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Introduction
This document describes how to alter the methods and ciphers that are used with Secure Socket Layer (SSL) or Transport Layer Security (TLS) configuration on the Cisco Email Security Appliance (ESA).
Contributed by James Noad and Robert Sherwin, Cisco TAC Engineers.
How to alter the methods and ciphers used with SSL/TLS
Note: SSL/TLS methods and ciphers should be set based on your company's specific security policies and preferences. For a third party reference regarding ciphers, please consider Mozilla's Security/Server Side TLS for recommended server configurations and detailed information.
With AsyncOS for Email Security, an administrator can configure the SSL or TLS protocols for the methods and ciphers used for GUI communication, advertised for inbound connections, and requested for outbound connections with the sslconfig command:
example.com (SERVICE)> sslconfig
sslconfig settings:
GUI HTTPS method: sslv3tlsv1
GUI HTTPS ciphers: RC4-SHA:RC4-MD5:ALL
Inbound SMTP method: sslv3tlsv1
Inbound SMTP ciphers: RC4-SHA:RC4-MD5:ALL
Outbound SMTP method: sslv3tlsv1
Outbound SMTP ciphers: RC4-SHA:RC4-MD5:ALL
Choose the operation you want to perform:
- INBOUND - Edit inbound SMTP ssl settings.
- OUTBOUND - Edit outbound SMTP ssl settings.
- VERIFY - Verify and show ssl cipher list.
[]> inbound
Enter the inbound SMTP ssl method you want to use.
1. SSL v2.
2. SSL v3
3. TLS v1
4. SSL v2 and v3
5. SSL v3 and TLS v1
6. SSL v2, v3 and TLS v1
[5]>
Enter the inbound SMTP ssl cipher you want to use.
[RC4-SHA:RC4-MD5:ALL]> MEDIUM:HIGH:-SSLv2:-aNULL:@STRENGTH
sslconfig settings:
GUI HTTPS method: sslv3tlsv1
GUI HTTPS ciphers: RC4-SHA:RC4-MD5:ALL
Inbound SMTP method: sslv3tlsv1
Inbound SMTP ciphers: MEDIUM:HIGH:-SSLv2:-aNULL:@STRENGTH
Outbound SMTP method: sslv3tlsv1
Outbound SMTP ciphers: RC4-SHA:RC4-MD5:ALL
Choose the operation you want to perform:
- INBOUND - Edit inbound SMTP ssl settings.
- OUTBOUND - Edit outbound SMTP ssl settings.
- VERIFY - Verify and show ssl cipher list.
[]>
If changes are made to the SSL configuration, be sure to commit any and all changes.
SSL Methods
As of version 9.6 of AsyncOS for Email Security, the ESA is set by default for TLS v1/TLS v1.2 for method. TLSv1.2 will take precedent in communication, if in use by both sending and receiving parties. In order to establish a TLS connection, both sides must have at least one enabled method that matches, and at least one enabled cipher that matches.
Note: On versions prior to 9.6 of AsyncOS for Email Security, the default had two methods: SSL v3 and TLS v1. Some administrators may want to disable SSL v3 because of recent vulnerabilities, if SSL v3 is enabled.
SSL Ciphers
When you view the default cipher listed from the previous example, it is important to understand the reason that it shows two ciphers followed by the word ALL. Although ALL includes the two ciphers that precede it, the order of the ciphers in the cipher list determines the preference. Thus, when a TLS connection is made, the client picks the first cipher that both sides support based on the order of appearance in the list.
Note: RC4 ciphers are enabled by default on the ESA. In the above example, the MEDIUM:HIGH is based on our Technote, Prevent Negotiations for Null or Anonymous Ciphers on the ESA and SMA. For further information regarding RC4 specifically, please considerMozilla's Security/Server Side TLS, and also On the Security of RC4 in TLS and WPA presented from the USENIX Security Symposium 2013. In order to remove RC4 ciphers from use, please see the examples below.
By manipulating the cipher list, you can influence which cipher gets chosen. In addition to listing specific ciphers or cipher ranges, you can also re-order them by strength by including the @STRENGTH option in the cipher string, as shown:
Enter the inbound SMTP ssl cipher you want to use.
[RC4-SHA:RC4-MD5:ALL]> MEDIUM:HIGH:-SSLv2:-aNULL:@STRENGTH
Please be sure to review the entire ciphers and ranges available on the ESA. This can be seen by running the sslconfig command and then running the verify sub-command. Options for SSL cipher categories are LOW, MEDIUM, HIGH, and ALL:
[]> verify
Enter the ssl cipher you want to verify.
