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Banking Trojan Uses NSA-Linked Exploit

Newly observed Retefe banking Trojan samples have implemented the National Security Agency-related EternalBlue exploit, Proofpoint security researchers have discovered.

Unlike previous malware attacks that exploited EternalBlue, however, the new campaign doesn’t abuse it to spread in an infinite loop. In fact, the exploit-carrying samples are distributed via spam emails, while the version dropped via EternalBlue lacks the exploit.

EternalBlue is a NSA-linked tool that became public in April, one month after Microsoft released a patch for it. The exploit leverages a vulnerability in Windows’ Server Message Block (SMB) on port 445, allowing attackers to have malicious code automatically executed on vulnerable systems.

The exploit became highly popular after being abused in the massive WannaCry ransomware campaign that unfolded in May this year. Other malware, however had been abusing it for weeks.

In a recent campaign targeting users in Switzerland, some of the Retefe samples Proofpoint has collected starting with September 5 revealed the use of EternalBlue for lateral movement capabilities.

Retefe is a banking Trojan active since 2013, well known for its continuous focus on users in Austria, Sweden, Switzerland and Japan. The malware operates by routing traffic to and from the targeted banks through proxy servers, often hosted on the TOR network.

Recently, the malware has been mostly distributed through spam email campaigns where a Microsoft Office document was attached to the messages. Leveraging social engineering, the attackers use the attachments to trick users into downloading a malicious payload,

In recent campaigns, a self-extracting Zip archive containing a multiply-obfuscated JavaScript installer was downloaded. While analyzing the installer code, the security researchers have discovered that recent samples contained a new parameter designed to implement the EternalBlue exploit.

The code was borrowed from a publicly available proof-of-concept posted on GitHub, but functionality to log the installation and victim configuration details was also included. Last week, the parameter was replaced with a new one that contains only the logging functions.

“The EternalBlue exploit downloads a PowerShell script from a remote server, which itself includes an embedded executable that installs Retefe. This installation, however, lacks the module responsible for further lateral spread via EternalBlue, thus avoiding an infinite spreading loop,” Proofpoint says.

The security researchers also note that malware versions compatible with Mac OS have been distributed between June and August this year.

“While far less widespread than other banking Trojans like Dridex or The Trick, the focus on Swiss banks provides the Retefe group with potential high-profile targets. In addition, we are observing increasingly targeted attacks from this group that, with the addition of the EternalBlue exploit, creates opportunities for effective propagation within networks once initial targets have been compromised,” Proofpoint notes.

Related: NSA's EternalBlue Exploit Fully Ported to Metasploit

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