HTTPS proxy load balancers should only permit strong cipher suites

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Description

Secure Sockets Layer (SSL) policies determine what port Transport Layer Security (TLS) features clients are permitted to use when connecting to load balancers. To prevent usage of insecure features, SSL policies should use one of the following:

At least TLS 1.2 with the MODERN profile;
The RESTRICTED profile, because it effectively requires clients to use TLS 1.2 regardless of the chosen minimum TLS version; or

a CUSTOM profile that does not support any of the following features:

TLS_RSA_WITH_AES_128_GCM_SHA256
TLS_RSA_WITH_AES_256_GCM_SHA384
TLS_RSA_WITH_AES_128_CBC_SHA
TLS_RSA_WITH_AES_256_CBC_SHA
TLS_RSA_WITH_3DES_EDE_CBC_SHA

Rationale

Load balancers are used to efficiently distribute traffic across multiple servers. Both SSL proxy and HTTPS load balancers are external load balancers, meaning they distribute traffic from the Internet to a GCP network. GCP customers can configure load balancer SSL policies with a minimum TLS version (1.0, 1.1, or 1.2) that clients can use to establish a connection, along with a profile (Compatible, Modern, Restricted, or Custom) that specifies permissible cipher suites. To comply with users using outdated protocols, GCP load balancers can be configured to permit insecure cipher suites. The GCP default SSL policy uses a minimum TLS version of 1.0 and a Compatible profile, which allows the widest range of insecure cipher suites. As a result, it is easy for customers to configure a load balancer without knowing that they are permitting outdated cipher suites.

Impact

Creating more secure SSL policies has the potential to prevent clients using older TLS versions from establishing a connection.

Default value

The GCP default SSL policy is the least secure setting: Min TLS 1.0 and Compatible profile

Remediation

From the console

If the TargetSSLProxy or TargetHttpsProxy does not have an SSL policy configured, create a new SSL policy. Otherwise, modify the existing insecure policy.

Navigate to the SSL Policies page by visiting: https://console.cloud.google.com/net-security/sslpolicies
Click on the name of the insecure policy to go to its SSL policy details page.
Click EDIT.
Set Minimum TLS version to TLS 1.2.
Set Profile to Modern or Restricted.

Alternatively, if the user selects the profile Custom, make sure that the following features are disabled:

TLS_RSA_WITH_AES_128_GCM_SHA256
TLS_RSA_WITH_AES_256_GCM_SHA384
TLS_RSA_WITH_AES_128_CBC_SHA
TLS_RSA_WITH_AES_256_CBC_SHA
TLS_RSA_WITH_3DES_EDE_CBC_SHA

From the command line

For each insecure SSL policy, update it to use secure ciphers:

gcloud compute ssl-policies update NAME [--profile COMPATIBLE|MODERN|RESTRICTED|CUSTOM] --min-tls-version 1.2 [--custom-features FEATURES]

If the target proxy has a GCP default SSL policy, use the following command corresponding to the proxy type to update it.

gcloud compute target-ssl-proxies update TARGET_SSL_PROXY_NAME --ssl-policy SSL_POLICY_NAME
gcloud compute target-https-proxies update TARGET_HTTPS_POLICY_NAME --ssl-policy SSL_POLICY_NAME