DokuWiki is a wiki targeted at developer teams, workgroups and small companies. It does not use a database backend.
"rgod" discovered that DokuWiki doesn't sanitize the X-FORWARDED-FOR HTTP header, allowing the injection of arbitrary contents - such as PHP commands - into a file. Additionally, the accessory scripts installed in the "bin" DokuWiki directory are vulnerable to directory traversal attacks, allowing to copy and execute the previously injected code.
A remote attacker may execute arbitrary PHP (and thus probably system) commands with the permissions of the user running the process serving DokuWiki pages.
Disable remote access to the "bin" subdirectory of the DokuWiki installation. Remove the directory if you don't use the scripts in there.
All DokuWiki users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=www-apps/dokuwiki-20060309d"