Cobalt Gang targets banks and financial service providers by sneaking PDFs past staff

News by Rene Millman

Security analysts use new techniques to expose attackers' commodity builders and tools and their infrastructures.

A gang of hackers targeting financial institutions are trying to avoid detection by slipping PDFs files in emails, but getting bank staff to click on malware links.

According to researchers at Palo Alto, the hackers who are part of the Cobalt gang have sent PDFs to employees. These files do not contain and exploits, but what happens instead is a social engineering attack where the criminals fool victims into clicking on a link within the PDF to download malicious macros.

The Colbalt gang has been active despite the arrest of its leader earlier this year. In August, the group targeted two banks in Romania and Russia.

In a blog post, researchers said the technique, in order to be effective against static analysis tools, has a specially crafted PDF to make it look more authentic.

"It contains empty pages as well as some text pages that help in not raising red flags during analysis," said researchers. "By employing these two techniques the PDF avoids almost all traditional AV detection, resulting in a very effective transport of the first stage of the attack via email."

As the attack progresses, the victim is taken to a download of an MS Word document containing malicious macros that has very low detection rate at the moment of this campaign delivery. The researchers said that from a metadata standpoint, the document does not include any specific signal or characteristic that would help in tracking documents from the same author.

The downloaded malicious macro uses cmstp.exe to run a scriptlet, a technique known to bypass AppLocker, and continues with the next steps in payload delivery. The goal of recent research activities was not payload analysis. Instead, researchers focused on all possible aspects of the attack to further track the campaign and its associated infrastructure.

The researchers said that as the attack has achieved low detection rates, the focus of the investigation into the Colbalt group has been the identification of a possible underlying macro builder. While a builder was identified, investigators couldn’t guarantee that this builder is only used by this specific Cobalt Gang group and its campaigns against those industries.

"However, using this in combination with other aspects such as the target, payload, or dropper characteristics, becomes very useful in tracking this group’s campaigns," said researchers.

With the builder identified as well as fining common signals in the PDFs used in the attack, researchers were then able to find the gang’s infrastructure pieces based on multiple aspects, such as the hunting rules defined in previous sections, session data obtained by the investigator’s telemetry, or public WHOIS registrar data.

Researchers said that commodity attacks are widely used for both criminal and more targeted attacks, making identification difficult for networks defenders and threat hunters.

"By focusing on specific aspects of the macro builders and metadata the actors left behind we were able to develop new mechanisms to track and hunt Cobalt Gang activity and infrastructure," said researchers.

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