Re: [qmailtoaster] SSL Problem Dovecot

[Gary Bowling]

Yes it's a bit tricky for sure. Phones for email, which I have a lot of. I have a customer with a fax machine that emails faxes, so it has an email account configured in it. All these things run TLSv1 and aren't things I can dictate go away. I also found that squirrelmail uses TLSv1 and ECDHE-RSA-AES256-SHA. Since it's logging in from 127.0.0.1 to 127.0.0.1 it's not a problem. But it IS a problem for setting these things in the server. At this point, I have NO ssl_cipher_list configured in dovecot, so it's using whatever the default is. I set it back this way (that's what it was when I started this exercise) because everything I configured caused me problems. I need to leave the users alone for a bit so they can get some work done :) With it set this way, I scanned my server using https://www.immuniweb.com/ssl/ Looks like it scans both the mail protocols and the web protocols. The only big problem is shows is the use of TLSv1, which I'm not sure I can do anything about at this point. There are a few other things it points out that I need to look in to.. - Doesn't support TLSv1.3. Not sure I can do anything about this one as I would assume it requires an update to openssl. - The server does not prefer cipher suites. Need to do some research on this one. - The server does not enforce HTTP Strict Transport Security. FIXED by adding the following to my virtualhost. Header always set Strict-Transport-Security "max-age=63072000; includeSubdomains;" Gary On 9/4/2019 10:01 AM, CarlC Internet Services Service Desk wrote: The problem is, you have to walk a fine line with your “customers”. If they are on an old version of Outlook on Windows 7, it’s possible they can’t do TLS 1.2 or even 1.1… I had a few clients like that and explained that they had to run Windows Update to get the W7 system up to TLS 1.1/1.2. The reason they don’t see this using web based? The browser can have its own library of TLS. Another email client, Thunderbird, has issues as well and you have to be careful as to how high you make the settings [unless you can dictate the email client to your customer base, then you can demand a client that supports all high level TLS 1.2].   And, just putting it on “HIGH” will result in some breakable ciphers being used. You really need to run a TLS/SSL scan against your ports to see which ones you still have open or in use to make sure you lock down the system correctly.   Carl   From: Gary Bowling [mailto:g...@gbco.us] Sent: Wednesday, September 04, 2019 09:19 AM To: qmailtoaster-list@qmailtoaster.com Subject: Re: [qmailtoaster] SSL Problem Dovecot     FYI. I wanted to see in the log files, what version people were using prior to making changes. To do that you need to add a %k to the login_log_format_elements line in the dovecot configuration. So I added this to the /etc/dovecot/local.conf file on my toaster.   login_log_format_elements = user=<%u> method=%m rip=%r lip=%l mpid=%e %c %k   After doing this, and watching the logs. I was surprised to find quite a few logins with TLSv1. The log file shows "TLSv1 with cipher ECDHE-RSA-AES256-SHA (256/256 bits)"   So much for using HIGH:-SSLv3 in the dovecot config!   I'm not sure how to log the version in qmail. Or even which log it would be in.   Which also brings up another question. When you require (as we all do now) verification of user/password on "send" in the clients. Which is an SMTP outgoing server config in most clients. Will that show up in the dovecot logs or in the qmail/smtp/send logs? I'm not sure which application does that verification.   Thanks, Gary   On 9/4/2019 8:04 AM, Gary Bowling wrote:   That's excellent info Andy, many thanks for that!! I'm going to have to go back and read it about 10 times and possibly go read the referenced material too!   Questions, I think you are saying that I can put either 'HIGH:-SSLv3' in the tlsserverciphers file (and also in the dovecot.conf file) or I can do openssl ciphers -v 'HIGH:-SSLv3' > tlsserverciphers to put the full individual ciphers in the list?   Can I also put the full individual ciphers in the dovecot.conf? I probably wouldn't, but just curious.   I understand the info about the client/server negotiation. But then you talk about other servers, I suppose the server to server delivery over smtp. In that scenario, does the sending server send the list of ciphers and the receiving server match that to what it has and pick the first overlapping cipher to use?   In the case of dovecot, if you specify a cipher list and also a min protocol, I'm assuming it won't use a cipher for something lower than the specified protocol, even if it's in the list? Maybe it doesn't offer up a cipher that doesn't meet the min protocol spec?   For my server, I'm not sure I care whether I receive mail from a Centos 5 server. I realize many here are still using them, but it's been out of support for a while so it should be either patched or upgraded. I guess bottom line is I need to try something like the following:   tlsservercipher contains ''ECDHE:DHE:-SSLv3'   (without the quotes?) toaster.conf (in the /etc/dovecot/ dir) contains ssl_cipher_list = ECDHE:DHE:-SSLv3 ssl_min_protocol = TLSv1.2   Then I need to watch logs to see if I have problems. I'm guessing problems would show up in both the dovecot.log and the /var/log/qmail/smtp or /var/log/qmail/send logs.   