From: Hiroshi Nakamura <nakahiro@...>
Date: 2012-02-10T22:13:47+09:00
Subject: [ruby-core:42479] [ruby-trunk - Bug #5353] TLS v1.0 and less - Attack on CBC mode


Issue #5353 has been updated by Hiroshi Nakamura.


At first, I misunderstood the message from Martin that he just want to turn off the flag by default. I thought we can turn off the SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS flag if we want.

Based on Apple's report at January, I realized that we didn't offer the feature from the beginning (I confirmed it to Gotoyuzo, the author of original code.) So we added the feature.  Please see the linked commit for more detail.

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