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Date: Tue, 10 Mar 2015 19:11:41 +0000
From: Marek Kroemeke <>
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). 


On Tue, Mar 10, 2015 at 02:21:26PM -0400, wrote:
> 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:
> 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 ]
> Version: GnuPG v1.4.14 (SunOS)
> rOAsm0IzgUYYDf0+061CwTzyZrNV3IjLYBs6P2/t+Qvh1Z9F2+sDcf1cWn7dJORo
> 6cc7m6hUBBaFBbYItDKp4UvnBMiyEKeC3bMEnMPdB6Z/Ukev2tO8Go6RwJL0jnDP
> Ry48HSNhiJMtBmMf+PXsq4rOFz3VSvJhL0iv105URg/h8hBM0WZqhjfVL0MGuGDu
> jdvlz3GR3xl7rPgaPDsKN/jdVcDVkKKvsbjD6yeJ6G28WuSg29VRDVQZX+utm5LE
> MwNZzEyTqffDtSJzx1nieTczLK7wcg+bD8wmb6rwFYf/Tsy0OZGOdyf6vaJsTPU=
> =tBwy

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