In what way could Java applets pose a security threat?

Their transport can interrupt the secure distribution of World Wide Web pages over the Internet by removing SSL and S-HTTP

Java interpreters do not provide the ability to limit system access that an applet could have on a client system.

Executables from the Internet may attempt an intentional attack when they are downloaded on a client system.

Java does not check the bytecode at runtime or provide other safety mechanisms for program isolation from the client system.