On Fri, Oct 24, 2014 at 11:18 AM, Richard Könning < richard.koenn...@ts.fujitsu.com> wrote:
> At 24.10.2014 19:03, David Li wrote: > >> I am still a little unclear by what exactly TLS_FALLBACK_SCSV option >> would do. >> >> What if the server only supports SSLv3 + TLSv1 and client only connects >> with SSLv3? Without the patch, both would agree to SSLv3. So this is a >> problem. >> > Well I thought it's the CBC cipher mode used by SSLv3 that has the problem. So we should avoid SSLv3 all together. Maybe this is is my confusion. Will the SSLv3 alone cause the attack? Or will a "downgrade process" from TLS 1.0 or later to the SSLv3 expose the vulnerability? > > Where is the problem? When the client only supports SSLv3 and therefore > right away starts with SSLv3, then you get an SSLv3 connection as wanted. > When you don't want any SSLv3 connections, remove the SSLv3 support in the > server and enhance the client so it speaks also TLSv1 or better. > > What happens with the patch only on the server? And what happens with >> the both server and client patched? >> > > First question: nothing, because the client can't say to the server that > the second handshake with SSLv3 is a fallback of a previous handshake > announcing the availability of TLSv1 or better. > > Second question: When the client starts due to a MITM attack a second > handshake announcing SSLv3 and setting TLS_FALLBACK_SCSV option than the > server knows that the client supports something better than SSLv3 and quits > the handshake. > > Best regards, > Richard > > >> >> >> On Fri, Oct 24, 2014 at 9:30 AM, Jakob Bohm <jb-open...@wisemo.com >> <mailto:jb-open...@wisemo.com>> wrote: >> >> 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 >> <tel:%2B45%2031%2013%2016%2010> >> This public discussion message is non-binding and may contain errors. >> WiseMo - Remote Service Management for PCs, Phones and Embedded >> >> >> > ______________________________________________________________________ > OpenSSL Project http://www.openssl.org > User Support Mailing List openssl-users@openssl.org > Automated List Manager majord...@openssl.org >
