Most people use an operating system like Windows or OS X on their desktop and laptop computers.
These OSes are popular because they tend to be easy to use and usually come pre-installed on the computers people buy.
However, they present problems when it comes to security.
For example, you might open an innocent-looking email attachment or website, not realizing that you're actually allowing malware (malicious software) to run on your computer.
Depending on what kind of malware it is, it might do anything from showing you unwanted advertisements to logging your keystrokes to taking over your entire computer.
This could jeopardize all the information stored on or accessed by this computer, such as health records, confidential communications, or thoughts written in a private journal.
Malware can also interfere with the activities you perform with your computer.
For example, if you use your computer to conduct financial transactions, the malware might allow its creator to make fraudulent transactions in your name.
Unfortunately, conventional security approaches like antivirus programs and (software and/or hardware) firewalls are no longer enough to keep out sophisticated attackers.
For example, nowadays it's common for malware creators to check to see if their malware is recognized by any signature-based antivirus programs.
If it's recognized, they scramble their code until it's no longer recognizable by the antivirus programs, then send it out.
The best of these programs will subsequently get updated once the antivirus programmers discover the new threat, but this usually occurs at least a few days after the new attacks start to appear in the wild.
By then, it's too late for those who have already been compromised.
More advanced antivirus software may perform better in this regard, but it's still limited to a detection-based approach.
New zero-day vulnerabilities are constantly being discovered in the common software we all use, such as our web browsers, and no antivirus program or firewall can prevent all of these vulnerabilities from being exploited.
This approach allows you to keep the different things you do on your computer securely separated from each other in isolated qubes so that one qube getting compromised won't affect the others.
For example, you might have one qube for visiting untrusted websites and a different qube for doing online banking.
This way, if your untrusted browsing qube gets compromised by a malware-laden website, your online banking activities won't be at risk.
Similarly, if you're concerned about malicious email attachments, Qubes can make it so that every attachment gets opened in its own single-use [disposable qube].
In this way, Qubes allows you to do everything on the same physical computer without having to worry about a single successful cyberattack taking down your entire digital life in one fell swoop.
Moreover, all of these isolated qubes are integrated into a single, usable system.
Programs are isolated in their own separate qubes, but all windows are displayed in a single, unified desktop environment with unforgeable colored window borders so that you can easily identify windows from different security levels.
Common attack vectors like network cards and USB controllers are isolated in their own hardware qubes while their functionality is preserved through secure [networking][network], [firewalls], and [USB device management][USB].
Integrated [file] and [clipboard] copy and paste operations make it easy to work across various qubes without compromising security.
The innovative [Template] system separates software installation from software use, allowing qubes to share a root filesystem without sacrificing security (and saving disk space, to boot).
Qubes even allows you to sanitize PDFs and images in a few clicks.
For example, a standard [Fedora](/doc/templates/fedora/) qube is expected to have basically the same privacy properties as that upstream Fedora distribution, enhanced to some degree by the control Qubes provides over that qube.
For most users, this level of privacy may be good enough for many common activities.
However, users seeking more advanced privacy features should use [Whonix][Qubes-Whonix] qubes.
Privacy is far more difficult than is commonly understood.
In addition to the [web browser](https://www.torproject.org/projects/torbrowser/design/), there is also [VM fingerprinting](https://www.whonix.org/wiki/VM_Fingerprinting) and [advanced deanonymization attacks](https://www.whonix.org/wiki/Advanced_Deanonymization_Attacks) that most users have never considered (and this is just to mention a few examples).
The [Whonix Project](https://www.whonix.org/) specializes in [protecting against these risks](https://www.whonix.org/wiki/Protocol-Leak-Protection_and_Fingerprinting-Protection).
Booting your computer from a live CD (or DVD) when you need to perform sensitive activities can certainly be more secure than simply using your main OS, but this method still preserves many of the risks of conventional OSes.
For example, popular live OSes (such as [Tails] and other Linux distributions) are still **monolithic** in the sense that all software is still running in the same OS.
This means, once again, that if your session is compromised, then all the data and activities performed within that same session are also potentially compromised.
Not all virtual machine software is equal when it comes to security.
You may have used or heard of VMs in relation to software like VirtualBox or VMware Workstation.
These are known as "Type 2" or "hosted" hypervisors.
(The **hypervisor** is the software, firmware, or hardware that creates and runs virtual machines.)
These programs are popular because they're designed primarily to be easy to use and run under popular OSes like Windows (which is called the **host** OS, since it "hosts" the VMs).
However, the fact that Type 2 hypervisors run under the host OS means that they're really only as secure as the host OS itself.
