Subscriber orgs can interact securely with external web applications by following these measures:
1. **Secure Credential Storage**: Use protected custom settings or metadata to store sensitive credentials. Provide a custom UI (e.g., Visualforce, Aura, or LWC) for privileged users to input and manage credentials securely.
2. **Named Credentials**: Use Named Credentials to package metadata and configuration, allowing subscribers to set up actual credentials after installation.
3. **Access Control**: Enforce CRUD (Create, Read, Update, Delete) and FLS (Field-Level Security) checks in Apex code to restrict access based on user permissions.
4. **Authentication**: Use secure authentication mechanisms like JWT or OAuth flows. Subscribers should complete the OAuth flow to provide consent for data access.
5. **Data Protection**: Avoid storing sensitive data in public custom objects, settings, or metadata, as these can be accessed by untrusted code.
6. **Input Validation**: Validate and sanitize user inputs to prevent injection attacks or unintended exposure of sensitive information.
7. **Session Management**: Ensure proper session termination and use CSRF tokens for sensitive operations to prevent unauthorized access.
8. **Secure Coding Practices**: Follow secure coding practices and conduct regular security reviews to identify and address vulnerabilities.
By implementing these practices, subscriber orgs can ensure secure interactions with external web applications.