Security Fix for Ruby OpenSSL module: Allow "0/n splitting" as a prevention for the TLS BEAST attack.

In OpenSSL, SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS option for SSL connection is used to prevent TLS-CBC-IV vulnerability described at [1]. It's known issue of TLSv1/SSLv3 but it attracts lots of attention these days as BEAST attack [2] (CVE-2011-3389). Ruby related topics are at our issue tracker [3].

Until now Ruby OpenSSL extension uses SSL_OP_ALL option, the combined option of OpenSSL for various bug workarounds that should be rather harmless, for all SSL connection by default. And it only allows users to add other options so you could not remove the feature that is included in SSL_OP_ALL option, like SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS. It was intentional as it didn't expose constants like SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS, but we should allow to unset the option so that Ruby OpenSSL extension inserts empty fragments at the beginning of SSL connections (named "0/n splitting") to prevent the TLS BEAST attack.

This release defines additional constants and allows users to unset options in SSL_OP_ALL. Default option is still SSL_OP_ALL but you can setup the SSLSocket with doing "0/n splitting" for BEAST prevention like this.

ctx =
ctx.verify_mode = OpenSSL::SSL::VERIFY_PEER, ctx)

Credit to Apple for reporting this issue.

NOTE: Some SSL endpoints are known to have a bug that cannot handle "0/n splitting" correctly so we (and OpenSSL) do not set this as a default option. Please test SSL connectivity before enabling this in production environment. If the other endpoint you're connecting cannot handle "0/n splitting", you must use another workaround for preventing the TLS BEAST attack, like enforcing ciphersuite to use RC4. For details, please find discussions and resources around CVE-2011-3389.