After receiving multiple updates, the Stegano exploit kit (EK) recently adopted the Diffie-Hellman algorithm to hinder analysis, Trend Micro security researchers warn.

Also known as Astrum, Stegano was previously associated with a massive AdGholas malvertising campaign that delivered Trojans such as Gozi and RAMNIT. The EK was also seen being used in the Seamless malvertising campaign, which normally employs RIG instead.

In late March, Proofpoint security researcher Kafeine discovered the EK abusing CVE-2017-0022, an information disclosure vulnerability in Windows that was patched on March 14. The exploit was used to look for antivirus apps on the system to evade detection and analysis.

In April, the threat received an update that prevented security researchers from replaying the malicious network traffic. The feature abuses the Diffie-Hellman algorithm, which is widely used for encrypting and securing network protocols.

“Implementing the Diffie-Hellman key exchange prevents malware analysts and security researchers from getting a hold of the secret key Astrum uses to encrypt and decrypt their payloads. Consequently, obtaining the original payload by solely capturing its network traffic can be very difficult,” Trend Micro notes.

In addition to the CVE-2017-0022 flaw, Astrum/Stegano is using exploits for a series of vulnerabilities in Adobe Flash, including CVE-2015-8651 (a code execution vulnerability patched December 28, 2015), CVE-2016-1019 (a remote code execution flaw patched April 7, 2016), and CVE-2016-4117 (an out-of-bound read bug in Flash patched May 10, 2016).

At the moment, the EK isn’t distributing established malware, and the threat is maintaining very low traffic, which Trend Micro believes can be seen as dry runs for their future attacks.

“It wouldn’t be a surprise if [Astrum/Stegano’s] operators turn it into an exclusive tool of the trade—like Magnitude and Neutrino did—or go beyond leveraging security flaws in Adobe Flash. Emulating capabilities from its predecessors such as fileless infections that can fingerprint its targets and deliver encrypted payloads shouldn’t be far off,” the researchers note.

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