If the host OS is ever compromised, then any VMs it hosts are also effectively compromised.
By contrast, Qubes uses a "Type 1" or "bare metal" hypervisor called [Xen].
Instead of running inside an OS, Type 1 hypervisors run directly on the "bare metal" of the hardware.
This means that an attacker must be capable of subverting the hypervisor itself in order to compromise the entire system, which is vastly more difficult.
Qubes makes it so that multiple VMs running under a Type 1 hypervisor can be securely used as an integrated OS.
For example, it puts all of your application windows on the same desktop with special colored borders indicating the trust levels of their respective VMs.
It also allows for things like secure copy/paste operations between VMs, securely copying and transferring files between VMs, and secure networking between VMs and the Internet.
Using a separate physical computer for sensitive activities can certainly be more secure than using one computer with a conventional OS for everything, but there are still risks to consider.
Briefly, here are some of the main pros and cons of this approach relative to Qubes:
* Physical separation doesn't rely on a hypervisor. (It's very unlikely that an attacker will break out of Qubes' hypervisor, but if one were to manage to do so, one could potentially gain control over the entire system.)
* Physical separation can be a natural complement to physical security.
(For example, you might find it natural to lock your secure laptop in a safe when you take your unsecure laptop out with you.)
* Physical separation can be cumbersome and expensive, since we may have to obtain and set up a separate physical machine for each security level we need.
* There's generally no secure way to transfer data between physically separate computers running conventional OSes.
(Qubes has a secure inter-VM file transfer system to handle this.)
* Physically separate computers running conventional OSes are still independently vulnerable to most conventional attacks due to their monolithic nature.
* Malware which can bridge air gaps has existed for several years now and is becoming increasingly common.
We don't believe either of these approaches are capable of providing reasonable security today, nor do we believe that they will be capable of doing so in the foreseeable future.
We believe that this is currently the only practically viable approach to implementing strong isolation while simultaneously providing compatibility with existing applications and drivers.
In short: we believe the Xen architecture allows for the creation of more secure systems (i.e. with a much smaller TCB, which translates to a smaller attack surface).
### What about this other/new (micro)kernel/hypervisor?
Whenever starting a discussion about another (micro)kernel or hypervisor in relation to Qubes, we strongly suggest including answers to the following questions first:
1. What kinds of containers does it use for isolation? Processes? PV VMs? Fully virtualized VMs (HVMs)? And what underlying h/w technology is used (ring0/3, VT-x)?
2. Does it require specially written/built applications (e.g. patched Firefox)?
3. Does it require custom drivers, or can it use Linux/Windows ones?
4. Does it support VT-d, and does it allow for the creation of untrusted driver domains?
5. Does it support S3 sleep?
6. Does it work on multiple CPUs/Chipsets?
7. What are the performance costs, more or less? (e.g. "XYZ prevents concurrent execution of two domains/processes on shared cores of a single processor", etc.)
8. Other special features? E.g. eliminates cooperative covert channels between VMs?
Here are the answers for Xen 4.1 (which we use as of 2014-04-28):
1. PV and HVM Virtual Machines (ring0/3 for PV domains, VT-x/AMD-v for HVMs).
2. Runs unmodified usermode apps (binaries).
3. Runs unmodified Linux drivers (dom0 and driver domains). PV VMs require special written pvdrivers.
4. Full VT-d support including untrusted driver domains.
We have designed the GUI virtualization subsystem with two primary goals: security and performance.
Our GUI infrastructure introduces only about 2,500 lines of C code (LOC) into the privileged domain (Dom0), which is very little, and thus leaves little space for bugs and potential attacks.
At the same time, due to the smart use of Xen shared memory, our GUI implementation is very efficient, so most virtualized applications really feel as if they were executed natively.
Instead, if you want to obtain your own trustworthy copy of this website in a secure way, you should clone our [website repo], [verify the PGP signatures on the commits and/or tags] signed by the [doc-signing keys] (which indicates that the content has undergone review per our [documentation guidelines]), then either [render the site on your local machine][render] or simply read the source, the vast majority of which was [intentionally written in Markdown so as to be readable as plain text for this very reason][Markdown].
We've gone to special effort to set all of this up so that no one has to trust the infrastructure and so that the contents of this website are maximally available and accessible.
A core tenet of the Qubes philosophy is "distrust the infrastructure," where "the infrastructure" refers to things like hosting providers, CDNs, DNS services, package repositories, email servers, PGP keyservers, etc.
As a project, we focus on securing endpoints instead of attempting to secure "the middle" (i.e., the infrastructure), since one of our primary goals is to free users from being forced to entrust their security to unknown third parties.
