ShanxT - Removing Insecure SSL Ciphers

   KB 20479        Last updated on 09/1/2017  

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During a security scan you may get a list of insecure/non-compliant ciphers. The most common one is the BEAST vulnerability. Disabling certain cipher-suites sorts this out. Zimbra uses OpenSSL, so we just have to obtain the cipher name with the OpenSSL equivalent of it, and then disable it accordingly.

We use the attribute 'zimbraSSLExcludeCipherSuites' for this.

Getting the attribute for the respective cipher suite

Here is a mapping of the cipher name, and the OpenSSL equivalents: or can check with

So for example, if the security scan shows 'AES256-SHA' has to be disabled, on scrolling down the list, (or doing a quick ctrl+f for AES256-SHA), this line comes up:

TLS_RSA_WITH_AES_256_CBC_SHA            AES256-SHA

This is how we can obtain the relevant argument for '+zimbraSSLExcludeCipherSuites'.

The actual command to disable it would be:

zmprov mcf +zimbraSSLExcludeCipherSuites TLS_RSA_WITH_AES_256_CBC_SHA

And then we restart mailbox, with:

zmmailboxdctl restart

Please note the '+' sign in front of 'zimbraSSLExcludeCipherSuites. That adds to the list of cipher suites to be excluded. Not including the '+' sign is dangerous, as it causes the list to be overwritten, and thus only the last cipher suite is included in the list.

Testing if it is enabled or disabled:

You can perform a quick test to see if a cipher suite is supported, by running the following on the Zimbra server:

openssl s_time -connect localhost:443 -new -cipher AES256-SHA

Note that the argument for '-cipher' is the original cipher suite name, and not the OpenSSL equivalent. If the cipher is enabled, the output would be similar to:

$ openssl s_time -connect localhost:443 -new -cipher AES256-SHA
Collecting connection statistics for 30 seconds

And the asterisks would continue for 30 seconds. In some cases it would be a random letter instead of an asterisk, but the bottom line being that the connection is successful.

If the cipher is disabled, you'll just get an error on connection:

$ openssl s_time -connect localhost:443 -new -cipher AES256-SHA
Collecting connection statistics for 30 seconds

Disabling all known ones at once

Now, since mailbox needs to be restarted, you will need to do it during a downtime. The problem here is that as there can be multiple ciphers that fail the scan, the process you would have to follow is: Do security scan -> find name of cipher -> disable cipher -> do scan again, and so on and so forth.

So to make it a little easier, this is a list of cipher suites that clients have disabled to pass their tests. This includes the BEAST vulnerability.

Note: This disables most block ciphers, because those were the ones affected by BEAST. Disabling some of these may break some services[1].

zmprov gacf zimbraSSLExcludeCipherSuites

zimbraSSLExcludeCipherSuites: TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384
zimbraSSLExcludeCipherSuites: TLS_DHE_RSA_WITH_AES_256_CBC_SHA
zimbraSSLExcludeCipherSuites: DHE-RSA-AES256-SHA
zimbraSSLExcludeCipherSuites: TLS_DHE_RSA_WITH_AES_256_CBC_SHA256
zimbraSSLExcludeCipherSuites: DHE-RSA-AES256-SHA256
zimbraSSLExcludeCipherSuites: TLS_RSA_WITH_AES_256_CBC_SHA
zimbraSSLExcludeCipherSuites: SSL_RSA_WITH_DES_CBC_SHA
zimbraSSLExcludeCipherSuites: SSL_DHE_RSA_WITH_DES_CBC_SHA
zimbraSSLExcludeCipherSuites: SSL_DHE_DSS_WITH_DES_CBC_SHA
zimbraSSLExcludeCipherSuites: SSL_RSA_EXPORT_WITH_RC4_40_MD5
zimbraSSLExcludeCipherSuites: SSL_RSA_EXPORT_WITH_DES40_CBC_SHA
zimbraSSLExcludeCipherSuites: TLS_DHE_RSA_WITH_AES_128_CBC_SHA
zimbraSSLExcludeCipherSuites: TLS_RSA_WITH_AES_128_CBC_SHA
zimbraSSLExcludeCipherSuites: SSL_RSA_WITH_3DES_EDE_CBC_SHA
zimbraSSLExcludeCipherSuites: TLS_RSA_EXPORT_WITH_DES40_CBC_SHA
zimbraSSLExcludeCipherSuites: TLS_RSA_WITH_DES_CBC_SHA
zimbraSSLExcludeCipherSuites: TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA
zimbraSSLExcludeCipherSuites: SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA
zimbraSSLExcludeCipherSuites: TLS_DHE_RSA_WITH_AES_128_CBC_SHA256
zimbraSSLExcludeCipherSuites: TLS_RSA_WITH_AES_128_CBC_SHA256
zimbraSSLExcludeCipherSuites: TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA
zimbraSSLExcludeCipherSuites: TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256
zimbraSSLExcludeCipherSuites: TLS_RSA_WITH_AES_256_CBC_SHA256

