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Re: YUM security issues...



On 25 July 2008, Matt Domsch wrote:
> On Fri, Jul 25, 2008 at 01:52:26PM -0400, Josh Bressers wrote:
> > That's a lot of IPs though.  Can I request multiple /16s, or only one?
> 
> As many as you like.  And recall, such changes are made using your FAS
> credentials.

Are these ever checked?  Does say a mail get generated every time someone
adds one of these?  My fear would be that someone could blanket quite a
large IP space without anyone noticing.  Granted that would no doubt
generate a huge volume of traffic, but if they're serving up a frozen repo,
they probably won't be pushing all that much data.

>  
> > How many mirrors are doing this?
> 
> 374 total Hosts
> 185 have at least 1 netblock entry
> 94 of these are "private" - don't serve the public
> 

wow, that's quite a few.  I wasn't expecting numbers this high honestly.

> 
> > Does the mirror have to be part of the /16 to request it?
> 
> no.  Take for example Dell's mirrors.  Netblock 143.166/16 is Dell US,
> but the mirror IPs are located inside the 10/8 private space.
> 

OK, so here is the problem the way I see it, signing the repository won't
fix it.  I'll try to explain this clearly, Justin can yell at me if I've
gotten any of this wrong.

So let's say Mallory (the bad guy) decides that he wants to host a
malicious mirror and wait for a nasty security flaw.  He sets up his mirror
and even claims some IP subnets to serve.  Bob and Alice are happily
installing valid updates from him for some period of time.  Since Mallory
has claimed to serve a specific subnet, he has a rather impressive view of
what Bob and Alice have installed.

Now let's say there is a horrible security bug found in a mail server.
Mallory knows for a fact that Bob and Alice both have it installed as he's
been their mirror for a while.  Mallory stops updating his mirror, so none
of the users being served will get the mail server updates.  Mallory also
knows the IP address of the vulnerable clients and can easily break into
their systems.

So from what I understand MirrorManager will check on the mirrors to ensure
they're not out of date.  Mallory knows this and makes sure that when
MirrorManager connects to his mirror, it lies and serves up current
metadata.


So here is the problem.  The repodata was valid.  The packages are signed.
Even if we sign the repodata, this attack works.  Being able to acquire an
IP block simply makes this attack easier to do.  It's still very possible
that a bad mirror will wait for users to connect, serve up old content then
use this knowledge to break into their system.

What this problem boils down to, is we need a way for clients to ask
MirrorManager what the current valid repo data is.  Ideally we want the
results to be signed in some manner so it can't be spoofed.

Some thoughts I've had are:

1) Have MirrorManager use https and return some repo verification data.
2) Sign the repo data, and if it's older than X, don't use it (I don't like
    this solution, but it's probably the easiest, just push out a new
    signed repo file once a day, even if nothing changes.)
3) Always get repo data from fedoraproject.org (probably not practical due
    to resource issues)
4) use DNS, have the client query
    <repodata sha1sum>.repo.fedoraproject.org
    if the lookup fails, the repo is invalid.  (this is really cheap from a
    resource standpoint, but hard to do technically)
5) ???

Thanks.

-- 
    JB


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