Instead, our aim is for users to be required to trust as few entities as possible (ideally, only themselves and any known persons whom they voluntarily decide to trust).
Users can never fully control all the infrastructure they rely upon, and they can never fully trust all the entities who do control it.
Therefore, we believe the best solution is not to attempt to make the infrastructure trustworthy, but instead to concentrate on solutions that obviate the need to do so.
We believe that many attempts to make the infrastructure appear trustworthy actually provide only the illusion of security and are ultimately a disservice to real users.
Since we don't want to encourage or endorse this, we make our distrust of the infrastructure explicit.
1. We [distrust the infrastructure] including GitHub (though there are aspects we're still [working on](https://github.com/QubesOS/qubes-issues/issues/3958)).
2. It's free (as in beer). We'd have to spend either time or money to implement a solution ourselves or pay someone to do so, and we can't spare either one right now.
3. It has low admin/overhead requirements, which is very important, given how little time we have to spare.
Although we caution users against [placing undue trust in this website][Should I trust this website?] because we [distrust the infrastructure], we have no objection to enabling website security features when doing so is relatively costless and provides some marginal benefit to website visitors.
This is mostly a security decision, as implementing such a feature would most likely introduce a great deal of complexity into the GUI virtualization infrastructure.
However, Qubes does allow for the use of accelerated graphics (e.g. OpenGL) in dom0’s Window Manager, so all the fancy desktop effects should still work.
Each qube is created from a TemplateVM and shares the root filesystem with this TemplateVM (in a read-only manner).
This means that each qube needs only as much disk space as is necessary to store its own private data.
This also means that it is possible to update the software for several qubes simultaneously by running a single update process in the TemplateVM upon which those qubes are based.
(These qubes will then have to be restarted in order for the update to take effect in them.)
If you are receiving an error message on install saying your "hardware lacks the features required to proceed", check to make sure the virtualization options are enabled in your BIOS/UEFI configuration.
You may be able to install without the required CPU features for testing purposes only, but VMs (in particular, sys-net) may not function correctly and there will be no security isolation.
On a system without VT-d/ADM-Vi/AMD IOMMU, there will be no real security benefit to having a separate NetVM, as an attacker could always use a simple [DMA attack](#what-is-a-dma-attack) to go from the NetVM to Dom0.
Nonetheless, all of Qubes' other security mechanisms, such as qube separation, work without VT-d/ADM-Vi/AMD IOMMU.
Therefore, a system running Qubes without VT-d/ADM-Vi/AMD IOMMU would still be significantly more secure than one running Windows, Mac, or Linux.
Most PCI devices allow the driver to request an arbitrary DMA operation (like "put received network packets at this address in memory", or "get this memory area and send it to the network").
The above attack requires access to a PCI device, which means that it can be performed only from the NetVM or USB VM, so someone must first break into one of those VMs.
There is a history of bugs in DHCP clients, DNS clients, etc.
Most attacks on the NetVM and USB VM (but not all of them!) require being somewhat close to the target system, for example, being connected to the same Wi-Fi network, or in the case of a USB VM, having physical access to a USB port.
Assuming XFCE desktop: in `Q` → `System Tools` → `Keyboard` → `Layout`, leave the checkbox "`Use system defaults`" checked. Do not customize the keyboard layout here.
Enable "debug mode" in the qube's settings, either by checking the box labeled "Run in debug mode" in the Qubes VM Manager qube settings menu or by running the `qvm-prefs` command.
See the [USB Troubleshooting guide](/doc/usb-troubleshooting/#usbvm-does-not-boot-after-creating-and-assigning-usb-controllers-to-it).
### I assigned a PCI device to a qube, then unassigned it/shut down the qube. Why isn't the device available in dom0?
This is an intended feature.
A device which was previously assigned to a less trusted qube could attack dom0 if it were automatically reassigned there.
In order to re-enable the device in dom0, either:
* Reboot the physical machine.
or
* Go to the sysfs (`/sys/bus/pci`), find the right device, detach it from the pciback driver and attach back to the original driver. Replace `<BDF>` with your device, for example `00:1c.2`:
In Qubes 4.0, this is accomplished with the Devices Widget located in the tool tray (default top right corner, look for an icon with a yellow square).
From the top part of the list, click on the drive you want to attach, then select the qube to attach it to.
Although you can also attach the entire USB device to a qube by selecting it from the bottom part of the list, in general this approach should not be used because you are exposing the target qube to unnecessary additional attack surface.
