Issue #5353 has been updated by Martin Bosslet.


Hiroshi Nakamura wrote:

> The original proposal from Martin, turning off the SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS bit by default, is still open.

Yes, to follow up on this: it remains to decide how this 
should be handled in libraries that use OpenSSL::SSL, such
as Net::HTTP. In Net::HTTP's case (and I could imagine
probably in most of the other cases, too), the SSLContext
object is not directly accessible, so we can't configure
0/n splitting there now.

Two paths could be chosen to enable the functionality.
Either patching each of the libraries by offering some
way to configure 0/n splitting - or we could simply
make 0/n splitting the default. The latter would only
require one central change, but bears the potential to
break existing installations. 

Generally we are in favor of staying as compatible as
possible for 2.0, but it would also mean that things
like the "BEAST" attack will remain feasible in the future.
So should we make this the default in trunk? The time
until 2.0 gets released should give incompatible setups
enough time to patch their environment?


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Bug #5353: TLS v1.0 and less - Attack on CBC mode
https://bugs.ruby-lang.org/issues/5353

Author: Martin Bosslet
Status: Open
Priority: High
Assignee: 
Category: ext
Target version: 2.0.0
ruby -v: -


A well-known vulnerability of TLS v1.0 and earlier has recently gained some attention:

http://www.theregister.co.uk/2011/09/19/beast_exploits_paypal_ssl/

Although this has been known for a long time (http://www.openssl.org/~bodo/tls-cbc.txt),
and a fix for this has been provided, in reality most applications seem to be working with

SSL_OP_ALL

which is a flag that enables some bug workarounds that were considered harmless. 

We, too, use this in ossl_sslctx_s_alloc(VALUE klass) in ossl_ssl.c. Unfortunately, 
this flag also includes

SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS

which disables the fix for the "CBC vulnerability". Here is what a comment says 
about the flag (OpenSSL 1.0.0d)

    /* Disable SSL 3.0/TLS 1.0 CBC vulnerability workaround that was added
     * in OpenSSL 0.9.6d.  Usually (depending on the application protocol)
     * the workaround is not needed.  Unfortunately some broken SSL/TLS
     * implementations cannot handle it at all, which is why we include
     * it in SSL_OP_ALL. */

If I understand http://www.openssl.org/~bodo/tls-cbc.txt correctly, the most
notable implementation that does not play well with these empty fragments
was (is?) IE - I don't know how this has evolved over time, I would have to 
research further.

An easy fix for the situation would be to discard SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS,
but this would risk affecting existing installations.

What do you propose? Should we solve this before the 1.9.3 release? 

(PS: The actual attack and fix are outlined in 

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.61.5887&rep=rep1&type=pdf

The attack to be presented by Thai Duong and Juliano Rizzo at 

http://ekoparty.org/cronograma.php (caution: currently the site is victim to the "reddit effect")

is very likely to be based on what was already known and should therefore hopefully
require no further fixes.) 

 


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