What is TLS/SSL in Azure App Service?

Transport Layer Security (TLS) is a widely adopted security protocol that is designed to secure connections and communications between servers and clients. In Azure App Service, you can use TLS and Secure Sockets Layer (SSL) certificates to help secure incoming requests in your web apps.

App Service supports TLS to help ensure:

• Encryption of data in transit.
• Authentication of web apps by using trusted certificates.
• Integrity to prevent tampering of data during transmission.

Azure App Service supports the following TLS versions for incoming requests to your web app:

• TLS 1.3: The latest and most secure version, now fully supported.
• TLS 1.2: The default minimum TLS version for new web apps.
• TLS 1.1 and TLS 1.0: Versions supported for backward compatibility, but not recommended.

You can configure the minimum TLS version for incoming requests to your web app and its Source Control Manager (SCM) site. By default, the minimum is set to TLS 1.2.

You can use Azure Policy to help audit your resources and minimum TLS version. Go to App Service apps should use the latest TLS version policy definition and change the values to the minimum TLS version you want your web apps to use. For related policy definitions for other App Service resources, see List of built-in policy definitions - Azure Policy for App Service.

TLS 1.3 is fully supported on App Service and introduces several improvements over TLS 1.2:

• Stronger security, with simplified cipher suites and forward secrecy.
• Faster handshakes for reduced latency.
• Encrypted handshake messages for enhanced privacy.

To require TLS 1.3 for all inbound requests, set Minimum Inbound TLS Version to TLS 1.3 in the Azure portal, the Azure CLI, or your Azure Resource Manager template (ARM template).

TLS 1.3 supports the following cipher suites, which are fixed and can't be customized:

• TLS_AES_256_GCM_SHA384
• TLS_AES_128_GCM_SHA256

These suites provide strong encryption and are automatically used when TLS 1.3 is negotiated.

TLS 1.2 is the default TLS version for App Service. It provides strong encryption and broad compatibility while meeting compliance standards like the Payment Card Industry Data Security Standard (PCI DSS). New web apps and SCM endpoints by default use TLS 1.2 unless you change them.

Azure App Service uses a secure set of TLS 1.2 cipher suites to help ensure encrypted connections and to protect against known vulnerabilities. Although you can enable TLS 1.1 and TLS 1.0 for backward compatibility, we recommend that you use a minimum version of TLS 1.2.

TLS 1.1 and TLS 1.0 are considered legacy protocols and are no longer considered secure. These versions are supported on App Service only for backward compatibility and should be avoided when possible. The default minimum TLS version for new apps is TLS 1.2, and we recommend that you migrate apps that use TLS 1.1 or TLS 1.0.

The minimum TLS cipher suite includes a fixed list of cipher suites with an optimal priority order that you can't change. Reordering or reprioritizing the cipher suites isn't recommended because it could expose your web apps to weaker encryption. You also can't add new or different cipher suites to this list. When you select a minimum cipher suite, the system automatically disables all less secure cipher suites for your web app. You can't selectively disable only some weaker cipher suites.

A cipher suite is a set of instructions that contains algorithms and protocols to help secure network connections between clients and servers. By default, the front-end OS chooses the most secure cipher suite that both App Service and the client support. However, if the client supports only weak cipher suites, the front-end OS would end up choosing a weak cipher suite. If your organization has restrictions on what cipher suites are allowed, you can update your web app’s minimum TLS cipher suite property to ensure that weak cipher suites are disabled for your web app.

For App Service Environments that have the FrontEndSSLCipherSuiteOrder cluster setting configured, update your settings to include the two TLS 1.3 cipher suites:

• TLS_AES_256_GCM_SHA384
• TLS_AES_128_GCM_SHA256

After you update your cluster setting, you must restart your front end for the changes to take effect. Also, you must still include the two required cipher suites described earlier, even when you update to support TLS 1.3. If you already use FrontEndSSLCipherSuiteOrder, we recommend that you don't also enable Minimum TLS Cipher Suite for your web app. The result might be conflicting configurations. Configure only one of these options to manage cipher suite preferences.

End-to-end (E2E) TLS encryption ensures that front-end to worker communication within Azure App Service is encrypted by using TLS. Without this feature, while incoming HTTPS requests are encrypted to the front ends, the traffic from front ends to workers running the application workloads would travel unencrypted inside Azure’s infrastructure.

E2E TLS helps ensure full encryption of traffic between:

• Clients and App Service front ends
• App Service front ends and worker processes hosting the application

This feature is available on:

• Premium App Service plans (recommended for new deployments)
• Legacy Standard App Service plans (existing users)

You can enable E2E TLS encryption via:

• Azure portal settings
• Azure CLI commands
• ARM templates for automation

After you enable E2E TLS encryption, all intra-cluster communications for your web app are encrypted by using TLS, ensuring end-to-end data protection.

To serve HTTPS traffic, App Service requires a TLS/SSL certificate that is bound to your custom domain.

• App Service certificates (ASC). Fully managed certificates issued and renewed automatically by Azure, stored securely in Azure Key Vault.

• Bring your own certificate (BYOC). Upload and manage certificates (in PFX format) issued by third-party Certificate Authorities (CAs).

After you upload or create a certificate, you bind it to a custom domain on your web app by using:

• SNI (Server Name Indication) SSL bindings for multitenant hosting
• IP SSL bindings for dedicated IP addresses

Azure App Service supports mutual TLS (mTLS) on both Linux and Windows App Service plans, so apps can require client certificates for added security.

• Clients present certificates that are validated against a trusted CA chain that you configure.
• Only clients that have valid certificates can connect.
• It's commonly used to secure APIs and internal apps.

• Enable mTLS by using the Azure portal, the Azure CLI, or ARM templates.
• Upload trusted CA certificates for client validation.
• Access client certificate information in app code via request headers.

Azure App Service provides built-in features to manage certificates automatically:

• App Service managed certificates (free). Automatically issued and renewed for custom domains. These certificates are limited to basic domain validation and don't support wildcard or exportable certificates.

• App Service certificates (paid). Fully managed certificates that support advanced scenarios, including wildcard domains and exportable certificates. These certificates are stored and managed in Azure Key Vault.

Azure App Service makes it easy to help secure your web apps by using TLS and SSL. With support for modern TLS versions, flexible certificate options, and advanced features like mutual TLS, App Service helps you protect data in transit and meet compliance requirements.

• Bind SSL certificates to your custom domain