Cipher suites

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Introduction

By default, the Zimbra mailbox server, zmmailboxd, supports both strong and weak SSL/TLS cipher suites for IMAPS, POP3S, and HTTPS. A typical security requirement is to disable weak ciphers which usually includes SSL versions prior to SSLv3 and any cipher not supporting at least 128 bit.

Enable Strong Ciphers

To enable strong ciphers, weak ciphers must be disabled. It is often best to run a SSL/TLS cipher scan first to see which ciphers your server currently supports. Once the supported weak ciphers are determined, they can be disabled one by one system wide using the zimbraSSLExcludeCipherSuites global directory attribute.

Disable weak ciphers using the zmprov command. Be sure to prefix the attribute name with "+" when using mcf to keep existing values.

su - zimbra
zmprov mcf +zimbraSSLExcludeCipherSuites <cipher1>
zmprov mcf +zimbraSSLExcludeCipherSuites <cipher2>
zmprov mcf +zimbraSSLExcludeCipherSuites <cipher...>
zmprov mcf +zimbraSSLExcludeCipherSuites <cipherN>
zmmailboxdctl restart

Here is ciphers list that excludes SSLv2 and ciphers less than 128 bit for IMAPS, POP3S, and HTTPS. This list should pass all security scans, so please let me know if it does not.

zimbraSSLExcludeCipherSuites SSL_DHE_DSS_WITH_DES_CBC_SHA
zimbraSSLExcludeCipherSuites SSL_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA
zimbraSSLExcludeCipherSuites SSL_DHE_RSA_WITH_DES_CBC_SHA
zimbraSSLExcludeCipherSuites SSL_RSA_EXPORT_WITH_DES40_CBC_SHA
zimbraSSLExcludeCipherSuites SSL_RSA_EXPORT_WITH_RC4_40_MD5
zimbraSSLExcludeCipherSuites SSL_RSA_WITH_3DES_EDE_CBC_SHA
zimbraSSLExcludeCipherSuites SSL_RSA_WITH_DES_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

Additional security scans will show ciphers not excluded from the configuration.

Bugs

Prior to ZCS 5.0.10, the zimbraSSLExcludeCipherSuites attribute values are not picked up by the Jetty configuration for HTTPS. To fix, replace all occurrences of zimbraSSLExcludeCipherSuites with zimbraSSLExcludeCipherSuitesXML in /opt/zimbra/jetty/etc/jetty.xml.in.

cd /opt/zimbra/jetty/etc
sed 's/%%zimbraSSLExcludeCipherSuites%%/%%zimbraSSLExcludeCipherSuitesXML%%/g' jetty.xml.in > /tmp/jetty.xml.in.new
cp jetty.xml.in /tmp/jetty.xml.in.old
mv /tmp/jetty.xml.in.new jetty.xml.in
zmmailboxdctl restart

Please see bug 30691 for more details.

References

J2SE cipher list http://java.sun.com/j2se/1.4.2/docs/guide/security/jsse/JSSERefGuide.html#SunJSSE

OpenSSL ciphers list http://openssl.org/docs/apps/ciphers.html#SSL_v3_0_cipher_suites_


Verified Against: Zimbra Collaboration Suite 5.0.9 Date Created: 10/1/2008
Article ID: http://wiki.zimbra.com/index.php?title=Cipher_suites Date Modified: 07/7/2011
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