Threat Groups Repurpose Banking Trojans into Backdoors

Ursnif, a one-time banking Trojan also known as Gozi, becomes the latest codebase to be repurposed as a more general backdoor, as malware developers trend toward modularity.

5 Min Read
white computer keyboard with red "backdoor" key opening like a door
Source: L. Millian via Shutterstock

Threat groups continue to recycle code from older tools into more generalized frameworks, a trend that will continue as the codebases incorporate more modularity, security experts said this week.

In the latest example, the threat group behind Ursnif — aka Gozi — recently moved the tool away from a focus on financial services to more general backdoor capabilities, cybersecurity services firm Mandiant stated in an analysis. The new variant, which the company has dubbed LDR4, is likely intended to facilitate the spread of ransomware and the theft of data for extortion.

The modular malware joins Trickbot, Emotet, Qakbot, IcedID, and Gootkit, among others, as tools that started as banking Trojans but have been repurposed as backdoors, without requiring the development effort of creating an entirely new codebase, says Jeremy Kennelly, senior manager for financial crime analysis at Mandiant.

"The developers working on banking Trojans have taken multiple approaches to retooling their malware as a backdoor to support intrusion operations, though a major code rewrite hasn’t generally been deemed necessary," he says. "These malware families — at their core — are just modular backdoors that have historically loaded secondary components enabling 'banker' functionality."

Mandiant's analysis of Ursnif points out that maintaining multiple codebases is a challenging task for malware developers, especially when one mistake could give defenders a way to block an attack and investigators a way to hunt down the attacker. Maintaining a single modular codebase is much more scalable, the company's analysis this week stated.

A Malware Movement Toward Backdoor Modularity

It's unsurprising that malware developers are moving to more general and modular code, says Max Gannon, a senior intelligence analyst at Cofense.

"In some cases, a purpose-built remote access Trojan (RAT), traditionally viewed as a backdoor, may be more conducive to the threat activity," he says. "However, a lot of threat actors want more than just a backdoor, and many commodity malware families have morphed to become multipurpose tools that simply include backdoor access."

The specialization of tools in the cybercriminal underground is also a reason why older codebases are being repurposed. By focusing specific tools on areas of attack — such as initial access, lateral movement, or data exfiltration — the developers of these tools are able to differentiate themselves against competitors and offer a unique set of features. Using existing codebases also saves time, and making such projects modular allows the tool to be customized for the customer's — read, "attacker's" — needs, says Jon Clay, vice president of threat intelligence at Trend Micro.

"The coders behind many of these toolkits create them and sell them within the cybercriminal underground markets, as they offer newbies and other malicious actors with a ready-made kits for executing attacks," he says. "Many of these offer automations now as well as GUI interfaces to manage the attacks and victim information/data."

The original Ursnif code appeared in the mid-2000s. The Zeus banking Trojan — used in thefts of tens of millions, and likely hundreds of millions, of dollars — has had a similar trajectory, with its adoption accelerated by a source code leak. Another banking Trojan, Emotet, has now become a general backdoor, allowing its development group to offer access as a service to other cybercriminals, a business relationship also demonstrated by Qakbot, another Trojan initially created as a banking Trojan.

All of these programs had the benefit of modularity, says Mandiant's Kennelly.

"All bankers that have been broadly repurposed as backdoors were already modular, which has the added benefit of limiting the complexity of the core malware while providing significant operational flexibility," he says. "These established malware families also had a proven track record and general familiarity to the actors using them."

Swiss Army Knife Malware Delivery

Rather than changes in functionality, a lot of the evolution in categorizing attackers tools has come about because labeling has had to catch up to changes in the malware design. By redesigning the codebases to be modular, defining a tool as a single thing — whether a banking Trojan, a spam bot, or a worm — becomes much more difficult. Adding a single new module would change the label for the code.

In the past, for example, computer viruses spread by infecting files, while worms used automated scanning and exploitation to spread quickly and more widely. However, a number of Trojans incorporated either or both functionality, leading to a more general term: malicious software, or malware.

A similar evolution has happened around the classification of attacker tools. Programs that were originally considered to be banking Trojans, RATs, or a scanning tools are now capabilities of more general frameworks, says Codefense's Gannon.

"If we think of a backdoor as software that sits on a machine to provide access that skirts normal security measures, banking Trojans inherently act as backdoors in order to perform their usual functions, so almost any banking Trojan can be used as one without the need for many changes," he says. "The difference is often simply in the intent of the user."

How to Protect Against Modular Malware

To combat the threat, companies should have tools that look for telltale signs that a backdoor or RAT are being used inside their network. Since phishing attacks are a common way to compromise end user's systems, multifactor authentication (MFA) and employee training can also help harden businesses against attacks.

Overall, having visibility into change to systems and anomalous traffic on the network can help immensely, Trend Micro's Clay says.

"The main thing to know is that in many cases there are early signs of these tools being used within the organization and that if seen," he says, "they should be taken very seriously that there is likely an active campaign against them."

About the Author(s)

Robert Lemos, Contributing Writer

Veteran technology journalist of more than 20 years. Former research engineer. Written for more than two dozen publications, including CNET, Dark Reading, MIT's Technology Review, Popular Science, and Wired News. Five awards for journalism, including Best Deadline Journalism (Online) in 2003 for coverage of the Blaster worm. Crunches numbers on various trends using Python and R. Recent reports include analyses of the shortage in cybersecurity workers and annual vulnerability trends.

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