Basic HTML version of Foils prepared April 7 1998

Foil 59 Why is type checking important!

From Basic Principles of Java and Internet Security CPS616 Web Technologies -- Spring 98. by Geoffrey C. Fox


1 If one has either deliberately or accidentally a "wild object pointer" that should be to a user defined on/off object but has somehow been applied to a sensitive object.
2 Then turning userobject.onoff to true is uncontroversial but this applied to appletprivilege could turn on the ability to write files!
  • Note setting userobject.onoff = true is really "go to location of this object and set its start address plus some many bytes to value true"!
3 Thus normal computer programs often overwrite themselves when you screw-up with a software error.
4 Java applets can obviously have software bugs but such errors do not let them ever overwrite themselves or anybody else.
  • Otherwise the overwriting can radically change security
5 Thus Java must guarantee types of objects precisely so operations can be stupid but never violate security.

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