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Understanding just-in-time (JIT) VM access

This page explains the principles behind Azure Security Center'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 Security Center or Azure Virtual Machines) or programmatically

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, Azure Security Center 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 security groups and Azure Firewall

When you enable just-in-time VM access, you can select the ports on the VM to which inbound traffic will be blocked. Security Center 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, Security Center checks that the user has Azure role-based access control (Azure RBAC) permissions for that VM. If the request is approved, Security Center 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, Security Center restores the NSGs to their previous states. Connections that are already established are not interrupted.

How Security Center identifies which VMs should have JIT applied

The diagram below shows the logic that Security Center applies when deciding how to categorize your supported VMs:

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