Missing CGI.pm and Other Mysteries
While we're on the subject of security, this is probably a good time for me to publicly wipe away some of the egg that remains on my face in the wake of my February column, in which I suggested that you should install CGI programs with permissions of 777, known to non-numeric types as “a+rwx”, or permission for all users on the system to read, write, and execute the program.
Suffice it to say that this is a grave error, as several readers noticed. Computer security depends on plugging as many holes as possible. On networked multiuser systems running programs that come from various sources, it's almost certainly a bad idea to install a program having permissions that let anyone on the system modify the contents of that program, particularly when a simple (and probably hard-to-notice) modification or two can turn a seemingly innocuous program into a ravenous bug-blatter beast. On a system not running one of the wrappers mentioned here, all CGI programs are run with the same permission, meaning that someone could write a program that can mess with the code or data of another.
If you are the only programmer working on a particular CGI program or Web site, then you can install your programs with 755 permission (u=rwx,ga+rx), so that others on the system—including the HTTP server, which is generally responsible for running CGI programs—can read and execute your code but cannot modify it.
If you are working with others on a site or CGI program, you can set the permissions to 775 (ug=rwx,a+rx), which lets everyone read and execute the program, but allows only the owner and members of the file's group to edit it.
There are probably times when it is appropriate to install a CGI program with 777 (a+rwx) permission, but these are rare.
That's it for the mailbag for this time. Next month, we'll return to a discussion of how to make life easier for non-programmers who might want to modify entries in tables on disk, by writing a few small CGI programs which can read and write files efficiently and easily.
Reuven M. Lerner has been playing with the Web since early 1993, when it seemed like more like a fun toy than the World's Next Great Medium. He currently works as a independent Internet and Web consultant from his apartment in Haifa, Israel. When not working on the Web or volunteering in informal educational programs, he enjoys reading on just about any subject, but particularly politics and philosophy, cooking, solving crossword puzzles and hiking. You can reach him at firstname.lastname@example.org or email@example.com.
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