Although external media such as external hard drives or flash drives plugged in via USB are available in the USB qube, it is not recommended to access them directly from inside the USB qube.
See [Block (Storage) Devices][storage](/doc/block-devices/) for more information.
You might even think that if a particular program or setting works so well for *you*, it would work well for *everyone*, so you'd actually be doing everyone a favor!
There is no particular configuration that will be ideal for everyone (despite how much you might feel that your preference would be better for everyone), so the best we can do is to put power in the hands of users to configure their Qubes installations the way they like (subject to security constraints, of course).
Please don't ask for your favorite program to be installed by default or for some setting that obviously varies by user preference to be changed so that it matches *your* preference.
This is an incredibly selfish attitude that demonstrates a complete lack of consideration for the thousands of other Qubes users who don't happen to share your preferences.
### Are there restrictions on the software that the Qubes developers are willing to use?
Yes.
In general, the Qubes developers will not use a piece of software unless there is an *easy* way to verify both its **integrity** and **authenticity**, preferably via PGP signatures (see [Verifying Signatures](/security/verifying-signatures/)).
Specifically:
* If PGP signatures are used, the signing key(s) should have well-publicized fingerprint(s) verifiable via multiple independent channels or be accessible to the developers through a web of trust.
* If the software is security-sensitive and requires communication with the outside world, a "split" implementation is highly preferred (for examples, see [Split GPG](/doc/split-gpg/) and [Split Bitcoin](/doc/split-bitcoin/)).
* If the software has dependencies, these should be packaged and available in repos for a [current, Qubes-supported version](/doc/supported-versions/#templatevms) of Fedora (preferred) or Debian (unless all the insecure dependencies can run in an untrusted VM in a "split" implementation).
* If the software must be built from source, the source code and any builders must be signed.
(Practically speaking, the more cumbersome and time-consuming it is to build from source, the less likely the developers are to use it.)
Since 2013 [Xen has not supported 32-bit x86 architecture](https://wiki.xenproject.org/wiki/Xen_Project_Release_Features) and Intel VT-d, which Qubes uses to isolate devices and drivers, is available on Intel 64-bit processors only.
In addition, with features like improved ASLR, it is often more difficult to exploit a bug on x64 Linux than x86 Linux.
While we designed Qubes from the beginning to limit potential attack vectors, we still realize that some of the code running in Dom0, e.g. our GUI daemon or xen-store daemon, however simple, might contain some bugs.
Plus since we haven't implemented a separate storage domain, the disk backends are in Dom0 and are "reachable" from the VMs, which adds up to the potential attack surface.
So, having faced a choice between 32-bit and 64-bit OS for Dom0, it was almost a no-brainer.
The 64-bit option provides some (little perhaps, but some) more protection against some classes of attacks, and at the same time does not have any disadvantages except the extra requirement of a 64 bit processor.
And even though Qubes now "needs" a 64 bit processor, it didn't make sense to run Qubes on a system without 3-4GB of memory, and those have 64-bit CPUs anyway.
### What is the recommended build environment for Qubes OS?
Any rpm-based, 64-bit environment, the preferred OS being Fedora.
### How do I build Qubes from sources?
See [these instructions](/doc/qubes-builder/).
### How do I submit a patch?
See the [Qubes Source Code Repositories](/doc/source-code/) article.
### What is Qubes' attitude toward changing guest distros?
We try to respect each distro's culture, where possible.
See the discussion on issue [#1014](https://github.com/QubesOS/qubes-issues/issues/1014) for an example.
The policy is there mostly to ease maintenance, on several levels:
* Less modifications means easier migration to new upstream distribution
releases.
* The upstream documentation matches the distribution running in the Qubes VM.
* We're less likely to introduce Qubes-specific issues.
* Each officially supported distribution (ideally) should offer the same set of
Qubes-specific features - a change in one supported distribution should be
followed also in others, including new future distributions.
No. Unlike many other virtualization systems, Qubes takes special effort to keep QEMU _outside_ of the TCB.
This has been achieved thanks to the careful use of Xen's stub domain feature.
For more details about how we improved on Xen's native stub domain use, see [here](https://blog.invisiblethings.org/2012/03/03/windows-support-coming-to-qubes.html).
See the Heads project [[1]](https://trmm.net/Heads) [[2]](http://osresearch.net/) for a better-designed non-UEFI-based secure boot scheme with very good support for Qubes.
(**Warning:** Since this is an external project that has not been reviewed or endorsed by the Qubes team, [allowing it to manage dom0 may be a security risk](https://github.com/Qubes-Community/Contents/blob/master/docs/security/security-guidelines.md#dom0-precautions).)
