Junio C Hamano wrote:
> On Thu, Feb 23, 2017 at 8:43 AM, Joey Hess <id@joeyh.name> wrote:
> >
> > Since we now have collisions in valid PDF files, collisions in valid git
> > commit and tree objects are probably able to be constructed.
> 
> That may be true, but
> https://public-inbox.org/git/Pine.LNX.4.58.0504291221250.18901@ppc970.osdl.org/

That's about someone replacing an valid object in Linus's repository
with an invalid random blob they found that collides. This SHA1
break doesn't allow generating such a blob anyway. Linus is right,
that's an impractical attack.

Attacks using this SHA1 break will look something more like:

* I push a "bad" object to a repo on github I set up under a
  pseudonym.
* I publish a "good" object in a commit and convince the maintainer to
  merge it.
* I wait for the maintainer to push to github.
* I wait for github to deduplicate and hope they'll replace the good
  object with the bad one I pre-uploaded, thus silently changing the
  content of the good commit the maintainer reviewed and pushed.
* The bad object is pulled from github and deployed.
* The maintainer still has the good object. They may not notice the bad
  object is out there for a long time.

Of course, it doesn't need to involve Github, and doesn't need to
rely on internal details of their deduplication[1]; 
that only let me publish the bad object under a psydonym.

-- 
see shy jo

[1] Which I'm only guessing about, but now that we have colliding
    objects, we can upload them to different repos and see if such
    dedupication happens.