Will is an attacker who is trying to craft an input string to gain shell access to a web server. What type of command injection attack is he pursuing?

The scenario describes an attacker attempting to gain shell access to a web server by crafting an input string. This action is indicative of a shell injection attack, which involves inserting commands into an input field that are executed by the system's shell. This type of attack is aimed at exploiting vulnerabilities in a web application that improperly handles user input, allowing the attacker to execute arbitrary shell commands on the server.

Shell injection specifically focuses on commands that are sent to the operating system's shell, which could lead to unauthorized actions such as viewing or modifying files, executing other scripts, or gaining higher privileges. By understanding this, it's clear that the attacker’s goal to gain shell access aligns directly with the nature of a shell injection attack, distinguishing it from other types of injection attacks that may not involve direct command execution at the operating system level.

In contrast, file injection refers to attacks targeting the upload or handling of files, which does not match Will's objective of gaining shell access. HTML embedding deals with the construction of malicious web pages or scripts inserted into HTML content, not the execution of commands. The option indicating 'none of these choices are correct' does not apply as shell injection fits the context perfectly.