VMware ESXi Version 5 Virtual Machine Security Technical Implementation Guide

Certain automated operations such as automated tools upgrades, use a component into the hypervisor called "Host Guest File System" and an attacker could potentially use this to transfer files inside the guest OS.

If the interface is not restricted, a VM can detect and be detected by all other VMs with the same option enabled within the same host. This might be the intended behavior, but custom-built software can have unexpected vulnerabilities that might potentially lead to an exploit. Additionally, it is possible for a VM to detect how many other VMs are within the same ESX system by simply registering the VM. This information might also be used for a potentially malicious objective. By default, the setting is FALSE. The VM can be exposed to other VMs within the same system as long as there is at least one program connected to the VMCI socket interface.

Excessive VM logging may degrade system performance. The following settings can be used to limit the total size and number of log files. Normally a new log file is created only when a host is rebooted, so the file can grow to be quite large. Ensure new log files are created more frequently by limiting the maximum size of the log files. To restrict the total size of logging data, VMware recommends saving 10 log files, each one limited to 1,000KB. Datastores are likely to be formatted with a block size of 2MB or 4MB, so a size limit too far below this size would result in unnecessary storage utilization. Each time an entry is written to the log, the size of the log is checked; if it is over the limit, the next entry is written to a new log. If the maximum number of log files already exists, when a new one is created, the oldest log file is deleted. A denial-of-service attack that avoids these limits might be attempted by writing an enormous log entry. But each log entry is limited to 4KB, so no log files are ever more than 4KB larger than the configured limit. A second option is to disable logging for the virtual machine. Disabling logging for a virtual machine makes troubleshooting challenging and support difficult. Do not consider disabling logging unless the log file rotation approach proves insufficient. Uncontrolled logging can lead to denial-of-service due to the datastore becoming filled.

When Virtual Machines are running on a hypervisor they are "aware" that they are running in a virtual environment and this information is available to tools inside the guest OS. This can give attackers information about the platform that they are running on that they may not get from a normal physical server. This option completely disables all hooks for a virtual machine and the guest OS will not be aware that it is running in a virtual environment at all. This feature may be enabled for short term diagnostics and troubleshooting, but must be disabled prior to resumption of normal operations.

Besides disabling unnecessary virtual devices from within the virtual machine, ensure no device is connected to a virtual machine if it is not required to be there. For example, serial and parallel ports are rarely used for virtual machines in a datacenter environment, and CD/DVD drives are usually connected only temporarily during software installation. For less commonly used devices that are not required, the parameter must be assigned a value of false. NOTE: The parameters listed are not sufficient to ensure a device is usable; other parameters are required to indicate specifically how each device is instantiated. Any enabled or connected device represents another potential attack channel.System Administrator

Besides disabling unnecessary virtual devices from within the virtual machine, ensure no device is connected to a virtual machine if it is not required to be there. For example, serial and parallel ports are rarely used for virtual machines in a datacenter environment, and CD/DVD drives are usually connected only temporarily during software installation. For less commonly used devices that are not required, either the parameter should not be present or its value must be FALSE. NOTE: The parameters listed are not sufficient to ensure a device is usable; other parameters are required to indicate specifically how each device is instantiated. Any enabled or connected device represents another potential attack channel.

Besides disabling unnecessary virtual devices from within the virtual machine, ensure no device is connected to a virtual machine if it is not required to be there. For example, serial and parallel ports are rarely used for virtual machines in a datacenter environment, and CD/DVD drives are usually connected only temporarily during software installation. For less commonly used devices that are not required, either the parameter should not be present or its value must be FALSE. NOTE: The parameters listed are not sufficient to ensure a device is usable; other parameters are required to indicate specifically how each device is instantiated. Any enabled or connected device represents another potential attack channel.

Besides disabling unnecessary virtual devices from within the virtual machine, ensure no device is connected to a virtual machine if it is not required to be there. For example, serial and parallel ports are rarely used for virtual machines in a datacenter environment, and CD/DVD drives are usually connected only temporarily during software installation. For less commonly used devices that are not required, either the parameter should not be present or its value must be FALSE. NOTE: The parameters listed are not sufficient to ensure a device is usable; other parameters are required to indicate specifically how each device is instantiated. Any enabled or connected device represents another potential attack channel.

Besides disabling unnecessary virtual devices from within the virtual machine, ensure no device is connected to a virtual machine if it is not required to be there. For example, serial and parallel ports are rarely used for virtual machines in a datacenter environment, and CD/DVD drives are usually connected only temporarily during software installation. For less commonly used devices that are not required, either the parameter should not be present or its value must be FALSE. NOTE: The parameters listed are not sufficient to ensure a device is usable; other parameters are required to indicate specifically how each device is instantiated. Any enabled or connected device represents another potential attack channel.

By default, remote console sessions can be connected to by more than one user at a time. When multiple sessions are activated, each terminal window gets a notification about the new session. If an administrator in the VM logs in using a VMware remote console during their session, a non-administrator in the VM might connect to the console and observe the administrator's actions. Also, this could result in an administrator losing console access to a virtual machine. For example, if a jump box is being used for an open console session and the admin loses connection to that box, then the console session remains open. Allowing two console sessions permits debugging via a shared session. For highest security, only one remote console session at a time should be allowed.

