The GrelosGTM cybercriminal group abuses the legitimate functionality of Google Tag Manager to compromise e-commerce websites, primarily those running Magento CMS. Since early 2020, they have injected malicious Google Tag Manager scripts that load multi-stage JavaScript payloads via WebSocket connections. The final payload is a heavily obfuscated JavaScript sniffer designed to steal payment card information during checkout through fake payment forms. Stolen data is exfiltrated to attacker-controlled servers. The campaign has affected at least seven websites across Belgium, Italy, the United Kingdom, and the United States.
