Tuesday, June 16, 2015

How to fix weak DH key in Zimbra 7

I just had to fix a problem of weak DH keys in Zimbra 7. Namely, Firefox and Chrome, after upgrade, don't want to connect to servers that use DH keys less than 1024 bits. This means that IMAP won't work either, as it uses SSL/TLS too (or at least it should ;)). Note that the solution is to upgrade to newest Zimbra version, but for me at the moment there is no way I can upgrade my server, i.e. the upgrade is planned but currently it isn't possible. Googling around gave nothing for Zimbra 7, but only for Zimbra 8. In the end, it also turned out that in order to fix the length of DH keys it is necessary to have Java 8, while in Zimbra 7 Java 6 is used.

After a lot of search, the solution turned out to be easy. The key is to disable cipher suites that use DH keys. I managed to do that using the following commands:
zmprov mcf +zimbraSSLExcludeCipherSuites TLS_DHE_RSA_WITH_AES_128_CBC_SHA
zmprov mcf +zimbraSSLExcludeCipherSuites TLS_DHE_RSA_WITH_AES_256_CBC_SHA
zmprov mcf +zimbraSSLExcludeCipherSuites TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA
zmprov mcf +zimbraSSLExcludeCipherSuites TLS_DHE_RSA_WITH_DES_CBC_SHA
zmprov mcf +zimbraSSLExcludeCipherSuites TLS_DHE_RSA_WITH_DES_CBC3_SHA
zmprov mcf +zimbraSSLExcludeCipherSuites TLS_EDH_RSA_WITH_3DES_EDE_CBC_SHA
zmprov mcf +zimbraSSLExcludeCipherSuites SSL_EDH_RSA_WITH_3DES_EDE_CBC_SHA
zmprov mcf +zimbraSSLExcludeCipherSuites TLS_DHE_DSS_WITH_AES_128_CBC_SHA
zmprov mcf +zimbraSSLExcludeCipherSuites TLS_DHE_DSS_WITH_AES_256_CBC_SHA
zmprov mcf +zimbraSSLExcludeCipherSuites SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA
zmmailboxdctl restart
After that, Webmail worked again. You can check supported ciphersuites using sslscan command, i.e. in my case after the given change I got the following ciphersuites:
$ sslscan webmail:443 | grep Accepted
    Accepted  SSLv3  256 bits  AES256-SHA
    Accepted  SSLv3  168 bits  EDH-RSA-DES-CBC3-SHA
    Accepted  SSLv3  168 bits  DES-CBC3-SHA
    Accepted  SSLv3  128 bits  AES128-SHA
    Accepted  SSLv3  128 bits  RC4-SHA
    Accepted  SSLv3  128 bits  RC4-MD5
    Accepted  TLSv1  256 bits  AES256-SHA
    Accepted  TLSv1  168 bits  EDH-RSA-DES-CBC3-SHA
    Accepted  TLSv1  168 bits  DES-CBC3-SHA
    Accepted  TLSv1  128 bits  AES128-SHA
    Accepted  TLSv1  128 bits  RC4-SHA
    Accepted  TLSv1  128 bits  RC4-MD5
Even though Webmail worked, Thunderbird didn't connect. Using Wireshark I found out that Thunderbird, for IMAP connection, tries to use EDH-RSA-DES-CBC3-SHA. I tried to disable that ciphersuite on the server side, but no matter what I've tried, it didn't work. In the end I disabled that cipher on the client side. I opened Thunderbird's configuration editor and there I manually disabled given cipher by setting configuration setting to false.

About Me

scientist, consultant, security specialist, networking guy, system administrator, philosopher ;)

Blog Archive