Securing access to Azure Linux VMs is a critical aspect of cloud security. Azure offers multiple authentication methods, including traditional SSH passwords, SSH key-based authentication, and the Entra ID SSH Access extension. Each has its own security implications, management overhead, and compliance considerations. In this post, we’ll compare these authentication methods, their pros and cons, and determine the best choice from a Well-Architected Framework (WAF) and Cloud Adoption Framework (CAF) perspective.
SSH Password Authentication
SSH password authentication allows users to log in using a username and password.
| Pros | Cons |
|---|---|
| Simple to set up: No pre-configuration required. | Security Risk: Susceptible to brute-force attacks if not protected by fail2ban or Azure Security Center recommendations. |
| Works well for short-lived or test environments. | Compliance Issues: Many security frameworks discourage or outright prohibit password authentication. |
| Does not require additional key management. | Management Overhead: Passwords must be rotated and managed securely. |
| No Centralized Access Control: Hard to enforce access policies centrally. |
WAF/CAF Considerations
Not Recommended for production workloads due to security concerns.
Requires Additional Hardening (e.g., Azure Firewall, Just-In-Time (JIT) VM Access, strong password policies).
SSH Key-Based Authentication
SSH keys provide a more secure alternative by using a public-private key pair.
| Pros | Cons |
|---|---|
| Strong Security: Resistant to brute-force attacks and eliminates password-based vulnerabilities. | Key Management Complexity: Private keys must be securely stored and rotated. |
| Easier to Automate: Can be used in DevOps pipelines and infrastructure automation. | User Onboarding/Offboarding: Requires manual distribution and revocation of keys if not managed via an identity-based solution. |
| Azure Integration: Azure allows key injection during VM creation and supports SSH key management via Azure Key Vault. | Risk of Key Leakage: If private keys are exposed, unauthorized access is possible. |
| Compliance-Friendly: Aligns with security best practices. |
WAF/CAF Considerations
Recommended for production workloads where Entra ID integration is not available.
Ensure Secure Storage of Private Keys (e.g., using hardware security modules (HSM) or Azure Key Vault).
Consider Automating Key Management via Azure DevOps or Ansible.
Configuration Guide
Use SSH keys with Azure Virtual Machines
Entra ID SSH Access Extension
The Entra ID (formerly Azure AD) SSH Access extension enables centralized identity-based authentication using Microsoft Entra ID.
| Pros | Cons |
|---|---|
| Centralized Access Management: Leverages Entra ID identities, reducing the need for local account management. | Requires Azure Entra ID Integration: Needs an enterprise setup with proper Entra ID RBAC policies. |
| Role-Based Access Control (RBAC): Access can be managed through Entra ID groups and roles. | Availability Considerations: Relies on Entra ID availability, which could be an issue in case of outages. |
| Auditability: Authentication logs are integrated with Azure Monitor and Entra ID logs. | Learning Curve: Teams must be familiar with Entra ID authentication models. |
| No Need for Key or Password Management: Eliminates the risk of key leakage and the need for password rotation. | |
| Short-Lived Credentials: Uses just-in-time authentication for enhanced security. |
WAF/CAF Considerations
Highly Recommended for enterprises with Entra ID adoption.
Ensures Centralized Governance and Compliance.
Best for Least Privilege Access Control and Automation.
Configuration Guide
Configure Entra ID-based SSH access for Azure Linux VMs
Conclusion: Which Authentication Method is Best?
| Authentication Method | Security | Management Overhead | Compliance | Best Use Case |
|---|---|---|---|---|
| SSH Passwords | Low | High | Poor | Temporary/test VMs |
| SSH Keys | High | Moderate | Good | General production workloads |
| Entra ID SSH Access | Very High | Low | Excellent | Enterprise environments with identity-based access management |
Recommendations based on WAF and CAF Perspective
Entra ID SSH Access is the optimal choice for organizations leveraging Entra ID. It simplifies access control, enhances auditability, and eliminates the complexity of managing keys or passwords.
SSH Keys remain a viable alternative for environments where Entra ID integration is not feasible. However, it introduces moderate management overhead and requires secure key management.
SSH Passwords should be avoided, except in scenarios involving temporary or test VMs. This method offers poor security and introduces high management overhead.
For more detailed guidance, refer to the Microsoft Well-Architected Framework and the Microsoft Cloud Adoption Framework.
Authentication in the Well-Architected Framework (WAF)
The WAF emphasizes the importance of robust identity and access management to ensure the security of your workloads. Key recommendations include:
Authentication and Authorization: Implement strong authentication mechanisms to verify user identities and enforce authorization policies to control access to resources. (Learn more)
Conditional Access: Utilize conditional access policies to enforce access controls based on user conditions, such as location, device compliance, and risk level. (Learn more)
Audit Logging: Enable comprehensive logging to monitor access and detect potential security incidents. (Learn more)
Authentication in the Cloud Adoption Framework (CAF)
The CAF offers detailed guidance on designing identity and access management strategies during cloud adoption:
Identity Strategy: Evaluate options for authenticating users and workload identities, assigning access to resources, and synchronizing hybrid identities with Microsoft Entra ID. (Learn more)
Application Access: Design access controls for applications, considering external users and integrating with Microsoft Entra B2B or Azure Active Directory B2C for secure external access. (Learn more)
Virtual Machine Access: Use Microsoft Entra ID identities to control access to Azure virtual machines, leveraging features like multifactor authentication and conditional access. (Learn more)
Next Steps
- For SSH Passwords: Immediately disable SSH password authentication and migrate to a more secure method.
- For SSH Keys: Implement Azure Key Vault to securely store and rotate SSH keys.
- For Entra ID: If you haven’t already, explore the SSH Access Extension to enhance security and simplify governance.
This post is part of an mini series on Linux authentication in Azure. Stay tuned for more deep dives into securing Linux workloads in the cloud!