Thanks, Gary     On 9/4/2019 1:46 AM, Andrew Swartz wrote: Some background: During the TLS negotiation, the client gives the server a list of ciphers which it supports, then from that list the server chooses which one to use. The server's cipher list is a list, in order of preference, of the ciphers it will use (from the client's list).  If there is no overlap between what the client offers and what the server requires, then the connection fails. The server dose not use the cipher list itself, but rather just passes the list to openssl when it requests establishment of the TLS connection.  Therefore essentially all servers/clients use the same format cipherlist. The next thing to know is that the list can specify individual ciphers or macros like "TLSv1.2".  Most people do not specify individual ciphers but rather just use the macros. There is no right or wrong for a cipher list, as the most appropriate list is the one which best meets your security requirements. The cipherlist "builds" a list of ciphers: 'ALL' adds all of the ciphers (including those with no encrpytion). 'ALL:-SSLv2' adds all the ciphers and then removes all of the SSLv2 ciphers. A reasonable cipherlist is: 'HIGH:-SSLv3' If you want "perfect forward secrecy", try this: 'ECDHE:DHE:-SSLv3' This will yield a subset of the TLSv1.2 ciphers which has the elliptic-curve diffie-hellman-ephemerel ciphers first and then standard diffie-hellman-ephemerel ciphers after that. If you put that into openssl ciphers ( openssl ciphers -v 'HIGH:-SSLv3') you will note that you only get TLSv1.2 ciphers.  That is because an important concept is the difference between ciphers and protocols.  TLS 1.0 and 1.1 updated the protocol but added no new ciphers.  (you can confirm this by comparing "openssl ciphers -v 'SSLv3' | md5sum" to "openssl ciphers -v 'TLSv1' | md5sum"; you'll get an error if you do it with TLSv1.1 because it does not even have a list of ciphers). But note that older servers, such as centos 5, will not be able to connect to you (if you use 'ECDHE:DHE:-SSLv3') because their old version of openssl does not support TLSv1.2.  In that case, for STARTTLS, it will fail, which will default to smtp transmission as cleartext.  SMTP is somewhat forgiving, as a failed STARTTLS connection will fall back to cleartext, whereas most other TLS protocols will fail to connect. This is a segway into the related topic of "protocols".  Many servers (like dovecot) have separate a setting for "TLS cipherlist" and "TLS protocol".  The protocol is the algorithm for establishing the connection, and it is independent of the ciphers.  You should avoid the SSLv3 or TLSv1 protocols, as the these protocols have been found to have weaknesses in how they negotiate the connection (completely unrelated to the strength of the ciphers). This manpage is a good explanation of all the macros and has examples at the end: https://www.openssl.org/docs/man1.0.2/man1/ciphers.html People with older versions of openssl (i.e. Centos 5) cannot do TLSv1.2 and will have no choice but to use ciphers/protocols with known weaknesses, and then hope that the other servers do not try to force a certain level of cipher/protocol.  That is not supposed to happen (per smtp/STARTTLS protocol), but I know for a fact that does:  I finally decided to upgrade from centos-5 because an important mail server started refusing to receive mail from mine, with a complaint about not accepting the SSLv3 ciphers.  I think it was Outlook Server, but I'm not sure. Hope this helps. -Andy PS: Someone running the old version of openssl will need to put '-SSLv2" at the end of the cipherlist, whereas the newer version no longer supports it so it doesn't require removing it.  And NO ONE should be using the SSLv2 protocol, as hacking it is trivial. On 9/3/2019 1:22 PM, CarlC Internet Services Service Desk wrote: Actually, doing the openssl ciphers > /var/qmail/control/tlsservercipher is a starting point. After I did that, I then ran my server through some tests. I happen to use OpenVAS [which tool you want to use to find insecure SSL connections is up to you]. It was able to tell me which ciphers to disable and why. Whichever product you use to test the SSL should be one that’s up to date [or can be brought up to date]. For example, I run the tests against my email server every week [for example, I test against port 25, 465 and 587]. In my case, I also use OpenVAS to test the HTTPS side as well. If you’re using dovecot, you will want to also put the ssl_cipher_list in /etc/dovecot/dovecot.conf as well as the ssl_protocols list. This protects your IMAPS and POP3S protocols. Again, OpenVAS is set to run against those protocols as well. Carl *From:*Gary Bowling [mailto:g...