MediaWiki is a collaborative editing software, used by big projects like Wikipedia.
MediaWiki fails to decode certain encoded URLs correctly.
By supplying specially crafted links, a remote attacker could exploit this vulnerability to inject malicious HTML or JavaScript code that will be executed in a user's browser session in the context of the vulnerable site.
There is no known workaround at this time.
All MediaWiki users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=www-apps/mediawiki-1.4.15"