lighttpd is a small-footprint, fast, compliant and very flexible web-server which is optimized for high-performance environments.
lighttpd uses file extensions to determine which elements are programs that should be executed and which are static pages that should be sent as-is. By appending %00 to the filename, you can evade the extension detection mechanism while still accessing the file.
A remote attacker could send specific queries and access the source of scripts that should have been executed as CGI or FastCGI applications.
There is no known workaround at this time.
All lighttpd users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=www-servers/lighttpd-1.3.10-r1"