Which type of device can react to network traffic and create or modify configuration rules to adapt?

The Network Security Configuration feature lets apps customize their network security settings in a safe, declarative configuration file without modifying app code. These settings can be configured for specific domains and for a specific app. The key capabilities of this feature are as follows:

• Custom trust anchors: Customize which Certificate Authorities (CA) are trusted for an app's secure connections. For example, trusting particular self-signed certificates or restricting the set of public CAs that the app trusts.
• Debug-only overrides: Safely debug secure connections in an app without added risk to the installed base.
• Cleartext traffic opt-out: Protect apps from accidental usage of cleartext traffic.
• Certificate pinning: Restrict an app's secure connection to particular certificates.

Add a Network Security Configuration file

The Network Security Configuration feature uses an XML file where you specify the settings for your app. You must include an entry in the manifest of your app to point to this file. The following code excerpt from a manifest demonstrates how to create this entry:

Customize trusted CAs

An app may want to trust a custom set of CAs instead of the platform default. The most common reasons of this are:

• Connecting to a host with a custom certificate authority, such as a CA that is self-signed or is issued internally within a company.
• Limiting the set of CAs to only the CAs you trust instead of every pre-installed CA.
• Trusting additional CAs not included in the system.

By default, secure connections (using protocols like TLS and HTTPS) from all apps trust the pre-installed system CAs, and apps targeting Android 6.0 (API level 23) and lower also trust the user-added CA store by default. An app can customize its own connections using base-config (for app-wide customization) or domain-config (for per-domain customization).

Configure a custom CA

Assume you want to connect to your host which uses a self-signed SSL certificate or to a host whose SSL certificate is issued by a non-public CA which you trust, such as your company's internal CA.

res/xml/network_security_config.xml:

Add the self-signed or non-public CA certificate, in PEM or DER format, to res/raw/my_ca.

Limit the set of trusted CAs

An app that does not want to trust all CAs trusted by system can instead specify its own reduced set of CAs to trust. This protects the app from fraudulent certificates issued by any of the other CAs.

The configuration to limit the set of trusted CAs is similar to trusting a custom CA for a specific domain except that multiple CAs are provided in the resource.

res/xml/network_security_config.xml:

Add the trusted CAs, in PEM or DER format, to res/raw/trusted_roots. Note that if using PEM format the file must contain only PEM data and no extra text. You can also provide multiple <certificates> elements instead of one.

Trust additional CAs

An app may want to trust additional CAs not trusted by the system, this could be due to the system not yet including the CA or a CA that does not meet the requirements for inclusion into the Android system. An app can do this by specifying multiple certificate sources for a configuration.

res/xml/network_security_config.xml:

Configure CAs for debugging

When debugging an app that connects over HTTPS, you may want to connect to a local development server, which does not have the SSL certificate for your production server. In order to support this without any modification to your app's code, you can specify debug-only CAs, which are trusted only when android:debuggable is true, by using debug-overrides. Normally, IDEs and build tools set this flag automatically for non-release builds.

This is safer than the usual conditional code because, as a security precaution, app stores do not accept apps which are marked debuggable.

res/xml/network_security_config.xml:

Opt out of cleartext traffic

Note: The guidance in this section applies only to apps that target Android 8.1 (API level 27) or lower. Starting with Android 9 (API level 28), cleartext support is disabled by default.

Applications intending to connect to destinations using only secure connections can opt-out of supporting cleartext (using the unencrypted HTTP protocol instead of HTTPS) to those destinations. This option helps prevent accidental regressions in apps due to changes in URLs provided by external sources such as backend servers. See NetworkSecurityPolicy.isCleartextTrafficPermitted() for more details.

For example, an app may want to ensure that all connections to secure.example.com are always done over HTTPS to protect sensitive traffic from hostile networks.

res/xml/network_security_config.xml:

Pin certificates

Normally, an app trusts all pre-installed CAs. If any of these CAs were to issue a fraudulent certificate, the app would be at risk from a man-in-the-middle attack. Some apps choose to limit the set of certificates they accept by either limiting the set of CAs they trust or by certificate pinning.

Certificate pinning is done by providing a set of certificates by hash of the public key (SubjectPublicKeyInfo of the X.509 certificate). A certificate chain is then valid only if the certificate chain contains at least one of the pinned public keys.

Note that, when using certificate pinning, you should always include a backup key so that if you are forced to switch to new keys or change CAs (when pinning to a CA certificate or an intermediate of that CA), your app's connectivity is unaffected. Otherwise, you must push out an update to the app to restore connectivity.

Additionally, it is possible to set an expiration time for pins after which pinning is not performed. This helps prevent connectivity issues in apps which have not been updated. However, setting an expiration time on pins may enable pinning bypass.

res/xml/network_security_config.xml:

Configuration inheritance behavior

Values not set in a specific configuration are inherited. This behavior allows more complex configurations while keeping the configuration file readable.

If a value is not set in a specific entry, then the value from the more general entry is used. For example, values not set in a domain-config are taken from the parent domain-config, if nested, or from the base-config if not. Values not set in the base-config use the platform default values.

For example, consider where all connections to subdomains of example.com must use a custom set of CAs. Additonally, cleartext traffic to these domains is permitted except when connecting to secure.example.com. By nesting the configuration for secure.example.com inside the configuration for example.com, the trust-anchors does not need to be duplicated.

res/xml/network_security_config.xml:

Configuration file format

The Network Security Configuration feature uses an XML file format. The overall structure of the file is shown in the following code sample:

The following sections describe the syntax and other details of the file format.

<network-security-config>

<base-config>

Any values that are not set use the platform default values.

The default configuration for apps targeting Android 9 (API level 28) and higher is as follows:

The default configuration for apps targeting Android 7.0 (API level 24) to Android 8.1 (API level 27) is as follows:

The default configuration for apps targeting Android 6.0 (API level 23) and lower is as follows:

<domain-config>

Note that if multiple domain-config elements cover a destination, the configuration with the most specific (longest) matching domain rule is used.

<domain>

<debug-overrides>

<trust-anchors>

<certificates>

• a raw resource ID pointing to a file containing X.509 certificates. Certificates must be encoded in DER or PEM format. In the case of PEM certificates, the file must not contain extra non-PEM data such as comments.
• "system" for the pre-installed system CA certificates
• "user" for user-added CA certificates

Specifies if the CAs from this source bypass certificate pinning. If "true", then pinning is not performed on certificate chains which are signed by one of the CAs from this source. This can be useful for debugging CAs or for testing man-in-the-middle attacks on your app's secure traffic.

Default is "false" unless specified in a debug-overrides element, in which case the default is "true".

<pin-set>

Expiration helps prevent connectivity issues in apps which do not get updates to their pin set, such as when the user disables app updates.

<pin>

Additional resources

For more information about Network Security Configuration, consult the following resources.

Codelabs

• Android Network Security Configuration

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