Kronolith is a web-based calendar which relies on the Horde Framework for integration with other applications.
Kronolith contains a mistake in lib/FBView.php where a raw, unfiltered string is used instead of a sanitized string to view local files.
An authenticated attacker could craft an HTTP GET request that uses directory traversal techniques to execute any file on the web server as PHP code, which could allow information disclosure or arbitrary code execution with the rights of the user running the PHP application (usually the webserver user).
There is no known workaround at this time.
All horde-kronolith users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=www-apps/horde-kronolith-2.1.4"