GPG and Software Packages
- Always keep the system up-to-date in dom0, template VMs and standalone VMs.
- Check gpg is enabled in config files (gpgcheck=1) if new Fedora repositories are installed.
- Safely import new signing keys by checking it is the same from multiple sources.
- Preferably only install packages from trusted sources, for example pre-configured Fedora, Debian, Whonix and Qubes sources.
- Untrusted or unverifiable programs should be installed in Standalone VMs or less trusted, cloned templates.
Hardware / Hardware Settings
- Enable VT-d/IOMMU via BIOS to have DMA protection,  effective network isolation and the ability to assign PCIe devices to a HVM. Check it is running via dom0 (qubes-hcl-report).
- Ensure Intel VT-x or AMD-V is available, since it is required for running HVM domains, such as Windows-based AppVMs.
- Prepare and utilize a USB qube to protect against side-channel attacks. 
- Use a Yubikey to enhance the security of Qubes user authentication, mitigate the risk of password snooping and to improve USB keyboard security.
- Prefer Intel Integrated Graphics Processing (IGP) units for greater system stability and security. 
- Ensure computer hardware meets all other Qubes-Whonix requirements for the best security, functionality and future compatibility with Qubes 4.X releases.
ISO and Qubes Version
- Verify the authenticity and integrity of the Qubes iso download.
- Prefer Qubes R4.0 over earlier releases, as fully virtualized (HVM or PVH) VMs provide greater protection against processor speculative execution bugs like the Meltdown and Spectre attacks, and other exploits.  
Protecting User Data and Activities
- For critical user data, protect against unintentional leaks by setting an empty NetVM field (set to "none") for the corresponding qube.
- For sensitive activities, do not run trusted, high-value VMs in paralell with untrusted VMs. 
- Observe the security context of colored windows borders in Qubes before running applications or manipulating data.
- If paying in cryptocurrencies, utilize a “split” bitcoin wallet which creates an offline “cold storage” wallet and an online “watching only” wallet.
- Avoid dual / multi-boot configurations in Qubes. The other OS could modify the unprotected /boot partition or firmware to maliciously compromise Qubes and/or spy on user activities.
- Be careful when running command line operations. Refer to a suitable resource first, then proceed.
- Use split-GPG for email to reduce the risk of key theft used for encryption / decryption and signing.
- Do not allow Qubes-Whonix or other VMs to completely "own" the full screen. 
- Disable previews (thumbnails) when using a file manager like Nautilus, as this is a known attack vector.
Template and Other VMs
- Never run applications in TemplateVMs or dom0, except updating tools or editors for configuration purposes (running applications poses security risks).
- Avoid configuring network traffic between two qubes for security reasons.
- Consider leveraging the non-persistence of Qubes' templates to fend off malware by locking-down, quarantining and checking the contents of /rw private storage.   
- Consider split dm-crypt to isolate device-mapper based secondary storage encryption (not the root filesystem) and LUKS header processing to DisposableVMs.
- Consider running
sys-usbas Static DisposableVMs.
- Qubes FAQ:
DMA is mechanism for PCI devices to access system memory (read/write). Without VT-d, any PCI device can access all the memory, regardless to which VM it is assigned (or if it is left in dom0). 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”). So, without VT-d, it gives unlimited access to the whole system. Now, it is only a matter of knowing where to read/write to take over the system, instead of just crashing. But since you can read the whole memory, it isn’t that hard.
- Automatically configured in Qubes R4, but an optional configuration in R3.2.
- Proprietary binary blobs of other GPU manufacturers pose security risks and the available open source drivers are notoriously unstable in Qubes.
- Qubes R3.2 and earlier versions rely on para-virtualized (PV) VMs.
- For instance, this recent security bug allows an attacker to escape from a PV domain and exploit the dom0 hypervisor. It only affects Qubes R3.2, since Qubes R4 only runs untrusted code in PVH or HVM domains by default.
- A successful exploit of the untrusted qube provides an avenue for attacking the sensitive qube.
- Without visible, colored decorations drawn by each VM window, a malicious application might only pretend to release the full screen (while the screen appears normal), or the full desktop may be emulated so users are tricked into entering sensitive information into false "trusted" domains.
vm-boot-protect.serviceis suitable for standalone VMs, AppVMs, netVMs, Whonix AppVMs and others. Do not use the
vm-boot-protect-rootservice for Whonix AppVMs.
- This service: starts before the private volume is mounted, protects /home desktop and shell startup executables, quarantines /rw configs and scripts (with whitelisting), re-deploys custom / default files to /rw on each boot, conducts SHA256 checking against unwanted changes, and more.
- Disabling of Qubes' default passwordless-root is also required.
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