Java Bytecode Modification and Applet Security

While the Java Virtual Machine includes a bytecode verifier that checks bytecode programs before execution, and a bytecode interpreter that performs run-time tests such as array bounds and null-pointer checks, Java applets may still behave in ways that are annoying or potentially harmful to users. For example, applets may mount denial-of-service attacks, forge email or display misleading information in order to trick users. With these concerns in mind, we present techniques that may be used to insert additional run-time tests into Java applets. These techniques may be used to restrict applet behavior or, potentially, insert code appropriate to profiling or other monitoring efforts. The main techniques are class-level modification, involving subclassing non-final classes, and method-level modification, which may be used when control over objects from final classes is desired