Mobile code security by Java bytecode instrumentation

Mobile code provides significant opportunities and risks. Java bytecode is used to provide executable content to Web pages and is the basis for dynamic service configuration in the Jini framework. While the Java Virtual Machine includes a bytecode verifier that checks bytecode programs before execution, and a bytecode interpreter that performs run-time tests, mobile code may still behave in ways that are harmful to users. We present techniques that insert run-time tests into Java code, illustrating them for Java applets and Jini proxy bytecodes. These techniques may be used to contain mobile code behavior or, potentially, insert code that is appropriate to profiling or other monitoring efforts. The main techniques are class modification, involving subclassing non-final classes, and method-level modifications that may be used when control over objects from final classes is desired.