ProjectDiscovery Nuclei is a vulnerability scanning tool that uses a template-based approach to identify security issues across applications, cloud infrastructure, and networks. The tool operates with a templating library containing over 12,000 templates contributed by a community of 900+ contributors. Nuclei scans for known vulnerabilities including CVEs, misconfigurations, exposed services, and security weaknesses. The scanner processes over 50 million monthly scans and maintains an actively updated template library that responds to newly disclosed vulnerabilities. Templates cover various vulnerability types including SQL injection, path traversal, default credentials, information disclosure, and authentication bypasses. The tool supports both internal and external scanning capabilities. Users can scan targets from outside networks or within private infrastructures. Nuclei integrates with ProjectDiscovery's cloud platform for automated scanning workflows and remediation processes. Templates are written in YAML format and can be created manually or using AI-powered assistance. The template library includes detection rules for web applications, cloud services, network devices, and enterprise software. Each template specifies severity levels following CVSS scoring standards. The scanner is available as an open source tool with 30,000+ GitHub stars. ProjectDiscovery also offers a commercial cloud platform that provides real-time automated scans, remediation workflows, and access to early access templates for emerging vulnerabilities.