Understanding just-in-time (JIT) VM access
This page explains the principles behind Microsoft Defender for Cloud's just-in-time (JIT) VM access feature and the logic behind the recommendation.
To learn how to apply JIT to your VMs using the Azure portal (either Defender for Cloud or Azure Virtual Machines) or programmatically, see How to secure your management ports with JIT.
The risk of open management ports on a virtual machine
Threat actors actively hunt accessible machines with open management ports, like RDP or SSH. All of your virtual machines are potential targets for an attack. When a VM is successfully compromised, it's used as the entry point to attack further resources within your environment.
Why JIT VM access is the solution
As with all cybersecurity prevention techniques, your goal should be to reduce the attack surface. In this case that means having fewer open ports especially management ports.
Your legitimate users also use these ports, so it's not practical to keep them closed.
To solve this dilemma, Microsoft Defender for Cloud offers JIT. With JIT, you can lock down the inbound traffic to your VMs, reducing exposure to attacks while providing easy access to connect to VMs when needed.
How JIT operates with network resources in Azure
In Azure, you can block inbound traffic on specific ports, by enabling just-in-time VM access. Defender for Cloud ensures "deny all inbound traffic" rules exist for your selected ports in the network security group (NSG) and Azure Firewall rules. These rules restrict access to your Azure VMs’ management ports and defend them from attack.
If other rules already exist for the selected ports, then those existing rules take priority over the new "deny all inbound traffic" rules. If there are no existing rules on the selected ports, then the new rules take top priority in the NSG and Azure Firewall.
When a user requests access to a VM, Defender for Cloud checks that the user has Azure role-based access control (Azure RBAC) permissions for that VM. If the request is approved, Defender for Cloud configures the NSGs and Azure Firewall to allow inbound traffic to the selected ports from the relevant IP address (or range), for the amount of time that was specified. After the time has expired, Defender for Cloud restores the NSGs to their previous states. Connections that are already established aren't interrupted.
Note
JIT does not support VMs protected by Azure Firewalls controlled by Azure Firewall Manager. The Azure Firewall must be configured with Rules (Classic) and cannot use Firewall policies.
How Defender for Cloud identifies which VMs should have JIT applied
The following diagram shows the logic that Defender for Cloud applies when deciding how to categorize your supported VMs:
When Defender for Cloud finds a machine that can benefit from JIT, it adds that machine to the recommendation's Unhealthy resources tab.
Next step
This page explained why just-in-time (JIT) virtual machine (VM) access should be used. To learn how to enable JIT and request access to your JIT-enabled VMs: