Security requirements for actionable messages in Office 365

Securing actionable email is simple and easy. There are two phases within the end-to-end experience that impose security requirements on your service when supporting actionable messages with Office 365. The phases and their corresponding requirements are as follows.

1. Send phase: The pre-requisites for your service to send actionable messages are as follows:
   • If you're using actionable email, you'll need to enable sender verification. Note that this does not apply to connector messages.
   • Your service must be registered with Microsoft.
   • The Action URL must support HTTPS.
2. Action processing phase: When processing an action, your service should:
   • Verify the bearer token (a JSON Web token) included in the header of the HTTP POST request. Verification can also be done leveraging the sample libraries provided by Microsoft.
   • Include Limited Purpose Token from your service as part of the target URL, which can be used by your service to correlate the service URL with the intended request & user. This is optional, but highly recommended.

Sender verification

Office 365 requires sender verification in order to enable actionable messages via email. Your actionable message emails must either originate from servers that implement DomainKeys Identified Mail (DKIM) and Sender Policy Framework (SPF), or you must implement signed cards.

While DKIM and SPF are sufficient for some scenarios, that solution will not work in some situations where emails are sent via an external provider, which can lead to recipients not experiencing the enhanced actionable message. For this reason, we recommend always implementing signed cards which work in all cases and are fundamentally more secure since they do not rely on DNS records.

Implementing DKIM and SPF

DKIM and SPF are industry standard ways to prove a sender's identity when sending emails over SMTP. Many companies already implement these standards to secure the emails they are already sending. To learn more about SPF/DKIM and how to implement them, see:

• DomainKeys Identified Mail (DKIM)
• Sender Policy Framework

Signed card payloads

Actionable messages sent via email support an alternative verification method: signing the card payload with an RSA key or X509 certificate. This method is required in the following scenarios:

• SPF/DKIM failure caused by sender setup or recipient tenant set custom security services in front of Office 365 services.
• Your scenario for actionable messages requires sending from multiple email accounts.

To use signed cards, you must register your public key in the email developer dashboard, and use the corresponding private key to sign the card.

SignedCard

Signed actionable message cards are available when sending via email. Use this format to include a signed card in the HTML body of an email. This payload is serialized in Microdata format appended in the end of HTML body.

<section itemscope itemtype="http://schema.org/SignedAdaptiveCard">
    <meta itemprop="@context" content="http://schema.org/extensions" />
    <meta itemprop="@type" content="SignedAdaptiveCard" />
    <div itemprop="signedAdaptiveCard" style="mso-hide:all;display:none;max-height:0px;overflow:hidden;">[SignedCardPayload]</div>
</section>

Note: Partners who prefer to use the legacy MessageCard entity may create a SignedMessageCard entity in place of a SignedAdaptiveCard.

SignedCardPayload

SignedCardPayload is a string encoded by JSON Web Signature (JWS) standard. RFC7515 describes JWS, and RFC7519 describes JSON Web Token (JWT). Given no claim is required in JWT, JWT libraries can be used to build JWS signature.

Note: The term "JWT" can be used interchangeably in practice. However, we prefer the term "JWS" here.

Here is an example of SignedCardPayload. The encoded Adaptive Card appears in the form of [header].[payload].[signature] as per JWS specification.

eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.eyJzZW5kZXIiOiJzZXJ2aWNlLWFjY291bnRAY29udG9zby5jb20iLCJvcmlnaW5hdG9yIjoiNjVjNjgwZWYtMzZhNi00YTFiLWI4NGMtYTdiNWM2MTk4NzkyIiwicmVjaXBpZW50c1NlcmlhbGl6ZWQiOiJbXCJqb2huQGNvbnRvc28uY29tXCIsXCJqYW5lQGNvbnRvc28uY29tXCJdIiwiYWRhcHRpdmVDYXJkU2VyaWFsaXplZCI6IntcIiRzY2hlbWFcIjpcImh0dHA6Ly9hZGFwdGl2ZWNhcmRzLmlvL3NjaGVtYXMvYWRhcHRpdmUtY2FyZC5qc29uXCIsXCJ0eXBlXCI6XCJBZGFwdGl2ZUNhcmRcIixcInZlcnNpb25cIjpcIjEuMFwiLFwiYm9keVwiOlt7XCJzaXplXCI6XCJsYXJnZVwiLFwidGV4dFwiOlwiSGVsbG8gQWN0aW9uYWJsZSBtZXNzYWdlXCIsXCJ3cmFwXCI6dHJ1ZSxcInR5cGVcIjpcIlRleHRCbG9ja1wifV0sXCJhY3Rpb25zXCI6W3tcInR5cGVcIjpcIkFjdGlvbi5JbnZva2VBZGRJbkNvbW1hbmRcIixcInRpdGxlXCI6XCJPcGVuIEFjdGlvbmFibGUgTWVzc2FnZXMgRGVidWdnZXJcIixcImFkZEluSWRcIjpcIjNkMTQwOGY2LWFmYjMtNGJhZi1hYWNkLTU1Y2Q4NjdiYjBmYVwiLFwiZGVza3RvcENvbW1hbmRJZFwiOlwiYW1EZWJ1Z2dlck9wZW5QYW5lQnV0dG9uXCJ9XX0iLCJpYXQiOjE1NDUzNDgxNTN9.BP9mK33S1VZyjtWZd-lNTTjvueyeeoitygw9bl17TeQFTUDh9Kg5cB3fB7BeZYQs6IiWa1VGRdiiR4q9EkAB1qDsmIcJnw6aYwDUZ1KY4lNoYgQCH__FxEPHViGidNGtq1vAC6ODw0oIfaTUWTa5cF5MfiRBIhpQ530mbRNnA0QSrBYtyB54EDJxjBF1vNSKOeVHAl2d4gqcGxsytQA0PA7XMbrZ8B7fEU2uNjSiLQpoh6A1tevpla2C7W6h-Wekgsmjpw2YToAOX67VZ1TcS5oZAHmjv2RhqsfX5DlN-ZsTRErU4Hs5d92NY9ijJPDunSLyUFNCw7HLNPFqqPmZsw

