These releases fix a security flaw (CVE-2018-17456), which allowed an
attacker to execute arbitrary code by crafting a malicious .gitmodules
file in a project cloned with --recurse-submodules.
When running "git clone --recurse-submodules", Git parses the supplied
.gitmodules file for a URL field and blindly passes it as an argument
to a "git clone" subprocess. If the URL field is set to a string that
begins with a dash, this "git clone" subprocess interprets the URL as
an option. This can lead to executing an arbitrary script shipped in
the superproject as the user who ran "git clone".
In addition to fixing the security issue for the user running "clone",
the 2.17.2, 2.18.1 and 2.19.1 releases have an "fsck" check which can
be used to detect such malicious repository content when fetching or
accepting a push. See "transfer.fsckObjects" in git-config(1).
Credit for finding and fixing this vulnerability goes to joernchen
and Jeff King, respectively.
P.S. Folks at Microsoft tried to follow the known exploit recipe on
Git for Windows (but not Cygwin or other Git implementations on
Windows) and found that the recipe (or its variants they can think
of) would not make their system vulnerable. This is due to the fact
that the type of submodule path require by the known exploit recipe
cannot be created on Windows. Nonetheless, it is possible we have
missed some exploitation path and users are encouraged to upgrade.