Local application hang via unchecked menu input

Input-validation failure in SourceCodester Phone Contact Manager System 1.0 allows local users to hang the application through its user menu. UserInterface::MenuDisplayStart reads an excessively long decimal string into an int without checking conversion state, length, or the allowed menu range; the invalid value then falls through a perpetual main loop. Access to the local application is required, and the demonstrated consequence is repeated menu rendering with high CPU use and loss of normal interaction rather than a proven memory overwrite.