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Date: Tue, 10 Mar 2015 19:11:41 +0000
From: Marek Kroemeke <kroemeke@...il.com>
To: cve-assign@...re.org
Cc: oss-security@...ts.openwall.com
Subject: Re: Varnish 4.0.3 heap-buffer-overflow while parsing backend server
HTTP response.
Hi there,
It sounds like the same discussion that we had - we tried to avoid it by adding that
last line of the report *cough* ;-)
You don't need DNS spoofing - a php script which returns required "rouge" headers
is enough right? When reporting the previous bugs, the thinking was more about cases like :
* CDNs using varnish (in the same way cloudflare uses nginx, akamai uses "ghost" etc...)
* Shared hosting environments that put varnish in front
* Cases where backend app's response splitting bug is escalated to DoS
or in this case, possibly to have RCE on server in between the client
and the backend app.
* Cases where you have backends with different level of "trust"
And again - it all depends on how you treat reverse proxy server. Our understanding
is that rfc doesn't specify or describe "reverse http proxy" , and essentially
this can be treated as a server (when looking from browser perspective) or a client
when looking from the backend server perspective. Question was - is a case where "client"
may end up in in that undefined state (DoS , RCE due to some buffer overflow..) be treated
as a security problem. Conclusion was that DoS is acceptable, not sure what about other
types of bugs though.
To be clear - we think that if vendor clearly says that you have to fully trust your
backend server when you use the product, then this is what it is, but we can't/couldn't
find anything like that on vendor's website (should we? - don't know, open question).
regards,
On Tue, Mar 10, 2015 at 02:21:26PM -0400, cve-assign@...re.org wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> > Our understanding after previous reports is that varnish security model assumes full
> > trust of the backend, so this is not considered a security problem
>
> We'll try to infer CVE inclusion based on:
>
> http://openwall.com/lists/oss-security/2014/07/08/13
>
> Our understanding is that Varnish Cache trusts the backend HTTP
> servers for two specific properties:
>
> 1. integrity of the web content
> 2. availability of the externally offered web service
>
> In the July 2014 discussion, the scenario was that a single use of a
> rogue backend server, caused by DNS spoofing, could cause a long-lived
> denial of service of the externally offered web service. There wasn't
> a CVE ID assignment because (as a rough summary) the product resolves
> DNS names only once, and the administrator is able to verify the IP
> addresses before those addresses are used.
>
> More generally, no privilege boundary is crossed if a backend HTTP
> server arbitrarily interferes with the intended behavior of the
> externally offered web service.
>
> There's a separate question of whether a privilege boundary is crossed
> if a backend HTTP server can take control of the Varnish Cache server
> machine. As far as we know, the Varnish Cache vendor has not directly
> commented on that.
>
> So, we expect that the outcome would be:
>
> - if the AddressSanitizer report corresponds to a buffer overflow or
> buffer over-read that we understand is exploitable only for a
> crash, then there won't be a CVE
>
> - if the AddressSanitizer report corresponds to a remote code
> execution vulnerability, then it's up to the vendor to clarify
> their perspective on trusting backend HTTP servers. If a system
> administrator decides to use a DNS domain name in a backend
> definition, and this results in reaching a wrong backend server,
> and that backend server launches a successful code-execution
> attack, this is perhaps sufficiently outside the bounds of
> expected or reasonable behavior that a CVE is required.
>
> - --
> CVE assignment team, MITRE CVE Numbering Authority
> M/S M300
> 202 Burlington Road, Bedford, MA 01730 USA
> [ PGP key available through http://cve.mitre.org/cve/request_id.html ]
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