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4.1.5 Disable weak ciphers

Information

The ssl_ciphers directive should be used to configure the available ciphers on your web server, and the proxy_ssl_ciphers directive should be used to configure the available ciphers for your proxy. Weak ciphers should be disabled based on your company's policy or an industry best practice compliance profile.

The ssl_prefer_server_ciphers should be used to ensure the user agent respects the server's preferred cipher order and does not set its own. If you are using a proxy or load balancer, you should use the proxy_ssl_ciphers directive to ensure your upstream connections are negotiated using secure ciphers.

Rationale:

The use of strong ciphers is critical to maintaining strong encryption on your web server, load balancer, or proxy. Weak ciphers may compromise the security of your site or your users by allowing legacy user agents to connect to your site in a vulnerable way. You may also meet compliance concerns by ensuring that your upstream connections meet the same level of security if using a proxy or load balancer. The server should enforce the cipher preference on the server side to protect users from malicious actors on the client side.

Impact:

Strong cipher configurations may not allow legacy user agents or user agents with weak configurations to connect to your site. If your server must also pass to a legacy upstream server, this may prevent it from being able to negotiate a cipher upstream.

Solution

The following procedures may be used to implement industry standard cipher profiles if you have an existing profile defined. These profiles may be modified to meet the requirements defined in your company's policy. This procedure assumes that all server blocks will be in /etc/nginx/nginx.conf and not inside any included files in the configuration.
Set the ssl_cipher directive as part of your server block, and set the proxy_ssl_ciphers directive as part of the location block for your upstream server.
This should look similar to the below examples:
Server block configuration for client connectivity to web server, proxy, or load balancer:

server {
 ssl_ciphers ALL:!EXP:!NULL:!ADH:!LOW:!SSLv2:!SSLv3:!MD5:!RC4;
}

Proxy or load balancer configuration for defined upstream negotiation:

location / {
 proxy_pass https://cisecurity.org;
 proxy_ssl_ciphers ALL:!EXP:!NULL:!ADH:!LOW:!SSLv2:!SSLv3:!MD5:!RC4;
}

The below procedure assumes the default configuration profile. If you do not have ssl_ciphers or proxy_ssl_ciphers defined, add the directives to your proxy or web server configuration profile, then run the below commands to configure them to your selected profile.
No weak ciphers SSLLABS proxy configuration

sed -i 's/proxy_ssl_ciphers[^;]*;/proxy_ssl_ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384;/' /etc/nginx/nginx.conf

No weak ciphers SSLLABS web server configuration:

sed -i 's/ssl_ciphers[^;]*;/ssl_ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384;/' /etc/nginx/nginx.conf

For changes to take effect, you must recycle nginx:

systemctl restart nginx

Default Value:

These directives are not specified by default and are set to the default of HIGH:!aNULL:!MD5.

See Also

https://workbench.cisecurity.org/benchmarks/17381

Item Details

Category: ACCESS CONTROL, IDENTIFICATION AND AUTHENTICATION, SYSTEM AND COMMUNICATIONS PROTECTION

References: 800-53|AC-17(2), 800-53|IA-5, 800-53|IA-5(1), 800-53|SC-8, 800-53|SC-8(1), CSCv7|14.4

Plugin: Unix

Control ID: 8c84425944f6fa8a1ceef2f41cbe554dbfb1e8a0e10bb3063d4f37035b295e13