A malicious third-party can give a crafted "ssh://..." URL to an unsuspecting victim, and an attempt to visit the URL can result in any program that exists on the victim's machine being executed. Such a URL could be placed in the .gitmodules file of a malicious project, and an unsuspecting victim could be tricked into running "git clone --recurse-submodules" to trigger the vulnerability.
https://www.mail-archive.com/linux-kernel%40vger.kernel.org/msg1466490.html
https://www.exploit-db.com/exploits/42599/
https://support.apple.com/HT208103
https://security.gentoo.org/glsa/201709-10
https://access.redhat.com/errata/RHSA-2017:2675
https://access.redhat.com/errata/RHSA-2017:2674
https://access.redhat.com/errata/RHSA-2017:2491
https://access.redhat.com/errata/RHSA-2017:2485
https://access.redhat.com/errata/RHSA-2017:2484
http://www.securitytracker.com/id/1039131