Detect and prevent account-related fraudulent activities on mobile applications

This document shows you how to use reCAPTCHA account defender to detect and prevent account-related fraudulent activities on mobile applications.

reCAPTCHA helps you protect critical actions, such as login and checkout. However, there are many subtle forms of account abuse that can be detected by observing a specific user's behavior on a mobile application over a period of time. reCAPTCHA account defender helps in identifying these kinds of subtle abuse by creating a site-specific model for your mobile application to detect a trend of suspicious behavior or a change in activity. By using the site-specific model, reCAPTCHA account defender helps you detect the following:

  • Suspicious activities
  • Accounts with similar behaviors
  • Requests coming from devices that were marked as trusted for specific users

Based on the analysis of reCAPTCHA account defender and the site-specific model, you can take the following actions:

  • Restrict or disable fraudulent accounts.
  • Prevent account takeover attempts.
  • Mitigate the successful account takeovers.
  • Grant access only to the requests coming from legitimate user accounts.
  • Reduce friction for users logging in from one of their trusted devices.

Before you begin

  1. reCAPTCHA account defender for mobile applications is accessible after triggering an automatic security review by adding a billing account to your project. Add a billing account to your project to onboard your site to this feature.
  2. Prepare your environment for reCAPTCHA.
  3. Create a score-based key.

Configure your mobile application for reCAPTCHA account defender

reCAPTCHA account defender requires a comprehensive understanding of account activities to enable effective detection. To start feeding account-related activities to reCAPTCHA account defender, and to create and improve your site-specific model, do the following:

  1. Integrate reCAPTCHA with your mobile application.

  2. Report on critical user actions.
  3. Assess critical user events.
  4. Annotate user events to tune your site-specific model.

Report on critical user actions

To detect suspicious activity patterns and build a better understanding of typical activity patterns on your site, reCAPTCHA account defender needs the information about critical user actions. For each action of your app that is protected using reCAPTCHA, call the execute() method with RecaptchaAction. For more information about execute() and RecaptchaAction, see the following:

reCAPTCHA provides a built-in set of actions, and if necessary you can create custom actions.

The following table lists the action names that you can use when reporting the critical user actions.

Action name User initiated event or user action
LOGIN

Login to the mobile application.
SIGNUP

Sign up on the mobile application.

Assess critical user events

When you call execute() on a user action, it generates a token. For the critical user events, such as successful and failed logins, registrations, and actions of the logged-in users, create an assessment to assess the results of the execute() call. The assessment provides you a risk verdict, which you can use to make a decision about how to handle potentially fraudulent activities. Some of the actions you can take are blocking suspicious requests, challenging risky logins, and investigating accounts of interest.

reCAPTCHA account defender requires you to provide a stable account identifier to attribute user activity – such as login requests, logged-in requests, and signup requests – to a specific account. This helps reCAPTCHA account defender understand user activity patterns and build an activity model for each account to better detect anomalous and abusive traffic.

Choose a stable account identifier accountId that is not often changed by the user and provide it to the assessment in the projects.assessments.create method. This stable account identifier should have the same value for all the events related to the same user. You can provide the following as the account identifier:

User identifiers

If every account can be uniquely associated with a stable username, email address, or phone number, you can use it as the accountId. When you provide such cross-site identifiers (identifiers that can be reused across sites), reCAPTCHA uses this information to improve protection for your user accounts based on cross-site models by flagging abusive account identifiers and using knowledge of cross-site abuse patterns related to these identifiers.

Alternatively, if you have an internal user ID uniquely associated with each account, you can provide it as the accountId.

Hashed or encrypted

If you do not have an internal user ID uniquely associated with each account, you can turn any stable identifier into an opaque, site-specific account identifier. This identifier is still needed for reCAPTCHA account defender to understand user activity patterns and detect anomalous behaviour, but it is not shared across other sites.

Pick any stable account identifier and make it opaque before sending to reCAPTCHA by using encryption or hashing:

  • encryption (recommended): encrypt the account identifier using a deterministic encryption method that produces a stable ciphertext. For detailed instructions, see encrypt data deterministically. When you choose symmetric encryption over hashing, you don't need to keep a mapping between your user identifiers and the corresponding opaque user identifiers. Decrypt the opaque identifiers that are returned by reCAPTCHA to turn them into the user identifier.

  • hashing: we recommend hashing the account identifier using the SHA256-HMAC method with a custom salt of your choice. Because hashes are one-way only, you need to keep a mapping between the generated hashes and your user identifiers so that you can map the hashed account identifier that are returned back to the original accounts.

