What type of tool should Alex use to automate filling web application forms to test for format string vulnerabilities?

Using a fuzzer is particularly effective for automating the process of filling web application forms when testing for format string vulnerabilities. A fuzzer is designed to input large amounts of unpredictable data into a program or application to identify unexpected behavior, including security vulnerabilities. In the case of format string vulnerabilities, the fuzzer can generate a variety of input formats and manipulate the string formatting in a way that tests how the application handles these inputs.

By sending numerous test inputs and variations, a fuzzer can help uncover issues such as memory corruption, information leakage, or application crashes that may be indicative of format string vulnerabilities. Essentially, it automates a comprehensive and dynamic approach to testing inputs in a web application's forms, which is vital for identifying weaknesses that could be exploited by attackers.

Other tools like a vulnerability scanner may help identify existing known vulnerabilities in applications but are not as dynamic or adaptable as fuzzers. Proxy tools, while useful for intercepting web traffic and modifying requests, are more focused on session manipulation rather than automating input generation for vulnerability testing. Debuggers are primarily used for examining the runtime execution of applications to find bugs or issues but do not inherently automate testing of input for vulnerabilities like a fuzzer does.