The Spreadsheet Described


We developed a marks spreadsheet to allow teachers maintain student marks. The spreadsheet has several desirable features such as the ability to create graphs of specified columns, automatically maintain past copies of grades (is an append only changelog), compute the average, etc. But the supported functionality is of little importance. The overriding concern in such a tool is maintaining marks security. In particular, no new security holes should be introduced. How can marks be maintained on the WWW so that the teacher has access to the marks, but others are prevented from accessing the marks?

In all the tools that we developed, we decided to depart as little as possible from prior practice. Prior to January 1995, marks were kept (unecrypted) in local files. To prevent unauthorised access, teachers would set the file's read and write permissions appropriately. So marks security rested upon a combination of Unix account security and teacher action. Marks integrity could be comprimised either if an account was cracked or if permissions were set incorrectly.

Our initial solution was to hide the spreadsheet behind a password protection scheme. This solution is problematic because it requires teachers to remember yet another password, it erects an obvious target for crackers, and it creates a new securtiy hole since it requires the protections on the marks file to be shared by the teacher and the server. There is no protection against scripts, invoked through the Common Gateway Interface (CGI), from maliciously modifying or inadvertently damaging marks.

Our second solution built upon prior practice rather than departing from it. We wrote a shell script that the user places in a private directory to invoke the spreadsheet. The shell script calls reads a private mail file (the name of the file is an argument to the shell script) and dynamically creates a private, temporary HTML page. The page is created with the teacher's read and write permissions only and is deleted upon exit from the shell script. All marks information is maintained in the local page.

The script then starts a browser (the browser is another input variable to the shell script) on the temporary page. From that page a link to an executable program is dynamically created to other pages (so no pages are ever stored and accessible to outside users). Grades are passed as hidden variables from one browser state to the next and ARE NEVER WRITTEN TO DISK. All this is for security to ensure that noone but the user will ever see the grades. Since grades are never written, any changes made are lost when the user leaves the browser. However, an option in the grader allows the user to e-mail a copy of the grades to that user (the e-mail address is read from the environment USER variable in the shell that first invoked the grader and passed as a hidden variable from one state of the grader to the next). When the user is emailed copy of the grades, they can simply append it to the mail file using their mailer. All past versions of the grades will then be accesible to the user (but to noone else).

The grader is no more or less secure than is Unix/Sendmail since it depends on sendmail to communicate the grades. The following are the salient security features.

Possible security flaws are the following:

Copyright © 1995 Curtis E. Dyreson. All rights reserved.