In addition to providing a stable account identifier for all the account-related requests, you can provide additional account identifiers, potentially non-stable, for some specific requests. Context-specific account identifiers provided in addition to the accountId help reCAPTCHA account defender better understand user activity and detect account takeover attempts to keep your user accounts safe. When you provide additional identifiers, reCAPTCHA uses this information to improve protection for your user accounts based on cross-site models by flagging abusive account identifiers and using knowledge of cross-site abuse patterns related to these identifiers. For example, you can provide the following:

  • The username, email address, or phone number that was used as a login handle for login requests

  • The email address or phone number that was verified for a multi-factor authentication request

  • An email address or phone number (primary or secondary) that was provided by the user during an account update request

  • The email addresses and phone numbers that are provided by the user during a registration request

Append the chosen stable account identifier to the accountId parameter in the projects.assessments.create method for all the account-related requests. Optionally, provide additional account identifiers for the relevant requests by using the userIds field in the assessment.

Before using any of the request data, make the following replacements:

  • PROJECT_ID: your Google Cloud project ID
  • TOKEN: token returned from the execute() call
  • KEY_ID: reCAPTCHA key associated with the app
  • ACCOUNT_ID: the identifier that is uniquely associated with the user account for a user account to your app
  • EMAIL_ADDRESS: Optional. An email address that is associated with this request, if any
  • PHONE_NUMBER: Optional. A phone number that is associated with this request, if any
  • USERNAME: Optional. A username that is associated with this request, if any

HTTP method and URL: 

POST https://recaptchaenterprise.googleapis.com/v1/projects/PROJECT_ID/assessments

Request JSON body: 

{
  "event": {
    "token": "TOKEN",
    "siteKey": "KEY_ID",
    "userInfo": {
      "accountId": "ACCOUNT_ID",
      "userIds": [
        {
          "email": "EMAIL_ADDRESS"
        },
        {
          "phoneNumber": "PHONE_NUMBER"
        },
        {
          "username": "USERNAME"
        }
      ]
    }
  }
}

To send your request, choose one of these options:

curl

Save the request body in a file named request.json, and execute the following command: 

curl -X POST \
     -H "Authorization: Bearer $(gcloud auth print-access-token)" \
     -H "Content-Type: application/json; charset=utf-8" \
     -d @request.json \
     "https://recaptchaenterprise.googleapis.com/v1/projects/PROJECT_ID/assessments"

PowerShell

Save the request body in a file named request.json, and execute the following command: 

$cred = gcloud auth print-access-token
$headers = @{ "Authorization" = "Bearer $cred" }

Invoke-WebRequest `
    -Method POST `
    -Headers $headers `
    -ContentType: "application/json; charset=utf-8" `
    -InFile request.json `
    -Uri "https://recaptchaenterprise.googleapis.com/v1/projects/PROJECT_ID/assessments" | Select-Object -Expand Content

You should receive a JSON response similar to the following:

{
  "tokenProperties": {
    "valid": true,
    "androidPackageName": "com.example.app" or "iosBundleId": "com.example.app",
    "action": "login",
    "createTime": "2019-03-28T12:24:17.894Z"
   },
  "riskAnalysis": {
    "score": 0.6,
  },
 "event": {
    "token": "TOKEN",
    "siteKey": "KEY",
    "userInfo": {
      "accountId": "ACCOUNT_ID"
    }
  },
  "name": "projects/PROJECT_NUMBER/assessments/b6ac310000000000",
  "accountDefenderAssessment": {
    "labels": ["SUSPICIOUS_LOGIN_ACTIVITY"]
  }
}

Code sample

Java

To authenticate to reCAPTCHA, set up Application Default Credentials. For more information, see Set up authentication for a local development environment. 


import com.google.cloud.recaptchaenterprise.v1.RecaptchaEnterpriseServiceClient;
import com.google.protobuf.ByteString;
import com.google.recaptchaenterprise.v1.AccountDefenderAssessment.AccountDefenderLabel;
import com.google.recaptchaenterprise.v1.Assessment;
import com.google.recaptchaenterprise.v1.CreateAssessmentRequest;
import com.google.recaptchaenterprise.v1.Event;
import com.google.recaptchaenterprise.v1.ProjectName;
import com.google.recaptchaenterprise.v1.RiskAnalysis.ClassificationReason;
import com.google.recaptchaenterprise.v1.TokenProperties;
import com.google.recaptchaenterprise.v1.UserId;
import com.google.recaptchaenterprise.v1.UserInfo;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.util.List;
import java.util.UUID;
import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;

public class AccountDefenderAssessment {

  public static void main(String[] args)
      throws IOException, NoSuchAlgorithmException, InvalidKeyException {
    // TODO(developer): Replace these variables before running the sample.
    // projectId: Google Cloud Project ID
    String projectId = "project-id";

