The vulnerability stems from Composer's entry point script (bin/composer) being executable as a PHP file in web-accessible environments with register_argc_argv enabled. The core issue is not a specific function but rather the lack of environment validation in the main script flow. The patch adds a check at the script level (not within a specific function) to abort execution when running under non-CLI SAPIs with register_argc_argv enabled. No specific functions are mentioned in the advisory or commit diffs as being vulnerable - the risk arises from the interaction between PHP's argv handling and Composer's entry point execution context.