What tool is specifically designed to test a web browser's handling of unexpected data?

The tool specifically designed to test a web browser's handling of unexpected data is zzuf. This tool is a fuzz testing utility that generates malformed or unexpected inputs to assess how software, including web browsers, reacts to such data. Fuzz testing is crucial in identifying vulnerabilities related to how software processes input, helping discover potential issues that may lead to crashes, security breaches, or improper data handling.

Nessus is a widely used vulnerability scanner aimed at identifying potential vulnerabilities across various systems, but it is not specifically focused on testing the handling of unexpected data in web browsers. Nmap is primarily a network scanning tool used to discover hosts and services on a computer network, making it more focused on network security assessments rather than browser input testing. Nikto is a web server scanner that focuses on finding vulnerabilities within web servers rather than specifically testing how browsers handle unexpected data. Thus, zzuf stands out as the appropriate choice for this particular testing goal.