Changes to Trusted Certificate Authorities in Android Nougat

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Posted by Chad Brubaker, Android Security team

In Android Nougat (7.0), we’ve changed how Android handles trusted certificate
authorities (CAs) to provide safer defaults for secure app traffic. Most apps
and users should not be affected by these changes or need to take any action.
The changes include:

  • Safe and easy APIs to trust custom CAs.
  • Apps that target API Level 24 and above no longer trust user or admin-added
    CAs for secure connections, by default.

  • All devices running Android Nougat offer the same standardized set of system
    CAs—no device-specific customizations.

For more details on these changes and what to do if you’re affected by them,
read on.

Safe and easy APIs

Apps have always been able customize which certificate authorities they trust.
However, we saw apps making mistakes due to the complexities of the Java TLS
APIs. To address this we improved
the APIs for customizing trust.

User-added CAs

Protection of all application data is a key goal of the Android application
sandbox. Android Nougat changes how applications interact with user- and
admin-supplied CAs. By default, apps that target API level 24 will—by design—not
honor such CAs unless the app explicitly opts in. This safe-by-default setting
reduces application attack surface and encourages consistent handling of network
and file-based application data.

Customizing trusted CAs

Customizing the CAs your app trusts on Android Nougat is easy using the Network
Security Config. Trust can be specified across the whole app or only for
connections to certain domains, as needed. Below are some examples for trusting
a custom or user-added CA, in addition to the system CAs. For more examples and
details, see the
full documentation.

Trusting custom CAs for debugging

To allow your app to trust custom CAs only for local debugging, include
something like this in your Network Security Config. The CAs will only be
trusted while your app is marked as debuggable.

<network-security-config>
      <debug-overrides>
           <trust-anchors>
                <!-- Trust user added CAs while debuggable only -->
                <certificates src="user" />
           </trust-anchors>
      </domain-config>
 </network-security-config>

Trusting custom CAs for a domain

To allow your app to trust custom CAs for a specific domain, include something
like this in your Network Security Config.

<network-security-config>
      <domain-config>
           <domain includeSubdomains="true">internal.example.com</domain>
           <trust-anchors>
                <!-- Only trust the CAs included with the app
                     for connections to internal.example.com -->
                <certificates src="@raw/cas" />
           </trust-anchors>
      </domain-config>
 </network-security-config>

Trusting user-added CAs for some domains

To allow your app to trust user-added CAs for multiple domains, include
something like this in your Network Security Config.

<network-security-config>
      <domain-config>
           <domain includeSubdomains="true">userCaDomain.com</domain>
           <domain includeSubdomains="true">otherUserCaDomain.com</domain>
           <trust-anchors>
                  <!-- Trust preinstalled CAs -->
                  <certificates src="system" />
                  <!-- Additionally trust user added CAs -->
                  <certificates src="user" />
           </trust-anchors>
      </domain-config>
 </network-security-config>

Trusting user-added CAs for all domains except some

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To allow your app to trust user-added CAs for all domains, except for those
specified, include something like this in your Network Security Config.

<network-security-config>
      <base-config>
           <trust-anchors>
                <!-- Trust preinstalled CAs -->
                <certificates src="system" />
                <!-- Additionally trust user added CAs -->
                <certificates src="user" />
           </trust-anchors>
      </base-config>
      <domain-config>
           <domain includeSubdomains="true">sensitive.example.com</domain>
           <trust-anchors>
                <!-- Only allow sensitive content to be exchanged
             with the real server and not any user or
    admin configured MiTMs -->
                <certificates src="system" />
           <trust-anchors>
      </domain-config>
 </network-security-config>

Trusting user-added CAs for all secure connections

To allow your app to trust user-added CAs for all secure connections, add this
in your Network Security Config.

<network-security-config>
      <base-config>
            <trust-anchors>
                <!-- Trust preinstalled CAs -->
                <certificates src="system" />
                <!-- Additionally trust user added CAs -->
                <certificates src="user" />
           </trust-anchors>
      </base-config>
 </network-security-config>

Standardized set of system-trusted CAs

To provide a more consistent and more secure experience across the Android
ecosystem, beginning with Android Nougat, compatible devices trust only the
standardized system CAs maintained in AOSP.

Previously, the set of preinstalled CAs bundled with the system could vary from
device to device. This could lead to compatibility issues when some devices did
not include CAs that apps needed for connections as well as potential security
issues if CAs that did not meet our security requirements were included on some
devices.

What if I have a CA I believe should be included on Android?

First, be sure that your CA needs to be included in the system. The preinstalled
CAs are only for CAs that meet our security requirements
because they affect the secure connections of most apps on the device. If you
need to add a CA for connecting to hosts that use that CA, you should instead
customize your apps and services that connect to those hosts. For more
information, see the Customizing trusted CAs section above.

If you operate a CA that you believe should be included in Android, first
complete the Mozilla CA
Inclusion Process and then file a feature request
against Android to have the CA added to the standardized set of system CAs.


Android Developers Blog

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