@gbco.us] *Sent:* Tuesday, September 03, 2019 03:35 PM *To:* qmailtoaster-list@qmailtoaster.com *Subject:* Re: [qmailtoaster] SSL Problem Dovecot Thanks for that Carl. I'm running openssl-1.0.2k-16.el7_6.1.x86_64 Pretty much everything about my server is continuously updated stock Centos 7. Currently at CentOS Linux release 7.6.1810 (Core) I do have epel installed, which updates some things and the qmt repo. That's it, and I'm a stickler for NOT installing anything that isn't done through yum and those repos. I've done this long enough to know that it's much easier to maintain, migrate to a new server, etc. is you're running everything in a managed way. So installing the repos and doing yum installs is pretty much the only way anything ever changes on my server, sans config files. Would be very interested in knowing not only the proper tlsservercipher file for this type of server, but also how to create/recreate it if it's a command done from openssl. Looks like you can create it with the command. openssl ciphers > /var/qmail/control/tlsservercipher But what I'm reading is that your advice is to NOT do that due to security concerns. So what would you recommend? Thanks, Gary On 9/3/2019 3:28 PM, CarlC Internet Services Service Desk wrote:     Your real problem is that this file is different based on which     CentOS you’re on [or should I say, which openssl is loaded]. If you     have CentOS 7, with openssl 1.0.2k, you can tune this file to     include each cipher you want [the file can actually be 10+ lines     long wrapped]. This is so you can remove all the “hacked” ciphers,     especially to force your clients security to remain high. If your     running openssl 0.9.x, you don’t get the newer TLS ciphers you need     to be secure.     Using the default is way too low, and if you do, you will where     someone gets hacked over a ‘free’ WiFi connection [because you had     SSL 3.0/TLS 1.0 on].     Carl     *From:*Gary Bowling [mailto:g...@gbco.us]     *Sent:* Tuesday, September 03, 2019 02:58 PM     *To:* qmailtoaster-list@qmailtoaster.com     <mailto:qmailtoaster-list@qmailtoaster.com>     *Subject:* Re: [qmailtoaster] SSL Problem Dovecot     So this may be an issue of the tlsserverciphers file. Some times     it's interesting not knowing what your doing! haha     I guess the question I have is.. What is the proper tlsserverciphers     for a qmailtoaster with a letsencrypt certificate. If that even     makes sense.     And what is the proper way to actually do it. I've read multiple     things on various forums, including here.     One says to do:     echo     "!EDH:!DHE:!RC4:!ADH:!DSS:HIGH:+AES128:+AES256-SHA256:+AES128-SHA256:+SHA:!3DES:!NULL:!aNULL:!eNULL"      > /var/qmail/control/tlsserverciphers     One says to do:     openssl ciphers 'MEDIUM:HIGH:!SSLv2:!MD5:!RC4:!3DES' >     /var/qmail/control/tlsserverciphers     yet another says to create a sym link to the servercert.pem file.     ln -sf /var/qmail/control/servercert.pem     /var/qmail/control/tlsserverciphers     I guess it has to do with how tight you want security to be and     maybe tlsserverciphers can contain various forms of how to define     that. Just looking for what "most" people would use for an up to     date Centos 7 server.     Thanks, Gary     On 9/3/2019 11:04 AM, Gary Bowling wrote:         I had to get a new cert for my server, which I installed         yesterday. Now I'm having problems with certain clients logging         in. I get the following error in the dovecot.log.         TLS handshaking: SSL_accept() failed: error:1408A10B:SSL         routines: ssl3_get_client_hello:wrong version number         Any help would be appreciated.         Thanks, Gary         --         ____________________         Gary Bowling         ____________________         ---------------------------------------------------------------------         To unsubscribe, e-mail:         qmailtoaster-list-unsubscr...@qmailtoaster.com         <mailto:qmailtoaster-list-unsubscr...@qmailtoaster.com> For         additional commands, e-mail:         qmailtoaster-list-h...@qmailtoaster.com         <mailto:qmailtoaster-list-h...@qmailtoaster.com>     --------------------------------------------------------------------- To     unsubscribe, e-mail: qmailtoaster-list-unsubscr...@qmailtoaster.com     <mailto:qmailtoaster-list-unsubscr...@qmailtoaster.com> For     additional commands, e-mail: qmailtoaster-list-h...@qmailtoaster.com     <mailto:qmailtoaster-list-h...@qmailtoaster.com> --------------------------------------------------------------------- To unsubscribe, e-mail: qmailtoaster-list-unsubscr...@qmailtoaster.com <mailto:qmailtoaster-list-unsubscr...@qmailtoaster.com> For additional commands, e-mail: qmailtoaster-list-h...@qmailtoaster.com <mailto:qmailtoaster-list-h...@qmailtoaster.com> --------------------------------------------------------------------- To unsubscribe, e-mail: qmailtoaster-list-unsubscr...@qmailtoaster.com For additional commands, e-mail: qmailtoaster-list-h...@qmailtoaster.com --------------------------------------------------------------------- To unsubscribe, e-mail: qmailtoaster-list-unsubscr...@qmailtoaster.com For additional commands, e-mail: qmailtoaster-list-h...@qmailtoaster.com --------------------------------------------------------------------- To unsubscribe, e-mail: qmailtoaster-list-unsubscr...@qmailtoaster.com For additional commands, e-mail: qmailtoaster-list-h...@qmailtoaster.com --------------------------------------------------------------------- To unsubscribe, e-mail: qmailtoaster-list-unsubscr...@qmailtoaster.com For additional commands, e-mail: qmailtoaster-list-h...@qmailtoaster.com