Summary In early May, Unit 42 discovered an attack campaign against at least one defense company in Russia and one unidentified organization in South Korea delivering a variant of Bisonal malware. While not previously publicly documented, the variant has been in the wild since at least 2014. There are three primary differences between it and older Bisonal malware including a different cipher and encryption for C2 communication, and a large rewrite of the code for both network communication and maintaining persistence. To date, we have only collected 14 samples of this variant, indicating it may be sparingly used. The adversary behind these attacks lured the targets into launching the Microsoft Windows executable malware by masquerading it as a PDF file (using a fake PDF icon) and reusing publicly available data for the decoy PDF file’s contents. Attacks using Bisonal have been blogged about in the past. We believe it is likely these tools are being used by one group of attackers. Though Bisonal malware has been in the wild for at least seven years and frequently updated, the actors keep using same high-level playbooks. Common features of attacks involving Bisonal include