Obsidian Browser Extension for SaaS Spear Phishing Prevention is a lightweight browser extension designed to detect and block spear phishing attempts targeting SaaS application users. The extension uses visual and content analysis combined with applied threat intelligence to identify and block zero-day phishing attempts in real time. It prevents users from submitting credentials to phishing sites by intercepting and evaluating page content before form submission occurs. Detection is driven by pre-built rules informed by observed phishing campaigns, with the option to create custom rules tailored to specific environments or individual users. The extension supports a silent detection mode that can escalate to active user alerts as threats are confirmed. When blocking a phishing attempt, the extension displays real-time explanations to the end user, providing in-context security awareness without requiring separate training programs. Security teams gain visibility into phishing attempts, can identify high-risk users and applications, and can investigate abnormal login activities through the platform. Deployment is designed to be fast, with the extension described as rolling out in minutes across an organization. It operates without network tunneling or TLS interception, minimizing performance impact. Only corporate-relevant data is collected; no personal data is stored. The extension is compatible with major device and browser management solutions.