Use these settings to limit the total size and number of log files. Normally a new log file is created only when a host is rebooted, so the file can grow to be quite large. Ensure new log files are created more frequently by limiting the maximum size of the log files. If restricting the total size of logging data is wanted, VMware recommends saving 10 log files, each one limited to 1,000KB. Datastores are likely to be formatted with a block size of 2MB or 4MB, so a size limit too far below this size would result in unnecessary storage utilization. Each time an entry is written to the log, the size of the log is checked; if it is over the limit, the next entry is written to a new log. If the maximum number of log files already exists, when a new one is created, the oldest log file is deleted. A denial-of-service attack that avoids these limits might be attempted by writing an enormous log entry. But each log entry is limited to 4KB, so no log files are ever more than 4KB larger than the configured limit. A second option is to disable logging for the virtual machine. Disabling logging for a virtual machine makes troubleshooting challenging and support difficult. Do not consider disabling logging unless the log file rotation approach proves insufficient. Uncontrolled logging can lead to denial-of-service due to the datastore becoming filled.

Use these settings to limit the total size and number of log files. Normally a new log file is created only when a host is rebooted, so the file can grow to be quite large. Ensure new log files are created more frequently by limiting the maximum size of the log files. If restricting the total size of logging data is wanted, VMware recommends saving 10 log files, each one limited to 1,000KB. Datastores are likely to be formatted with a block size of 2MB or 4MB, so a size limit too far below this size would result in unnecessary storage utilization. Each time an entry is written to the log, the size of the log is checked; if it is over the limit, the next entry is written to a new log. If the maximum number of log files already exists, when a new one is created, the oldest log file is deleted. A denial-of-service attack that avoids these limits might be attempted by writing an enormous log entry. But each log entry is limited to 4KB, so no log files are ever more than 4KB larger than the configured limit. A second option is to disable logging for the virtual machine. Disabling logging for a virtual machine makes troubleshooting challenging and support difficult. Do not consider disabling logging unless the log file rotation approach proves insufficient. Uncontrolled logging can lead to denial-of-service due to the datastore becoming filled.

The VM console enables a connection to the console of a virtual machine, in effect seeing what a monitor on a physical server would show. The VM console also provides power management and removable device connectivity controls, which might potentially allow a malicious user to bring down a virtual machine. In addition, it also has a performance impact on the service console, especially if many VM console sessions are open simultaneously.

Normal users and processes-that is, users and processes without root or administrator privileges-within virtual machines have the capability to connect or disconnect devices, such as network adaptors and CD-ROM drives, as well as the ability to modify device settings. In general, the virtual machine settings should use editor or configuration editor to remove any unneeded or unused hardware devices. However, the device may need to be used again, so removing it is not always a good solution. In that case, prevent a user or running process in the virtual machine from connecting or disconnecting a device from within the guest operating system, as well as modifying devices, by adding the following parameters. By default, a rogue user with non-administrator privileges in a virtual machine can connect a disconnected CD-ROM drive and access sensitive information on the media left in the drive.

Normal users and processes-that is, users and processes without root or administrator privileges-within virtual machines have the capability to connect or disconnect devices, such as network adaptors and CD-ROM drives, as well as the ability to modify device settings. In general, the virtual machine settings should use editor or configuration editor to remove any unneeded or unused hardware devices. However, the device may need to be used again, so removing it is not always a good solution. In that case, prevent a user or running process in the virtual machine from connecting or disconnecting a device from within the guest operating system, as well as modifying devices, by adding the following parameters. By default, a rogue user with non-administrator privileges in a virtual machine can connect a disconnected CD-ROM drive and access sensitive information on the media left in the drive.

If enabled, a VM can obtain detailed information about the physical host. The default value for the parameter is FALSE. This setting should not be TRUE unless a particular VM requires this information for performance monitoring. An adversary potentially can use this information to inform further attacks on the host.

Serial ports are interfaces for connecting peripherals to the virtual machine. They are often used on physical systems to provide a direct, low-level connection to the console of a server, and a virtual serial port allows for the same access to a virtual machine. Serial ports allow for low-level access, which often does not have strong controls like logging or privileges.

The VMsafe CPU/memory API allows a security virtual machine to inspect and modify the contents of the memory and CPU registers on other VMs, for the purpose of detecting and preventing malware attacks. However, an attacker might compromise the VM by making use of this introspection channel; therefore it should be monitored for unauthorized usage of this API. A VM must be configured explicitly to accept access by the VMsafe CPU/memory API. This involves three parameters: one to enable the API, one to set the IP address used by the security virtual appliance on the introspection vSwitch, and one to set the port number for that IP address. If the VM is being protected by such a product, then make sure the latter two parameters are set correctly. This should be done only for specific VMs for which this protection is wanted.

The VMsafe CPU/memory API allows a security virtual machine to inspect and modify the contents of the memory and CPU registers on other VMs, for the purpose of detecting and preventing malware attacks. However, an attacker might compromise the VM by making use of this introspection channel; therefore it should be monitored for unauthorized usage of this API. A VM must be configured explicitly to accept access by the VMsafe CPU/memory API. This involves three parameters: one to enable the API, one to set the IP address used by the security virtual appliance on the introspection vSwitch, and one to set the port number for that IP address. If the VM is being protected by such a product, then make sure the latter two parameters are set correctly. This should be done only for specific VMs for which this protection is wanted.

The VMsafe CPU/memory API allows a security virtual machine to inspect and modify the contents of the memory and CPU registers on other VMs, for the purpose of detecting and preventing malware attacks. However, an attacker might compromise the VM by making use of this introspection channel; therefore it should be monitored for unauthorized usage of this API. A VM must be configured explicitly to accept access by the VMsafe CPU/memory API. This involves three parameters: one to enable the API, one to set the IP address used by the security virtual appliance on the introspection vSwitch, and one to set the port number for that IP address. If the VM is being protected by such a product, then make sure the latter two parameters are set correctly. This should be done only for specific VMs for which this protection is wanted.