Threat Groups Repurpose Banking Trojans into Backdoors

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."

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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."

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