Re: [TLS] Rizzo claims implementation attach, should be interesting

Martin Rex <mrex@sap.com> Thu, 29 September 2011 14:40 UTC

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From: Martin Rex <mrex@sap.com>
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To: yngve@opera.com (Yngve N. Pettersen)
Date: Thu, 29 Sep 2011 16:42:46 +0200 (MEST)
In-Reply-To: <op.v2kqrrnkkvaitl@lessa-ii.oslo.os> from "Yngve N. Pettersen" at Sep 29, 11 04:34:29 pm
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Subject: Re: [TLS] Rizzo claims implementation attach, should be interesting
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Thank you for the detailed and very helpful information!

I also believe that fragmenting application data is likely to break
brittle application design that silently assumes application data boundaries
(network read&writes, respectively SSL_read()&SSL_write()) will
be preserved.

So _not_ upsetting the peers TLS stack might not be a sufficient
precondition to make a mitigation work.  :-(

-Martin


Yngve N. Pettersen wrote:
> 
> The test only used a split record for application data, and was only  
> performed if the server supported on of the CBC ciphersuites supported by  
> the prober, and a successful HTTP request for / without split record had  
> been sent earlier in the test.
> 
> Determining failure required detecting two successive failures that either  
> did not return a response at all, or a 400+ error code that was not the  
> same as was received in the baseline request. (200/300 was considered a  
> success regardless )
> 
> Five different split policies were tested (0, 1, 2, blocksize-1, blocksize  
> -2 bytes), each with or without having the blocks in the same TCP record.  
> The above number was with the two records in the same TCP record (as that  
> had a significantly lower failure rate).
> 
> The above numbers is the combined failure rate for servers that support  
> AES, or only support 3DES. (AES is presumed to be selected by these  
> servers).
> 
> I have previously seen HTTPS servers mishandle two HTTP requests in the  
> same record (the second was ignored); my guess is that this is the same  
> thing: An assumption that a single SSL/TLS Application record contain just  
> a single, complete HTTPS request. My guess is that the responsible servers  
> are front-ends, firewalls or similar, not servers like Apache with modSSL  
> or IIS.
> 
> At least one server return "ET is not a recognized HTTP method",  
> apparently having dropped the record with the "G" part of the request.
> 
> BTW: 0.045% failed all 5 of the tests, so I consider it likely that there  
> is no split policy that will work for all servers.