gistfile1.txt

    # Serve .vtt files with Access-Control-Allow-Origin: * header if referer is the correct domain
    <IfModule mod_setenvif.c>
        <FilesMatch "\.vtt$">
            SetEnvIf Referer "^http(s)?://(.+\.)?somedomain\.com/" VALID_REFERER=$1
            Header set Access-Control-Allow-Origin "*" env=VALID_REFERER
        </FilesMatch>
    </IfModule>

You could also use origin

Persisting issue: this doesn't work properly if you're behind ATS/Squid/.. which caches requests without taking headers into account