Apache mod_jk is a connector for the Tomcat web server.
Apache mod_jk decodes the URL within Apache before passing them to Tomcat, which decodes them a second time.
A remote attacker could browse a specially crafted URL on an Apache server running mod_jk, possibly gaining access to restricted resources.
There is no known workaround at this time.
All Apache mod_jk users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=www-apache/mod_jk-1.2.23"