[]> MEDIUM
ADH-RC4-MD5 SSLv3 Kx=DH Au=None Enc=RC4(128) Mac=MD5
IDEA-CBC-SHA SSLv3 Kx=RSA Au=RSA Enc=IDEA(128) Mac=SHA1
RC4-SHA SSLv3 Kx=RSA Au=RSA Enc=RC4(128) Mac=SHA1
RC4-MD5 SSLv3 Kx=RSA Au=RSA Enc=RC4(128) Mac=MD5
IDEA-CBC-MD5 SSLv2 Kx=RSA Au=RSA Enc=IDEA(128) Mac=MD5
RC2-CBC-MD5 SSLv2 Kx=RSA Au=RSA Enc=RC2(128) Mac=MD5
RC4-MD5 SSLv2 Kx=RSA Au=RSA Enc=RC4(128) Mac=MD5
These can also be combined to include ranges:
[]> verify
Enter the ssl cipher you want to verify.
[]> MEDIUM:HIGH
ADH-RC4-MD5 SSLv3 Kx=DH Au=None Enc=RC4(128) Mac=MD5
IDEA-CBC-SHA SSLv3 Kx=RSA Au=RSA Enc=IDEA(128) Mac=SHA1
RC4-SHA SSLv3 Kx=RSA Au=RSA Enc=RC4(128) Mac=SHA1
RC4-MD5 SSLv3 Kx=RSA Au=RSA Enc=RC4(128) Mac=MD5
IDEA-CBC-MD5 SSLv2 Kx=RSA Au=RSA Enc=IDEA(128) Mac=MD5
RC2-CBC-MD5 SSLv2 Kx=RSA Au=RSA Enc=RC2(128) Mac=MD5
RC4-MD5 SSLv2 Kx=RSA Au=RSA Enc=RC4(128) Mac=MD5
ADH-CAMELLIA256-SHA SSLv3 Kx=DH Au=None Enc=Camellia(256) Mac=SHA1
DHE-RSA-CAMELLIA256-SHA SSLv3 Kx=DH Au=RSA Enc=Camellia(256) Mac=SHA1
DHE-DSS-CAMELLIA256-SHA SSLv3 Kx=DH Au=DSS Enc=Camellia(256) Mac=SHA1
CAMELLIA256-SHA SSLv3 Kx=RSA Au=RSA Enc=Camellia(256) Mac=SHA1
ADH-CAMELLIA128-SHA SSLv3 Kx=DH Au=None Enc=Camellia(128) Mac=SHA1
DHE-RSA-CAMELLIA128-SHA SSLv3 Kx=DH Au=RSA Enc=Camellia(128) Mac=SHA1
DHE-DSS-CAMELLIA128-SHA SSLv3 Kx=DH Au=DSS Enc=Camellia(128) Mac=SHA1
CAMELLIA128-SHA SSLv3 Kx=RSA Au=RSA Enc=Camellia(128) Mac=SHA1
ADH-AES256-SHA SSLv3 Kx=DH Au=None Enc=AES(256) Mac=SHA1
DHE-RSA-AES256-SHA SSLv3 Kx=DH Au=RSA Enc=AES(256) Mac=SHA1
DHE-DSS-AES256-SHA SSLv3 Kx=DH Au=DSS Enc=AES(256) Mac=SHA1
AES256-SHA SSLv3 Kx=RSA Au=RSA Enc=AES(256) Mac=SHA1
ADH-AES128-SHA SSLv3 Kx=DH Au=None Enc=AES(128) Mac=SHA1
DHE-RSA-AES128-SHA SSLv3 Kx=DH Au=RSA Enc=AES(128) Mac=SHA1
DHE-DSS-AES128-SHA SSLv3 Kx=DH Au=DSS Enc=AES(128) Mac=SHA1
AES128-SHA SSLv3 Kx=RSA Au=RSA Enc=AES(128) Mac=SHA1
ADH-DES-CBC3-SHA SSLv3 Kx=DH Au=None Enc=3DES(168) Mac=SHA1
EDH-RSA-DES-CBC3-SHA SSLv3 Kx=DH Au=RSA Enc=3DES(168) Mac=SHA1
EDH-DSS-DES-CBC3-SHA SSLv3 Kx=DH Au=DSS Enc=3DES(168) Mac=SHA1
DES-CBC3-SHA SSLv3 Kx=RSA Au=RSA Enc=3DES(168) Mac=SHA1
DES-CBC3-MD5 SSLv2 Kx=RSA Au=RSA Enc=3DES(168) Mac=MD5
Any SSL ciphers that you do not want configured and available should be removed by using the "-" option in front of the specific ciphers. A good example would be as follows:
[]> MEDIUM:HIGH:-SSLv2:-aNULL:@STRENGTH:-EDH-RSA-DES-CBC3-SHA:-EDH-DSS-DES-CBC3-SHA:-DES-CBC3-SHA
This example would negate NULL, EDH-RSA-DES-CBC3-SHA, EDH-DSS-DES-CBC3-SHA, and DES-CBC3-SHA ciphers from being advertised and used in SSL communication.
You can also accomplish similar with "!" in front of the cipher group, or string, that you wish to not have available:
[]> MEDIUM:HIGH:-SSLv2:-aNULL:!RC4:@STRENGTH
This example would remove ALL RC4 ciphers from use. The RC4-SHA and RC4-MD5 ciphers would be negated and not advertised in SSL communication.
If changes are made to the SSL configuration, be sure to commit any and all changes.