    // recaptchaSiteKey: Site key obtained by registering a domain/app to use recaptcha
    // services.
    String recaptchaSiteKey = "recaptcha-site-key";

    // token: The token obtained from the client on passing the recaptchaSiteKey.
    // To get the token, integrate the recaptchaSiteKey with frontend. See,
    // https://cloud.google.com/recaptcha-enterprise/docs/instrument-web-pages#frontend_integration_score
    String token = "recaptcha-token";

    // recaptchaAction: The action name corresponding to the token.
    String recaptchaAction = "recaptcha-action";

    // Unique ID of the user, such as email, customer ID, etc.
    String accountId = "default" + UUID.randomUUID().toString().split("-")[0];

    // User phone number
    String phoneNumber = "555-987-XXXX";

    // User email address
    String emailAddress = "john.doe@example.com";

    accountDefenderAssessment(projectId, recaptchaSiteKey, token, recaptchaAction, accountId, phoneNumber, emailAddress);
  }

  /**
   * This assessment detects account takeovers. See,
   * https://cloud.google.com/recaptcha-enterprise/docs/account-takeovers The input is the hashed
   * account id. Result tells if the action represents an account takeover. You can optionally
   * trigger a Multi-Factor Authentication based on the result.
   */
  public static void accountDefenderAssessment(
      String projectId,
      String recaptchaSiteKey,
      String token,
      String recaptchaAction,
      String accountId,
      String phoneNumber,
      String emailAddress)
      throws IOException {
    try (RecaptchaEnterpriseServiceClient client = RecaptchaEnterpriseServiceClient.create()) {

      // Set the properties of the event to be tracked.
      Event.Builder eventBuilder =
          Event.newBuilder()
              .setSiteKey(recaptchaSiteKey)
              .setToken(token);

      // Set the account id, email address and phone number (of the user).
      eventBuilder.setUserInfo(
        UserInfo.newBuilder()
          .setAccountId(accountId)
          .addUserIds(UserId.newBuilder().setEmail(emailAddress))
          .addUserIds(UserId.newBuilder().setPhoneNumber(phoneNumber)));

      Event event = eventBuilder.build();

      // Build the assessment request.
      CreateAssessmentRequest createAssessmentRequest =
          CreateAssessmentRequest.newBuilder()
              .setParent(ProjectName.of(projectId).toString())
              .setAssessment(Assessment.newBuilder().setEvent(event).build())
              .build();

      Assessment response = client.createAssessment(createAssessmentRequest);

      // Check integrity of the response token.
      if (!checkTokenIntegrity(response.getTokenProperties(), recaptchaAction)) {
        return;
      }

      // Get the reason(s) and the reCAPTCHA risk score.
      // For more information on interpreting the assessment,
      // see: https://cloud.google.com/recaptcha-enterprise/docs/interpret-assessment
      for (ClassificationReason reason : response.getRiskAnalysis().getReasonsList()) {
        System.out.println(reason);
      }
      float recaptchaScore = response.getRiskAnalysis().getScore();
      System.out.println("The reCAPTCHA score is: " + recaptchaScore);
      String assessmentName = response.getName();
      System.out.println(
          "Assessment name: " + assessmentName.substring(assessmentName.lastIndexOf("/") + 1));

      // Get the Account Defender result.
      com.google.recaptchaenterprise.v1.AccountDefenderAssessment accountDefenderAssessment =
          response.getAccountDefenderAssessment();
      System.out.println(accountDefenderAssessment);

      // Get Account Defender label.
      List<AccountDefenderLabel> defenderResult =
          response.getAccountDefenderAssessment().getLabelsList();
      // Based on the result, can you choose next steps.
      // If the 'defenderResult' field is empty, it indicates that Account Defender did not have
      // anything to add to the score.
      // Few result labels: ACCOUNT_DEFENDER_LABEL_UNSPECIFIED, PROFILE_MATCH,
      // SUSPICIOUS_LOGIN_ACTIVITY, SUSPICIOUS_ACCOUNT_CREATION, RELATED_ACCOUNTS_NUMBER_HIGH.
      // For more information on interpreting the assessment, see:
      // https://cloud.google.com/recaptcha-enterprise/docs/account-defender#interpret-assessment-details
      System.out.println("Account Defender Assessment Result: " + defenderResult);
    }
  }

  private static boolean checkTokenIntegrity(
      TokenProperties tokenProperties, String recaptchaAction) {
    // Check if the token is valid.
    if (!tokenProperties.getValid()) {
      System.out.println(
          "The Account Defender Assessment call failed because the token was: "
              + tokenProperties.getInvalidReason().name());
      return false;
    }

