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Dave Everitt
Asked: 2020-05-24 12:40:58 +0800 CST

Best way to secure world-writable .shtml files written to by Perl?

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The Perl script

I have a Perl script running on an old Debian 6 server (until I move to a new server) that opens an editable pane in an .shtml file, accepts only Creole markup input from a single user, and writes to two user-editable .shtml files and makes a backup copy of each when saving the edited .shtml file. The .shtml file permissions are 606 or -rw----rw-.

After editing, the non-tech-savvy user has to enter a password to trigger the script, and (in the unlikely event that he'd ever do this) 'script' tags come out in plain text.

Before switching servers these used to run under suEXEC (which I did not set up and don't want to use), but they now need the permissions change to run.

The writable files

The two editable .shtml files are SSI with the virtualHost block set to IncludesNoExec.

There is no .htaccess file in the user's public_html directory, the only other user only ever makes simple HTML edits and has no other knowledge, I manage the scripts in the cgi-bin above the public_html folder, and all files in the user directory are owner/group username.

My questions, taking into account the above:

  1. is there anything I should beware of if the files permissions are 606/-rw----rw-?
  2. what else could I do to further secure such an old Debian server for now?

I've considered changing the server port from 22 to something much higher, which I've done on another server, but for a couple of sites running on this old one I have no access as yet to their DNS or remote login settings, so it isn't yet an option. Like many people, I'm not really a server admin, but have to maintain things as my co-dev died.

security perl file-permissions server-side-includes

1 Answers

  1. Best Answer

    The most important questions are:

    • who runs the script?
    • who can modify the script?

    From your description it looks like the app is an HTML form and your program is a CGI script. So that script is launched by the HTTP server daemon user when the user clicks on the Submit button.

    In case of compromission of this CGI script or an other one, you do not want your script to be modified. So the script must be read+exec only by the HTTP daemon user, and the owner of the file must be another user so the daemon user can't change the permission.

    The .shtml files must be read+write by the daemon and also not owned by the daemon. The script must never delete them and instead just rewrite their content.

    The directories where the script and the .shtml files are stored must not be writable nor owned by the HTTP daemon user.

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