RubyGems before 1.8.23 does not verify an SSL certificate, which allows remote attackers to modify a gem during installation via a man-in-the-middle attack.
http://rhn.redhat.com/errata/RHSA-2013-1203.html
http://rhn.redhat.com/errata/RHSA-2013-1441.html
http://rhn.redhat.com/errata/RHSA-2013-1852.html
http://secunia.com/advisories/55381
http://www.openwall.com/lists/oss-security/2012/04/20/24
http://www.ubuntu.com/usn/USN-1582-1/