Security/Collab/86

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Security Settings for Zimbra Collaboration 8.6 series

Proxy

Ciphers

The single valued zimbraReverseProxySSLCiphers attribute configures what cipher suites the nginx proxy will allow to be negotiated over SSL. This affects HTTPS when the web proxy is enabled, and POP and IMAP when the mail proxy is enabled. It is only possible to set this value in globalconfig.

The current recommended setting is (removes RC4 from the default in 8.6):

ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128:AES256:HIGH:!aNULL:!eNULL:!EXPORT:!DES:!MD5:!PSK:!RC4

It can be set using the zmprov mcf command:

$ zmprov gcf zimbraReverseProxySSLCiphers ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128:AES256:HIGH:!aNULL:!eNULL:!EXPORT:!DES:!MD5:!PSK:!RC4
$ zmproxyctl restart # on all proxies

Ciphers: Potential Impacts

• To support some older clients, like Outlook 2011 for Mac, one would need to remove the !3DES from this setting (see: bug 97232).
• For 8.7 !RC4 will be in the default value for zimbraReverseProxySSLCiphers and an upgrade step will update the key if the key has not been changed from the old default (ref: bug 96852)

Protocols

The multi-valued attribute controlling the acceptable TLS protocol versions to be used by the nginx proxy is zimbraReverseProxySSLProtocols. It can be set at both the globalconfig and server level.

The current recommend values for this attribute are:

TLSv1
TLSv1.1
TLSv1.2

The values can be modified to either add or remove a protocol:

zmprov mcf zimbraReverseProxySSLProtocols +protocol # add a protocol at the globalconfig level

zmprov mcf zimbraReverseProxySSLProtocols -protocol # remove a protocol at the globalconfig level

If it is desired to have the servers have different protocol level settings, this can be done by setting the values at the server level:

zmprov ms hostname zimbraReverseProxySSLProtocols +protocol # add a protocol at the server level

zmprov ms hostname zimbraReverseProxySSLProtocols -protocol # remove a protocol at the server level

Mailboxd (Jetty)

Ciphers

The multi-valued attributes controlling the acceptable TLS ciphers to be used by mailboxd are zimbraSSLExcludeCipherSuites and zimbraSSLIncludeCipherSuites. They can be set at the globalconfig level.

zimbraSSLExcludeCipherSuites

excluded cipher suites

zimbraSSLIncludeCipherSuites

List of included cipher suites for Jetty. If any value is set only these ciphers will be used, in effect superseding zimbraSSLExcludeCipherSuites. Controlling the cipher suite order will be handled as part of bug 96928.

Default zimbraSSLExcludeCipherSuites settings in 8.6.0 (changing defaults to be addressed in bug 96921):

SSL_RSA_WITH_DES_CBC_SHA
SSL_DHE_RSA_WITH_DES_CBC_SHA
SSL_DHE_DSS_WITH_DES_CBC_SHA
SSL_RSA_EXPORT_WITH_RC4_40_MD5
SSL_RSA_EXPORT_WITH_DES40_CBC_SHA
SSL_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA
SSL_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA

Recommended (*may break compatibility with clients with limited capabilities):

.*_(3DES|RC4)_.*

Ciphers: Potential Impacts

• Excluding RC4 will likely cause problems for Windows XP users, and possibly other old clients.
• Keeping 3DES may still be desirable for interoperability with old clients, including Outlook 2011 for Mac.

References:

• http://docs.oracle.com/javase/8/docs/technotes/guides/security/SunProviders.html
• https://www.ssllabs.com/downloads/SSL_TLS_Deployment_Best_Practices.pdf
• http://www.symantec.com/connect/blogs/ssl-ciphers-beyond-private-key-and-certificate
• http://en.wikipedia.org/wiki/Transport_Layer_Security

Protocols

zimbraMailboxdSSLProtocols

contains a list of SSL/TLS protocols (as documented by SunJSSE Provider Protocols and used in setEnabledProtocols) to be enabled in Jetty for HTTPS, IMAPS, POP3S, and STARTTLS (including LMTP)

The current recommend values for this attribute are:

TLSv1
TLSv1.1
TLSv1.2

Protocols: Potential impacts

• Adding SSLv2Hello may be desirable for interoperability with old clients, including Outlook 2011 for Mac and Clients using Java 6 (see: bug 97332). Please note: Enabling SSLv2Hello does not mean that the server is actually allowing SSLv2, but it does mean that it allows the SSLv2Hello "introduction" before switching to TLSv1 or greater. One known downside of the SSLv2Hello is that it does not support the Server Name Indication TLS extension.

See also: http://www.oracle.com/technetwork/java/javase/documentation/cve-2014-3566-2342133.html

LDAP

The single-valued localconfig attributes controlling the acceptable TLS protocol versions and ciphers to be used by the OpenLDAP server are ldap_common_tlsprotocolmin and ldap_common_tlsciphersuite.

Protocols

ldap_common_tlsprotocolmin

Specifies minimum SSL/TLS protocol version that will be negotiated. To require TLS 1.x or higher, set this option to 3.(x+1), e.g., a value of 3.2 would require TLS 1.1.

Ciphers

ldap_common_tlsciphersuite

Configures what ciphers will be accepted and the preference order for OpenSSL. To check what ciphers a given spec selects in OpenSSL, use: openssl ciphers -v cipher-suite-spec

The current recommend values for these localconfig settings are:

ldap_common_tlsprotocolmin = "3.1"
ldap_common_tlsciphersuite = "MEDIUM:HIGH"

For more details see olcTLSCipherSuite and olcTLSProtocolMin in the OpenLDAP man page for slapd-config(5).

MTA

Protocols

The current recommend values for the MTA's protocol attributes are:

zimbraMtaSmtpdTlsProtocols = !SSLv2, !SSLv3
zimbraMtaLmtpTlsProtocols  = !SSLv2, !SSLv3

However, please note: bug 97186 exists to address the lack of support for controls on mandatory protocols in postfix:

zimbraMtaSmtpdTlsMandatoryProtocols (smtpd_tls_mandatory_protocols)
zimbraMtaSmtpTlsMandatoryProtocols  (smtp_tls_mandatory_protocols)
zimbraMtaLmtpTlsMandatoryProtocols  (lmtp_tls_mandatory_protocols)

Ciphers

The current recommend values for the MTA's ciphers attributes are:

zimbraMtaSmtpTlsCiphers = export
zimbraMtaSmtpTlsMandatoryCiphers = export,low,medium,high,null
zimbraMtaSmtpdTlsCiphers = export
zimbraMtaSmtpdTlsExcludeCiphers = (empty)
zimbraMtaSmtpdTlsMandatoryCiphers = medium
zimbraMtaLmtpTlsCiphers = export
zimbraMtaLmtpTlsExcludeCiphers  = (empty)
zimbraMtaLmtpTlsMandatoryCiphers = export,low,medium,high,null

For more details on Postfix TLS settings see http://www.postfix.org/TLS_README.html.

Further Research and Reading

• https://wiki.mozilla.org/Security/Server_Side_TLS
• https://bettercrypto.org/