On Mon, Sep 26, 2016 at 11:10:54AM -0700, Junio C Hamano wrote:

> Junio C Hamano <gits...@pobox.com> writes:
> > I am inclined to say that it has no security implications.  You have
> > to be able to write a bogus loose object in an object store you
> > already have write access to in the first place, in order to cause
> > this ...
> Note that you could social-engineer others to fetch from you and
> feed a small enough update that results in loose objects created in
> their repositories, without you having a direct write access to the
> repository.
> The codepath under discussion in this thread however cannot be used
> as an attack vector via that route, because the "fetch from
> elsewhere" codepath runs verification of the incoming data stream
> before storing the results (either in loose object files, or in a
> packfile) on disk.

I don't think it could be used at all for anything that speaks the git
protocol, because the object header is not present at all in a packfile.
So even if you hit unpack-objects, it would be writing the (correct)
loose object header itself.

But when we grab loose objects _directly_ from a remote, as in dumb-http
fetch, I'd suspect that the code doing the verification calls
unpack_sha1_header() as part of it. So I didn't test, but I'd strongly
suspect that's a viable attack vector.

I'm not sure what the actual attack would look like, though, aside from
locally accessing memory in a read-only way.


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