The "/ext" page:

Relay an HTTP request to secondary CGI after first checking the login credentials and setting auxiliary environment variables so that the secondary CGI can be aware of the credentials and capabilities of the Fossil user.

The /ext page is only functional if the "extroot: DIR" setting is found in the CGI script that launched Fossil, or if the "--extroot DIR" flag is present when Fossil is launched using the "server", "ui", or "http" commands. DIR must be an absolute pathname (relative to the chroot jail) of the root of the file hierarchy that implements the CGI functionality. Executable files are CGI. Non-executable files are static content.

The path after the /ext is the path to the CGI script or static file relative to DIR. For security, this path may not contain characters other than ASCII letters or digits, ".", "-", "/", and "_". If the "." or "-" characters are present in the path then they may not follow a "/".

If the path after /ext ends with "/" and is the name of a directory then that directory is searched for files named "index.html", "index.wiki", and "index.md" (in that order) and if found, those filenames are appended to the path.