To run all the above in a single command, run the following:

zmprov mcf +zimbraSSLExcludeCipherSuites TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384 \
 +zimbraSSLExcludeCipherSuites TLS_DHE_RSA_WITH_AES_256_CBC_SHA \
 +zimbraSSLExcludeCipherSuites DHE-RSA-AES256-SHA  \
 +zimbraSSLExcludeCipherSuites TLS_DHE_RSA_WITH_AES_256_CBC_SHA256 \
 +zimbraSSLExcludeCipherSuites DHE-RSA-AES256-SHA256 \
 +zimbraSSLExcludeCipherSuites TLS_RSA_WITH_AES_256_CBC_SHA \
 +zimbraSSLExcludeCipherSuites SSL_RSA_WITH_DES_CBC_SHA  \
 +zimbraSSLExcludeCipherSuites SSL_DHE_RSA_WITH_DES_CBC_SHA \
 +zimbraSSLExcludeCipherSuites SSL_DHE_DSS_WITH_DES_CBC_SHA \
 +zimbraSSLExcludeCipherSuites SSL_RSA_EXPORT_WITH_RC4_40_MD5 \
 +zimbraSSLExcludeCipherSuites SSL_RSA_EXPORT_WITH_DES40_CBC_SHA \
 +zimbraSSLExcludeCipherSuites SSL_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA \
 +zimbraSSLExcludeCipherSuites SSL_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA \
 +zimbraSSLExcludeCipherSuites TLS_DHE_RSA_WITH_AES_128_CBC_SHA \
 +zimbraSSLExcludeCipherSuites TLS_RSA_WITH_AES_128_CBC_SHA \
 +zimbraSSLExcludeCipherSuites SSL_RSA_WITH_3DES_EDE_CBC_SHA \
 +zimbraSSLExcludeCipherSuites TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA \
 +zimbraSSLExcludeCipherSuites TLS_RSA_EXPORT_WITH_DES40_CBC_SHA \
 +zimbraSSLExcludeCipherSuites TLS_RSA_WITH_DES_CBC_SHA \
 +zimbraSSLExcludeCipherSuites TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA \
 +zimbraSSLExcludeCipherSuites SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA \
 +zimbraSSLExcludeCipherSuites TLS_DHE_RSA_WITH_AES_128_CBC_SHA256 \
 +zimbraSSLExcludeCipherSuites TLS_RSA_WITH_AES_128_CBC_SHA256 \
 +zimbraSSLExcludeCipherSuites TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA \
 +zimbraSSLExcludeCipherSuites TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256 \
 +zimbraSSLExcludeCipherSuites TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA \
 +zimbraSSLExcludeCipherSuites TLS_RSA_WITH_AES_256_CBC_SHA256 

After it runs, restart mailbox.

Bug 80133

We have also disabled a lot of insecure ciphers by default in 8.0.4. Please read the release notes of 8.0.4, and see the bug here:

Verified Against: Zimbra Collaboration Suite 8,7 Date Created: 06/24/2013
Article ID: Date Modified: 09/1/2017

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