    // Check if the expected action was executed.
    if (!tokenProperties.getAction().equals(recaptchaAction)) {
      System.out.printf(
          "The action attribute in the reCAPTCHA tag '%s' does not match "
              + "the action '%s' you are expecting to score",
          tokenProperties.getAction(), recaptchaAction);
      return false;
    }
    return true;
  }
}

Interpret the risk verdict of the critical user events

When you create an assessment with account defender enabled, account defender returns accountDefenderAssessment as part of the assessment response. The value of accountDefenderAssessment helps you assess whether the user activity is legitimate or fraudulent. It also returns an assessment ID that you need to use when annotating user events.

The following example is a sample JSON response:

{
  "tokenProperties": {
    "valid": true,
    "androidPackageName": "com.example.app" or "iosBundleId": "com.example.app",
    "action": "login",
    "createTime": "2019-03-28T12:24:17.894Z"
   },
  "riskAnalysis": {
    "score": 0.6,
  },
 "event": {
    "token": "TOKEN",
    "siteKey": "KEY_ID",
    "expectedAction": "USER_ACTION"
  },
  "name": "projects/PROJECT_ID/assessments/b6ac310000000000X",
  "accountDefenderAssessment": {
    labels: ["SUSPICIOUS_LOGIN_ACTIVITY"]
  }
}

The accountDefenderAssessment field can have any of the following values:

Value Description
SUSPICIOUS_LOGIN_ACTIVITY Indicates that the request represents a high risk of credential stuffing or account takeover.
SUSPICIOUS_ACCOUNT_CREATION Indicates that the request represents a high risk of abusive account creation.
PROFILE_MATCH

Indicates that the attributes of the user match the attributes that have been seen earlier for this particular user. This value is an indicator that this user is on a trusted device that was used before to access your mobile application.

PROFILE_MATCH is returned only in the following scenarios:

  • You use Multi-factor(MFA) or Two-factor authentication(2FA) and reCAPTCHA account defender marks user profiles as trusted after users pass the MFA or 2FA challenge.
  • You annotate assessments as LEGITIMATE or PASSED_TWO_FACTOR and reCAPTCHA account defender marks the corresponding user profile as trusted.

Annotate events to tune your site-specific model

To provide more information to reCAPTCHA account defender and improve your site-specific detection model, you must annotate the events that you assessed by creating assessments.

You annotate an assessment by sending a request to the projects.assessments.annotate method with the assessment ID. In the body of that request, you include labels providing additional information about an event described in the assessment.

To annotate an assessment, do the following:

  1. Determine the information and labels to add in the request JSON body depending on your use case.

    The following table lists the labels and values that you can use to annotate events:

    Label Description Request example
    reasons Required. A label to support your assessments.

    Provide real-time event details in the reasons label in a few seconds or minutes after the event because they influence real-time detection.

    For the list of possible values, see reasons values.

    Example: To detect account takeovers, annotate if the entered password was correct with CORRECT_PASSWORD or INCORRECT_PASSWORD values. If you deployed your own MFA, you can add the following values: INITIATED_TWO_FACTOR, and PASSED_TWO_FACTOR or FAILED_TWO_FACTOR. 

          {
      "reasons": ["INCORRECT_PASSWORD"]
      }
    annotation Optional. A label to indicate the legitimacy of assessments.

    Provide facts about login and registration events to validate or correct your risk assessments in the annotation label.

    Possible values: LEGITIMATE or FRAUDULENT.

    You can send this information at any time or as part of a batch job. However, we recommend sending this information in a few seconds or minutes after the event because they influence real-time detection.

    		 
          {
       "annotation": "LEGITIMATE"
      }
    accountId

    Optional. A label to associate an account ID with an event.

    If you created an assessment without an account ID, use this label to provide the account ID of an event whenever it is available.

    		 
      {
   "accountId": "ACCOUNT_ID"
  }

  2. Create an annotate request with the appropriate labels.

    Before using any of the request data, make the following replacements:

    • ASSESSMENT_ID: Value of the name field returned from the projects.assessments.create call.
    • ANNOTATION: Optional. A label to indicate whether the assessment is legitimate or fraudulent.
    • REASONS: Optional. Reasons that support your annotation. For the list of possible values, see reasons values.
    • ACCOUNT_ID: Optional. the identifier that is uniquely associated with the user account on your app.

