On 24/10/2014 18:19, Aditya Kumar wrote:
Thanks Jakob for correcting my understanding. In short, can I conclude
the following about FALLBACK flag.
1. Whenever client is sending the FALLBACK flag in its request, an
updated Server will interpret it that this client supports a higher
version but since that higher version protocol request was refused,
its trying to connect using a lower version protocol.
2. The FALLBACK flag should only be set to communicate to those
extremely rare old SSLv3 servers which completely fail to accept a
request for (SSLv3 or TLSv1+, the best client have). In that case,
first client should attempt to connect with SSLAUTONEGOTIATE and if it
fails, then connect with SSLV3 FALLBACK enabled.
Much simpler: The FALLBACK flag should be set only to communicate that
the client has activated its manual fall back code (if any). If the
client doesn't contain manual fallback code, it doesn't need to do
anything.
3. Points 2 holds true even for the cases where clients connecting
using TLS 1.2 fail and then client need to connect using TLS 1.1,
TLS1.0 or SSLV3.0. Then client should attempt the next connections
using FALLBACK flag set.
Yes, SSLv3 is just an example, which happens to be important right now
because of poodle.
Hope this will clear all the confusions.
-Aditya
On Fri, Oct 24, 2014 at 5:35 PM, Jakob Bohm <jb-open...@wisemo.com
<mailto:jb-open...@wisemo.com>>wrote:
On 24/10/2014 13:33, Aditya Kumar wrote:
Hi All,
Thanks for your detailed responses, specially Florian Weimer
and Matt Caswell. For the benefit of everyone and me, I am
summarizing the thoughts which I have understood through all
your replies. Please correct me wherever I am wrong.
To summarize:
1. Best way to prevent POODLE attack is to disable
SSLV3 on both client and server side.
2. If for some reason, you cannot disable SSLv3 on server
side even if Server support TLS 1.0 or higher(e.g server
having SSLV23 set), Server definitely need to be patched to
prevent fallback. Once server is patched, it will prevent
updated clients from fallback attack.
3. After server is patched with OpenSSL FALLBACK flag fix,
Server’s behavior will not change for the clients which do not
send FALLBACK flag in their clienthello request. Server will
continue to work with older client as usual. Only if an
updated client sends FALLBACK flag into its clienthello
request, server will be able to prevent fallback.
4. If for some reason, client has to keep SSLV3 enable even
if it supports TLS 1.0 or higher version, client need to patch
itself and set FALLBACK flag so that it does not come under
fallback attack.
WRONG, See below
5. Clients should never set protocol as SSLV23 to support
both SSL3.0 and TLS Servers. Clients should always explicitly
first try to connect using its highest supported
version(TLS1.0 or higher) and if the server rejects the
connection, then clients should explicitly try to connect
using next supported lower version protocol.
WRONG, If client simply calls the SSL23_ (aka SSLAUTONEGOTIATE_) with
options to allow both SSLv3 and higher TLSvX.XX, it is already secure
and will never need to send the fallback flag.
6. While connecting to server using higher protocol like TLS1
or higher, client should set FALLBACK flag so that server do
not allow automatically downgrade to a lower version protocol.
WRONG, Client should always try its full range of enabled SSL/TLS
versions in one attempt, in which case the protocols themselves
(even without the latest patch) will automatically detect and
prevent a fallback MiTM attack.
However if client needs to work around some (extremely rare) old
SSLv3 servers which completely fail to accept a request for (SSLv3
r TLSv1+, the best you have), that client may use a workaround of:
Step 6.1: Attempt to connect with SSLAUTONEGOTIATE_(SSLv3 up to
TLSv1.2). Do not set/send FALLBACK flag.
Step 6.2: If Step 6.1 fails (either because of old broken server or
because of new fallback MiTM attack), try again with SSLV3ONLY_(),
and set the FALLBACK flag to tell the server that the maximum
version specified in this call is not the true maximum version of
the client (in case it is not an old server, but a MiTM attack
trying to trick this fallback code).
Step 6.3: Step 6.2 could be extended to do retries with TLSv1.1,
then TLSv1.0, then SSLv3 etc. all of which would need the FALLBACK
flag because the client would actually have wanted TLSv1.2 if it
could get it.
**
Few questions which still remains in my mind are:
As part of my question’s reply, Florian replied that following:
*Unconditionally setting SSL_MODE_SEND_FALLBACK_SCSV (if by
default or after user configuration) is a time bomb—your
client application will break once the server implements TLS
1.3 (or any newer TLS version than what is supported by the
OpenSSL version you use). Extremely few applications have to
deal with SSL_MODE_SEND_FALLBACK_SCSV.*
Why client application will break if FALLBACK flag is set and
the server is upgrade to TLS 1.3 or higher version? Isn’t that
the server should take care of this flag when it is updated
with higher version protocol?
Note: If client calls with SSLAUTONEGOTIATE_(SSLvX up to TLSv1.1)
and sets the FALLBACK flag, then a server which understands
TLSv1.2 will read this as "I know this call says I only understand
up to TLSv1.1, but that is only because I think you refused my
attempt to use TLSv1.2 or higher", and therefore the server will
REJECT the connection as if a MiTM attack in progress.
Note 2: If a client calls with SSLAUTONEGOTIATE_(SSLvX up to
TLSv1.2) and sets the FALLBACK flag, then a server which understands
TLSv1.3 will read this as "I know this call says I only understand
up to TLSv1.2, but that is only because I think you refused my
attempt to use TLSv1.3 or higher", and therefore the server will
REJECT the connection as if a MiTM attack is in progress.
Please let me know your opinion on this.
Once again thanks everyone for your response.
-Aditya
Enjoy
Jakob
--
Jakob Bohm, CIO, Partner, WiseMo A/S. http://www.wisemo.com
Transformervej 29, 2860 Søborg, Denmark. Direct +45 31 13 16 10
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