Re: [fetchmail-devel] PATCH: Clear SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS without SSL_CTX_clear_options()

[Earl C. <ear...@ya...>]

Matthias,

> The question I am asking myself is: What use is there in supporting
> "older implementations" (meaning before 0.9.8m)?  How many systems are
> affected (and I would tend to ignore enterprise distributions - they
> ought to patch OpenSSL instead).  I suppose those older implementations
> would likely also be vulnerable to several other attacks, which might
> subvert the effort of closing this (minor) hole.

Yes, a reasonable question to ask.

I think that there are two separate considerations:

a. The absence/presence of SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS
b. The absence/presence of SSL_CTX_clear_options()


My reading is that (a) is the key to closing the vulnerability. The availability of SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS
indicates that the vulnerability is closed and it is important that SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS
is _not_ set in this scenario (except when overridden by FETCHMAIL_DISABLE_CBC_IV_COUNTERMEASURE).

There are two obvious ways to achieve this. One way is to simply clear the bit in SSL_OP_ALL,
and for prior art, I refer you to:


http://www.opensource.apple.com/source/curl/curl-69.1/patches/curl-dont-insert-empty-fragments.patch


This brings me to (b) which use SSL_CTX_clear_options() as an alternate way to manipulate the options. But as
the prior art shows, it is entirely unnecessary in this scenario to use this new function.



> I suppose those older implementations
> would likely also be vulnerable to several other attacks, which might
> subvert the effort of closing this (minor) hole.


As I described above, the whole question of using SSL_CTX_clear_options() is a separate
one from the ability to close down the vulnerability.

The patch I submitted keeps the vulnerability closed, but does not require SSL_CTX_clear_options().

As you have probably figured out, one of the OpenSSL installations that I have (1.0.0-0.10.beta3.fc12) supports
SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS but does not support SSL_CTX_clear_options().


Earl