involved:

- Developer’s perspective: The malware author/s request a percentage per payment for use of the ransomware product. This way the developers have less risk than the affiliates spreading the malware. The developers can set certain targets for their affiliates regarding the amount of infections they need to produce. In a way, this is very similar to a modern sales organization in the corporate world.

Subsequently, a RaaS model offers malware authors a safe haven when they operate from a country that does not regard developing malware as a crime. If their own nation’s citizens are not victimized, the developers are not going to be prosecuted.

- Affiliate perspective: As an affiliate you do not have to write the ransomware code yourself; less technical skill is involved. RaaS makes ransomware more accessible to a greater number of users. An affiliate just needs to be accepted in the criminal network and reach the targets set by the developers. As a service model it also offers a level of decentralization where each party sticks to their own area of expertise.

Getting a Piece of the Pie

Affiliates want to get paid proportionate to the infections they made; they expose themselves to a large amount of risk by spreading ransomware and they want to reap the benefits. Mutual trust between the developer and the affiliate plays a huge role in joining a RaaS system. It is very much like the expression: “Trust, hard to build, and easy to lose” and this largely explains the general skepticism that cybercriminal forum members display when a new RaaS system is announced. For the RaaS service to grow and maintain their trust, proper administration of infections/earnings per affiliate plays an important part. Through this, the developers can ensure that everyone gets an honest piece of the proverbial “pie”. So how can this administration be achieved? One way is having hardcoded values in the ransomware.

Linking the Ransomware to Affiliates

Through our technical malware analysis, we established that, starting from version 4, GandCrab included certain hardcoded values in the ransomware source code:

• ID – The affiliate id number.

• Sub_ID – The Sub ID of the affiliate ID; we suspect that affiliates can sub rent infections to their own partners, identifiable via the sub_id number.

• Version – The internal version number of the malware.

Version 4 had significant changes overall and we believe that these changes were partly done by the authors to improve administration and make GandCrab more scalable to cope with its increased popularity.

Based on the hardcoded values it was possible for us, to a certain extent, to extract the administration information and create our own overview. We hunted for as many different GandCrab samples as we could find, using Yara rules, industry contacts and customer submissions. The sample list we gathered is quite extensive but not exhaustive. From the collected samples we extracted the hardcoded values and compile times automatically, using a custom build tool. We aggregated all these values together in one giant timeline from GandCrab version 4, all the way up to version 5.2.

Affiliate vs. Salesperson & Disruption

An active affiliate can be compared to a top salesperson in any normal commercial organization. Given that the income of the RaaS network is largely dependent on the performance of its top affiliates, identifying and disrupting a top affiliate’s activity can have a crippling effect on the income of the RaaS network, internal morale and overall RaaS performance. This can be achieved through arrests of an affiliate and/or co-conspirers.

Another way is disrupting the business model and lowering the ransomware’s profits through offering free decryption tools or building vaccines that prevent encryption. The disruption will increase the operational costs for the criminals, making the RaaS of less interest.

Lastly, for any future proceedings (suspect apprehension and legal) it is important to maintain a chain of custody linking victims, samples and affiliates together. Security providers as gatherers and owners of this data play a huge role in safeguarding this for the future.

Where are the All-Stars? Top Affiliates Missing in 5.2

At the time we were not realizing it fully but, looking back at the overview, it stands out that none of the top affiliates/ID numbers where present in the final version 5.2 of GandCrab which was released in February. We believe that this was an early indicator that the end of GandCrab was imminent.

This discovery might indicate that some kind of event had taken place that resulted in the most active affiliates not being present. The cause could have been internal or external.

But what puzzles us is why would a high performing affiliate leave? Maybe we will never hear the exact reason. Perhaps it is quite similar to why people leave regular jobs… feeling unhappy, a dispute or leaving for a better offer.

Goodbye GandCrab, Hello Sodinokibi/REvil

In our opening episode we described the technical similarities we have seen between GandCrab and REvil. We are not the only ones that noticed these similarities - security reporter Brian Krebs published an article where he highlights the similarities between GandCrab and a new ransomware named Sodinokibi or REvil, and certain postings that were made on several underground forums.

Affiliates Switching RaaS Families….

On two popular underground Forums a user named UNKN, aka unknown, placed an advertisement on the 4th of July 2019, for a private ransomware as a service (RaaS) he had been running for some time. Below is a screenshot of the posting. Interesting is the response from a user with the nickname Lalartu. In a reply to the advertisement, Lalartu mentions that he is working with UNKN and his team, as well as that they had been a former GandCrab affiliate, something that was noticed by Bleepingcomputer too. Lalartu’s post supports our earlier observations that some top GandCrab affiliates suddenly disappeared and might have moved to a different RaaS family.  This is something that was suspected but never confirmed with technical evidence.

We suspect that Lalartu is not the only GandCrab affiliate that has moved to Sodinokibi. If top affiliates have a solid and very profitable infection method available, then it does not make sense to retire with the developers.

Around February 2019, there was a noticeable change in some of GandCrab’s infections behavior. Managed Service Providers (MSP) were now targeted through vulnerable systems and their customers got infected with GandCrab on a large scale, something we had not seen performed before by any of the affiliates. Interestingly, shortly after the retirement of GandCrab, the MSP modus operandi was quickly adopted by Sodinokibi, another indication that a former GandCrab affiliate had moved to Sodinokibi.

This makes us suspect that Sodinokibi  is actively recruiting the top performing affiliates from other successful RaaS families, creating a sort of all-star team.

At the same time, the RaaS market is such where less proficient affiliates can hone their skills, improve their spreading capabilities and pivot to the more successful RaaS families. Combined with a climate where relatively few ransomware arrests are taking place, it allows for an alarming cybercriminal career path with dire consequences.

It needs to be said that the timespan for the GandCrab overview was generated over a long period of time with a larger total of samples than the Sodinokibi overview.

Nevertheless, the similarity is quit striking. The activity of both ID numbers displays a tree-shaped structure with the parent ID number at the root and branching out to the respective SUB numbers linked to multiple samples.

We believe that the activity above might be linked to a tiered affiliate group that is specialized in RDP brute forcing and infecting systems with Sodinokibi after each successful compromise.

PREVIEW: In episode 4 of our blog series we will explain more details of the group behind ID nr 99 based on our honeypot data

Both RaaS family structures are too large to effectively publish within the space of this blog. Our Complete overview for the Sodinokibi RaaS structure can be found on our McAfee GitHub.

Conclusion

When we started our journey with GandCrab we did not expect it would take us so far down the rabbit hole. Mass sample analysis and searching for administration indicators provided a way to get more insight in a multi-million-dollar criminal enterprise, determine key players and foresee future events through changes in the business structure. We believe that the retirement of GandCrab was not an overnight decision and, based on the data on the affiliates, it was clear that something was going to happen.

With the emergence of Sodinokibi and the few forum postings by a high profile former GandCrab affiliate, everything fell into place. We have strong indications that some of the top affiliates have found a new home with Sodinokibi to further their criminal business.

Given that the income of the RaaS network is largely dependent on the performance of its top affiliates, and it is run like a normal business, we (the security industry) should not only research the products the criminals develop, but also identify possible ways to successfully disrupt the criminal business.

In our next episode we dive deeper into the financial streams involved in the affiliate program and provide an estimate of how much money these actors are earning with the ransomware-as-a-service business model.

By John Fokker and Christiaan Beek