The (1) git-remote-ext and (2) unspecified other remote helper programs in Git before 2.3.10, 2.4.x before 2.4.10, 2.5.x before 2.5.4, and 2.6.x before 2.6.1 do not properly restrict the allowed protocols, which might allow remote attackers to execute arbitrary code via a URL in a (a) .gitmodules file or (b) unknown other sources in a submodule.
https://security.gentoo.org/glsa/201605-01
https://lkml.org/lkml/2015/10/5/683
https://kernel.googlesource.com/pub/scm/git/git/+/33cfccbbf35a56e190b79bdec5c85457c952a021
https://github.com/git/git/blob/master/Documentation/RelNotes/2.6.1.txt
https://github.com/git/git/blob/master/Documentation/RelNotes/2.5.4.txt
https://github.com/git/git/blob/master/Documentation/RelNotes/2.4.10.txt
https://github.com/git/git/blob/master/Documentation/RelNotes/2.3.10.txt
https://bugzilla.redhat.com/show_bug.cgi?id=1269794
http://www.ubuntu.com/usn/USN-2835-1
http://www.slackware.com/security/viewer.php?l=slackware-security&y=2016&m=slackware-security.533255
http://www.securitytracker.com/id/1034501
http://www.securityfocus.com/bid/78711
http://www.oracle.com/technetwork/topics/security/linuxbulletinoct2015-2719645.html
http://www.oracle.com/technetwork/topics/security/linuxbulletinjan2016-2867209.html
http://www.oracle.com/technetwork/topics/security/bulletinapr2016-2952098.html
http://www.openwall.com/lists/oss-security/2015/12/11/7
http://www.openwall.com/lists/oss-security/2015/12/09/8
http://www.openwall.com/lists/oss-security/2015/12/08/5
http://www.debian.org/security/2016/dsa-3435
http://rhn.redhat.com/errata/RHSA-2015-2515.html
http://lists.opensuse.org/opensuse-updates/2015-11/msg00066.html