In an era of rapidly evolving technology, organizations must provide access to their digital resources in a way that is secure, efficient, and user-friendly. While traditional methods like passwords have been the cornerstone of authentication for decades, they can be cumbersome, insecure, and hard to manage. For many organizations, especially those with frontline workers or external contractors, leveraging SMS Sign-In in Microsoft Entra ID provides a seamless and secure way to authenticate users without the complexities of password management.
SMS-based authentication enables passwordless sign-in by sending a one-time passcode (OTP) to a user’s mobile phone, which they can use to gain access to their corporate resources. Let’s explore how this technology works, the benefits, and how to set it up — with a closer look at the technical details.
Table of Contents
What Is SMS Sign-In?
SMS Sign-In is a method of authenticating users through their mobile phone numbers. It allows users to sign in to their accounts with a one-time passcode (OTP) sent via SMS, instead of using a traditional password. It’s part of a broader effort to reduce the reliance on passwords and move towards more secure, user-friendly authentication methods.
This solution is ideal for users who may not have access to a corporate device, lack internet connectivity to use apps like Microsoft Authenticator, or simply need a fast and simple way to authenticate. It also supports users in bring-your-own-device (BYOD) scenarios, where users are not using company-managed devices.
Benefits of SMS Sign-In
| Benefit | Description |
|---|---|
| Simplicity | Users are already familiar with text messages, making this method easy to adopt. |
| Scalability | Can be deployed across the organization without needing specialized infrastructure. |
| Accessibility | Works even on basic mobile phones that do not require internet access. |
| Passwordless Access | Reduces the need for complex passwords, minimizing password-related risks. |
How SMS Sign-In Works
The SMS Sign-In process works by associating a phone number with a user’s account, sending an OTP to that number, and verifying the user’s identity when they input the OTP. Below is the detailed flow of how the authentication process occurs:
| Step | Action |
|---|---|
| 1. User Initiates Login | User enters their phone number instead of a username. |
| 2. Phone Number Verification | Entra ID checks that the phone number is valid and associated with a registered user account. |
| 3. OTP Generation | Entra ID generates a unique, time-sensitive OTP for the user. |
| 4. OTP Delivery via SMS | The OTP is sent to the user’s phone via SMS. |
| 5. User Inputs OTP | The user enters the OTP into the login page. |
| 6. Access Granted | If the OTP is correct and within the time limit, the user gains access to their resources. |
This flow ensures that the authentication process is both simple and secure, leveraging something the user already has — their mobile phone — while avoiding the need for complex credentials like passwords.
Setting Up SMS Sign-In
To get SMS Sign-In up and running, administrators need to enable the feature and configure certain settings in the Microsoft Entra ID admin center. Here’s a detailed guide on how to set it up:
Step 1: Enable SMS Authentication Method
- Navigate to the Admin Center:
- Go to Security > Authentication Methods in the Microsoft Entra admin center.
- Enable SMS Authentication:
- Find and enable the SMS Sign-In method under Authentication Methods.
- Assign Authentication Method:
- Decide which users or groups will have access to SMS Sign-In. You can apply this to all users or specific groups based on organizational needs.
- Configure Registration:
- Ensure that users are able to register their mobile numbers via the My Security Info portal or, if needed, admin registrations via PowerShell.
Step 2: User Registration
| Method | Description |
|---|---|
| User Registration via My Security Info | Users register their phone numbers through the My Security Info portal. |
| Admin-Managed Registration | Admins can populate phone numbers using PowerShell or directly through the portal. |
Step 3: Configure Conditional Access (Optional)
For organizations that need additional layers of security, pairing SMS Sign-In with Conditional Access policies can ensure:
| Conditional Access Feature | Description |
|---|---|
| Multi-Factor Authentication (MFA) | Enforce MFA for critical applications, ensuring users authenticate with multiple methods. |
| Location-based Access Restrictions | Limit access to resources based on geographic location or network conditions. |
How SMS Sign-In Works Under the Hood
1. Backend Infrastructure
The SMS-based authentication flow in Microsoft Entra ID relies on a combination of secure cloud services and telecommunication systems to ensure the OTP is sent and received without compromise. Here’s how it works under the hood:
| Component | Description |
|---|---|
| Microsoft Entra ID (Identity Provider) | Entra ID manages user accounts, phone number registration, and authentication methods. |
| Telecommunication Providers | Entra ID integrates with SMS service providers to send OTPs to users via their mobile phone numbers. |
| OTP Generation and Validation | The OTP is generated using secure cryptographic methods and validated against the Entra ID backend. |
| Time-based Expiry | OTPs are time-sensitive, typically valid for 5 minutes to prevent misuse. |
2. Security Considerations:
While SMS-based authentication is convenient, it’s important to consider its security implications. The main risks associated with SMS-based authentication include:
| Risk | Description |
|---|---|
| SIM Swap Attack | An attacker may convince the mobile carrier to transfer the user’s phone number to another SIM card, gaining access to OTPs. |
| SMS Interception | Malicious actors may intercept SMS messages via network vulnerabilities, potentially obtaining OTPs. |
| Phishing Attacks | Users could be tricked into entering OTPs on phishing websites, allowing attackers to hijack their sessions. |
For these reasons, it’s advised to pair SMS Sign-In with other security controls, such as Conditional Access policies and user education on mobile security best practices.
Limitations and Considerations
While SMS Sign-In is a simple and accessible authentication method, there are some limitations to be aware of:
| Limitation | Description |
|---|---|
| Vulnerability to SIM Swap Attacks | SMS-based OTP is considered less secure than app-based authentication methods like Microsoft Authenticator or FIDO2 keys. |
| Limited Application Compatibility | Not all applications support SMS Sign-In. It may require additional configuration for third-party apps. |
| Licensing Requirements | SMS Sign-In is available with Microsoft Entra ID Free, P1, and P2, but some advanced features may require higher-tier licenses. |
Best Practices for Using SMS Sign-In
| Best Practice | Description |
|---|---|
| Pair SMS with Other MFA Methods | Always combine SMS Sign-In with other MFA methods, such as biometrics or hardware tokens, to increase security. |
| Use Conditional Access Policies | Implement policies that restrict access to sensitive resources based on user location, device health, and other factors. |
| Educate Users on Mobile Security | Train users on the importance of securing their mobile devices, enabling PINs, and understanding the risks of SIM swapping. |
Conclusion
SMS Sign-In in Microsoft Entra ID offers a practical, user-friendly way to authenticate users without requiring passwords. While it’s a valuable option for organizations looking to simplify user access, it’s essential to consider the security implications and combine SMS Sign-In with additional layers of protection. As organizations continue their journey towards a passwordless future, SMS Sign-In serves as an important bridge — making authentication easier today while building a foundation for stronger, more resilient identity systems tomorrow.
