RubyGems before 1.8.23 can redirect HTTPS connections to HTTP, which makes it easier for remote attackers to observe or modify a gem during installation via a man-in-the-middle attack.
rhn.redhat.com/errata/RHSA-2013-1203.html
rhn.redhat.com/errata/RHSA-2013-1441.html
rhn.redhat.com/errata/RHSA-2013-1852.html
secunia.com/advisories/55381
www.openwall.com/lists/oss-security/2012/04/20/24
www.ubuntu.com/usn/USN-1582-1/
bugzilla.redhat.com/show_bug.cgi?id=814718
github.com/rubygems/rubygems/blob/1.8/History.txt