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Guestbook CGI Remote Command Execution Vulnerability

When Guestbook is configured to allow HTML posts and you have enabled server-side includes for HTML, it may be possible for an attacker to embed SSI (server-side include) code in guestbook messages. The server-side includes allow for remote command execution, including displaying of any files to which the web server has read access (see the example):


<!--#exec cmd="cat /etc/group"

In an attempt to stop this from happening, guestbook.pl parses for SSI commands under the assumption that they are in this format:

<-- SSI command -->
^^ Does not need to be there.

Apache will accept different formats, which can evade the regular expression in guestbook.pl, executing commands on the target host as they would [if they were put there by the author].