The source code of the Morris Worm discussed in chapter 6 of Protocol is freely available to the public, at http://www.foo.be/docs-free/morris-worm/worm/ <– this link. It is available as a learning experience to those who are interested, and should not be used maliciously. Having said that, I do not believe that a piece of code like this would have any effect on computers today, what with more rigorous antivirus softwares, and namely the fact that it used exploits in (if memory serves) Unix and SunOS kernels. It also requires a quite dated C compiler to assemble the program in the first place. This code is not the original code, however. What is available to the public is a decompiled version with comments in the side put in by those at either Berkeley or MIT or Cornell who took the liberty of decompiling it.
It is interesting and rather fitting of him (Robert Morris jr.) that he targeted Unix systems, since his father (Robert Morris Sr.) was a large contributor to the Unix kernel back in the day, and one of the people at Bell Labs that Protocol discusses who started the "Core War" games.