Description

MATCHBOIL is a malicious C#-based downloader associated with UAC-0099, a cyberespionage group assessed with medium confidence to be aligned with Russian interests. The malware has undergone continuous development since at least April 2024, incorporating improved obfuscation, anti-analysis techniques, and persistent command-and-control (C2) communication. ESET researchers observed infections in Ukrainian transportation, manufacturing, and energy organizations between 2025 and 2026, indicating continued targeting of critical sectors. The infection chain begins with spear-phishing emails containing malicious links. When a recipient clicks the link, an archive containing a VBScript file is downloaded. The victim must manually execute the script, which downloads and launches MATCHBOIL on the compromised Windows system. Upon execution, the malware checks for existing installation directories, collects system information through Windows Management Instrumentation (WMI), including processor identifiers and BIOS serial numbers, and uses these details to identify the infected host during subsequent C2 communications. MATCHBOIL establishes HTTPS connections with attacker-controlled C2 servers, some concealed behind Cloudflare infrastructure. It makes three requests to retrieve a numeric value, download a payload embedded as hexadecimal data within an HTML-formatted response, and obtain configuration information. The payload is decoded and written to disk; in most observed cases, it is MATCHWOK, a C# backdoor associated with UAC-0099. Persistence is established through Windows Registry Run keys or scheduled tasks. Newer variants employ Eziriz .NET Reactor obfuscation, debugger and system-uptime checks to evade analysis, and repeated C2 communication every two minutes. Some variants also display misleading planner or text-search interfaces to disguise execution. Organizations should strengthen email filtering, block malicious attachments and scripts, and educate users against executing unexpected files. Endpoint detection and response solutions should monitor suspicious VBScript activity, unfamiliar C# processes, recurring HTTPS connections, and unauthorized registry or scheduled-task changes. Security teams should investigate systems for relevant indicators of compromise, isolate suspected infections, block confirmed malicious infrastructure, and remove persistence mechanisms. Regular patching, least-privilege access, centralized logging, and updated threat intelligence can further reduce the risk of compromise.