The header in above JWS is:

{
  "alg": "RS256",
  "typ": "JWT"
}

The payload in above JWS is:

{
  "sender": "[email protected]",
  "originator": "65c680ef-36a6-4a1b-b84c-a7b5c6198792",
  "recipientsSerialized": "[\"[email protected]\",\"[email protected]\"]",
  "adaptiveCardSerialized": "{\"$schema\":\"http://adaptivecards.io/schemas/adaptive-card.json\",\"type\":\"AdaptiveCard\",\"version\":\"1.0\",\"body\":[{\"size\":\"large\",\"text\":\"Hello Actionable message\",\"wrap\":true,\"type\":\"TextBlock\"}],\"actions\":[{\"type\":\"Action.InvokeAddInCommand\",\"title\":\"Open Actionable Messages Debugger\",\"addInId\":\"3d1408f6-afb3-4baf-aacd-55cd867bb0fa\",\"desktopCommandId\":\"amDebuggerOpenPaneButton\"}]}",
  "iat": 1545348153
}

Required Claims

Sample code generating signed card:

• .NET Sample
• Node.js Sample

Verifying that requests come from Microsoft

All action requests from Microsoft have a bearer token in the HTTP Authorization header. This token is a JSON Web Token (JWT) token signed by Microsoft, and it includes important claims that we strongly recommend should be verified by the service handling the associated request.

Typically, a service will perform the following verifications.

1. The token is signed by Microsoft. OpenID metadata is located at https://substrate.office.com/sts/common/.well-known/openid-configuration, which includes information regarding signing keys.
2. The aud claim corresponds to the service's base URL.

With all the above verifications done, the service can trust the sender and sub claims to be the identity of the sender and the user taking the action. The service can validate that the sender and sub claims match the sender and user it is expecting.

Please refer to the Microsoft code samples provided below, which show how to do these validations on the JWT token.

• .NET Sample
• Node.js Sample
• Java Sample
• Python Sample

Action-Authorization header

The use of Authorization header by Actionable messages may interfere with existing authentication/authorization mechanism for the target endpoint. In this case, developers can set the Authorization header to an empty string in the headers property of an Action.Http action. Actionable messages will then send the same bearer token via Action-Authorization header instead of using Authorization header.

Example Action.Http with Action-Authorization header

{
  "type": "Action.Http",
  "title": "Say hello",
  "method": "POST",
  "url": "https://api.contoso.com/sayhello",
  "body": "{{nameInput.value}}",
  "headers": [
    { "name": "Authorization", "value": "" }
  ]
}

Verifying the identity of the user

The bearer token included with all requests includes the Azure AD identity of the Office 365 user who took the action. If necessary, you can include your own token, specific to your service, in the URLs of your HTTP actions to represent the identity of the user in your system. Microsoft does not prescribe how the limited-access tokens should be designed or used by the service. This token is opaque to Microsoft, and is simply echoed back to the service.

Service-specific tokens can be used to correlate service URLs with specific messages and users. Microsoft recommends that if you use your own service-specific token, youinclude it as part of the action target URL, or in the body of the request coming back to the service. For example:

https://contoso.com/approve?requestId=abc&serviceToken=a1b2c3d4e5...

Service-specific tokens act as correlation IDs (for e.g. a hashed token using the userID, requestID, and salt). This allows the service provider to keep track of the action URLs it generates and sends out and match it with action requests coming in. In addition to correlation, the service provider may use the service-specific token to protect itself from replay attacks. For example, the service provider may choose to reject the request, if the action was already performed previously with the same token.