A Blob URL is a URL that refers to data created at runtime, often inside the browser. In many web APIs, a blob: URL points to in-memory content such as a file, image, or HTML page; in some environments, a similar trusted cloud-hosted URL may be used to deliver the same effect. The key idea is that the final content is not always a normal, permanent website.
In cyber security, Blob URLs matter because they can hide the last stage of a phishing page or malicious document behind ephemeral, session-bound content. That makes static blocklists, domain reputation, and takedown requests less effective. Defenders should look for browser runtime behavior, unusual redirects, and suspicious login prompts embedded in trusted services. Browser telemetry, identity monitoring, and endpoint controls are often better signals than the URL alone.