Microsoft Defender is the built-in anti-malware package that's included with modern Windows operating systems. It's alternatively known as Windows Security (it shows up under Settings > Privacy & security as Windows Security), Windows Defender, or Microsoft Defender Antivirus, as on this Microsoft Learn page. But whatever you want to call it, for many Windows users, this tool is the go-to default for handling security on their PCs.
The attack chains, per the cybersecurity company, leverage ZIP archives containing decoy PDF documents along with malicious shortcut (LNK) or executable files that are masked as PDF to trick users into opening them. When launched, the LNK file runs an embedded PowerShell script that reaches out to an external server to download a lure document, a PDF for a marketing job at Marriott.