This document describes how to alter the Cisco Email Security Appliance (ESA) and Cisco Security Management Appliance (SMA) cipher settings in order to prevent negotiations for null or anonymous ciphers. This document applies to both hardware based and virtual based appliances.
Cisco recommends that you have knowledge of these topics:
The information in this document is based on all versions of the Cisco ESA and Cisco SMA.
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.
Prevent Negotiations for Null or Anonymous Ciphers
This section describes how to prevent negotiations for null or anonymous ciphers on the Cisco ESA that runs AsyncOS for Email Security Versions 9.1 and later, and also on the Cisco SMA.
ESAs that Run AsyncOS for Email Security Version 9.5 or newer
With the introduction of AsyncOS for Email Security Version 9.5, TLS v1.2 is now supported. The commands that are described in the previous section still work; however, you will see the updates for TLS v1.2 included in the outputs.
Here is an example output from the CLI:
sslconfig settings: GUI HTTPS method: tlsv1/tlsv1.2 GUI HTTPS ciphers: MEDIUM HIGH -SSLv2 -aNULL @STRENGTH Inbound SMTP method: tlsv1/tlsv1.2 Inbound SMTP ciphers: MEDIUM HIGH -SSLv2 -aNULL @STRENGTH Outbound SMTP method: tlsv1/tlsv1.2 Outbound SMTP ciphers: MEDIUM HIGH -SSLv2 -aNULL @STRENGTH
Choose the operation you want to perform: - GUI - Edit GUI HTTPS ssl settings. - 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/TLS v1.2 4. SSL v2 and v3 5. SSL v3 and TLS v1/TLS v1.2 6. SSL v2, v3 and TLS v1/TLS v1.2 >
In order to reach these settings from the GUI, navigate to System Administration > SSL Configuration > Edit Settings...:
Tip: For complete information, refer to the appropriate ESA End-User Guide for Version 9.5 or later.
ESAs that Run AsyncOS for Email Security Version 9.1 or older
You can modify the ciphers that are used on the ESA with the sslconfig command. In order to prevent the ESA negotiations for null or anonymous ciphers, enter the sslconfig command into the ESA CLI and apply these settings:
Inbound Simple Mail Transfer Protocol (SMTP) method: sslv3tlsv1
Choose the operation you want to perform: - GUI - Edit GUI HTTPS ssl settings. - 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 > 3
Enter the inbound SMTP ssl cipher you want to use. [RC4-SHA:RC4-MD5:ALL]> MEDIUM:HIGH:-SSLv2:-aNULL:@STRENGTH
Note: Set the GUI, INBOUND, and OUTBOUND as needed for each cipher.
As of AsyncOS for Email Security Version 8.5, the sslconfig command is also available via the GUI. In order to reach these settings from the GUI, navigate to System Administration > SSL Configurations > Edit Settings:
Tip: Secure Sockets Laver (SSL) Version 3.0 (RFC-6101) is an obsolete and insecure protocol. There is a vulnerability in SSLv3 CVE-2014-3566 known as Padding Oracle On Downgraded Legacy Encryption (POODLE) attack, which is tracked by Cisco bug ID CSCur27131. Cisco recommends that you disable SSLv3 while you change the ciphers, use Transport Layer Security (TLS) only, and select option 3 (TLS v1). Refer to Cisco bug ID CSCur27131 for complete details.
SMAs that Run AsycnOS for Content Security Management 9.6 or newer
Similar to the ESA, run the sslconfig command on the CLI.
SMAs that Run AsyncOS for Content Security Management 9.5 or Later
The sslconfig command is not available for old versions of SMA.
Note: Older versions of AsyncOS for SMA only supported TLS v1. Please upgrade to 9.6 or newer on your SMA for up-to-date SSL management.
You must complete these steps from the SMA CLI in order to modify the SSL ciphers:
Save the SMA configuration file to your local computer.