    For more information, see labels for annotations.

    HTTP method and URL: 

    POST https://recaptchaenterprise.googleapis.com/v1/ASSESSMENT_ID:annotate

    Request JSON body: 

    {
  "annotation": ANNOTATION,
  "reasons": REASONS,
  "accountId": ACCOUNT_ID
}

    To send your request, choose one of these options:

    curl

    Save the request body in a file named request.json, and execute the following command: 

    curl -X POST \
         -H "Authorization: Bearer $(gcloud auth print-access-token)" \
         -H "Content-Type: application/json; charset=utf-8" \
         -d @request.json \
         "https://recaptchaenterprise.googleapis.com/v1/ASSESSMENT_ID:annotate"

    PowerShell

    Save the request body in a file named request.json, and execute the following command: 

    $cred = gcloud auth print-access-token
    $headers = @{ "Authorization" = "Bearer $cred" }

    Invoke-WebRequest `
        -Method POST `
        -Headers $headers `
        -ContentType: "application/json; charset=utf-8" `
        -InFile request.json `
        -Uri "https://recaptchaenterprise.googleapis.com/v1/ASSESSMENT_ID:annotate" | Select-Object -Expand Content

    You should receive a successful status code (2xx) and an empty response.

Code sample

Java

To authenticate to reCAPTCHA, set up Application Default Credentials. For more information, see Set up authentication for a local development environment. 


import com.google.cloud.recaptchaenterprise.v1.RecaptchaEnterpriseServiceClient;
import com.google.protobuf.ByteString;
import com.google.recaptchaenterprise.v1.AnnotateAssessmentRequest;
import com.google.recaptchaenterprise.v1.AnnotateAssessmentRequest.Annotation;
import com.google.recaptchaenterprise.v1.AnnotateAssessmentRequest.Reason;
import com.google.recaptchaenterprise.v1.AnnotateAssessmentResponse;
import com.google.recaptchaenterprise.v1.AssessmentName;
import java.io.IOException;
import java.security.NoSuchAlgorithmException;
import java.util.UUID;

public class AnnotateAccountDefenderAssessment {

  public static void main(String[] args) throws IOException, NoSuchAlgorithmException {
    // TODO(developer): Replace these variables before running the sample.
    // projectID: GCloud Project id.
    String projectID = "project-id";

    // assessmentId: Value of the 'name' field returned from the CreateAssessment call.
    String assessmentId = "account-defender-assessment-id";

    // accountId: Set the accountId corresponding to the assessment id.
    String accountId = "default" + UUID.randomUUID().toString().split("-")[0];

    annotateAssessment(projectID, assessmentId, accountId);
  }

  /**
   * Pre-requisite: Create an assessment before annotating. Annotate an assessment to provide
   * feedback on the correctness of recaptcha prediction.
   */
  public static void annotateAssessment(
      String projectID, String assessmentId, String accountId) throws IOException {

    try (RecaptchaEnterpriseServiceClient client = RecaptchaEnterpriseServiceClient.create()) {
      // Build the annotation request.
      // For more info on when/how to annotate, see:
      // https://cloud.google.com/recaptcha-enterprise/docs/annotate-assessment#when_to_annotate
      AnnotateAssessmentRequest annotateAssessmentRequest =
          AnnotateAssessmentRequest.newBuilder()
              .setName(AssessmentName.of(projectID, assessmentId).toString())
              .setAnnotation(Annotation.LEGITIMATE)
              .addReasons(Reason.PASSED_TWO_FACTOR)
              .setAccountId(accountId)
              .build();

      // Empty response is sent back.
      AnnotateAssessmentResponse response = client.annotateAssessment(annotateAssessmentRequest);
      System.out.println("Annotated response sent successfully ! " + response);
    }
  }
}

Enable reCAPTCHA account defender

After you configure your mobile application for reCAPTCHA account defender, you can enable reCAPTCHA account defender.

  1. In the Google Cloud console, go to the reCAPTCHA page.

    Go to reCAPTCHA

  2. Verify that the name of your project appears in the resource selector at the top of the page.

    If you don't see the name of your project, click the resource selector, then select your project.

  3. Click Settings.

  4. In the Account defender pane, click Configure.

  5. In the Configure account defender dialog, click Enable, and then click Save.

It might take a few hours for the reCAPTCHA account defender enablement to propagate to our systems. After the feature enablement is propagated to our systems, you should start receiving responses related to account defender as part of the assessments.

What's next