At Quarkslab, we don't only break software and exploit vulnerabilities, we also try to create innovative and efficient solutions to counter them. Cappsule is one of those solutions.

Have you ever been pwned because of a malicious document? Do you spend way too much time installing throw-away VMware or VirtualBox virtual machines to improve your OPSEC or to test random software found on the Internet? Do you trust your web browser? Users should be able to execute any software without putting the entire system security at stake. Cappsule virtualizes any software on the fly (e.g. web browser, office suite, media player) into lightweight VMs called cappsules thanks to hardware virtualization. Attacks are confined inside cappsules and therefore don't have any impact on the host OS. Applications don't need to be repackaged and their usage remains the same for the end user: it is completely transparent. Moreover, the OS doesn't need to be reinstalled nor modified.

Today, we release Cappsule under GPLv2. More information about the project can be found on its own website, and the source code is hosted on GitHub. Eager to test it? VMware and VirtualBox images as well as Ubuntu packages are available.

video screenshot (new tab)

What's new?

Traditional virtualization solutions (e.g. VMware, Xen, KVM) virtualize whole operating systems (such as Windows or Linux), whereas Cappsule virtualizes a running system. VMs launched by Cappsule don’t go through the boot step; they start directly on a running kernel. This particular feature allows an instantaneous launch of VMs. One can think of VMs as forks of the host operating system. In fact, the VMs’ kernel is a copy of the running host kernel. Another particularity is that no VM disk image is required. There’s no need to setup, configure, install, manage and keep new VMs up-to-date. The host filesystem is accessible as copy-on-write (with respect to a whitelist of files and folders accessible in read-only or read-write mode).

Enough of This Marketing Speech

Cappsule is a small hypervisor (roughly 15K lines of source code) designed for Linux x86-64. It has been heavily tested on Ubuntu 16.04, but any distro with a recent kernel should be able to use it. Since it relies on hardware virtualization, the CPU must support Intel VT-x and EPT. In order to operate, Cappsule inserts 2 kernel modules (cappsule.ko and cappsule_guest.ko). A userland daemon is responsible for the communication between the userland and the kernel module.

For the record, Intel VT-x and EPT are described in the Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volumes 3C: System Programming Guide Part 3 [1]. [2] provides an excellent introduction to Intel VT-x and the development of hypervisors.

For the sake of clarity and to avoid any confusion, a quick explanation about the terminology is required. Cappsule, with an upper-case C is the project name whereas cappsule(s) with a lower-case c is the VM's name.


Host Virtualization

The running system is bluepilled [0]: the hypervisor is launched from the kernel module and a new VM called «trusted guest» containing the running system is created. There are only two types of VM: trusted guest and cappsules.

The only instance without restrictions is the bluepilled OS (trusted guest). It's created during the launch of the hypervisor and stopped when the hypervisor is removed. This doesn't affect the correct operation of the running system. However, the hypervisor can still take control of the system when a VM exit occurs. A few x86 instructions unconditionally trigger VM exits (for instance: cpuid, vmcall, etc.). Privileged operations such as VM creation are initiated through ioctl from userland and through vmcall from kernelland. On the one hand ioctls are handled by the trusted guest kernel module and vmcalls by the hypervisor, on the other.

Cappsules (0 to n instances) are the lightweight VMs described below. They aren't trusted by the hypervisor: they aren't allowed to access to hardware and their memory is a copy-on-write version of the snapshot. The hypervisor kills the cappsule when a forbidden operation (for instance, I/O instructions) occurs.

Memory Snapshot

Once the running system is bluepilled, the userland daemon does an ioctl to initiate the memory snapshot, this is quite similar to the suspend-to-RAM feature of the Linux kernel. The average snapshot size is typically under the few hundred MB. The kernel module of the trusted guest does a vmcall which triggers a VM exit and the hypervisor finally does the snapshot (interrupts are disabled in VMX root mode, this avoids race conditions). A copy of the whole system memory is made except for:

  • Userland pages, because userland processes are frozen before the snapshot and won't be able to resume their execution in cappsules

  • Filesystem caches

  • The code of the hypervisor, because an attacker may take advantage of it if a vulnerability were to be found

  • Sensitive pages containing cryptographic keys should be zeroed, but we didn't implement this feature yet

However, there are a few subtleties related to locking. For instance, if a kernel thread locks a mutex before the snapshot phase and the thread isn't allowed to resume its execution, we just introduced a deadlock in the cappsule's kernel. We try hard to avoid this situation by taking these locks before the snapshot and unlocking them after.

Cappsule Creation

Once the snapshot is done, cappsules can finally be created. They are launched by users through a small executable (virt) which communicates with the userland daemon thanks to a JSON API. For example, the following request is sent to the daemon when virt exec --policy unrestricted --no-gui bash is executed:

  "cmd": "create",
    "params": {
      "cwd": "/home/user",
      "tty": "48x210",
      "env": [
      "basedir": "/home/user/.cappsule/unrestricted",
      "no-gui": true,
      "argv": [
      "fstype": "overlay",
      "groups": "4,24,27,30,46,114,115,1000",
      "memory": 1024,
      "policy": "unrestricted",
      "miscfs": [],
      "display": "",
      "rootfs": "/"

The daemon converts this request to an ioctl, which tells the hypervisor to create a new VM. The daemon also launches the device servers (netserver, fsserver, but no guiserver because of the --no-gui option) associated to these cappsule. A new kernel thread (called shadow process) associated to this VM is created:

$ ps fauxw | grep capsule
root       8856  2.3  0.0      0     0 ?        S    16:23   0:00  \_[capsule-5]

CPU Virtualization

Hardware virtualization allows fine grain control for the execution of privileged instruction by a guest. VM behavior is configured through a VM Control Structure called VMCS. In order to avoid any modification of the host, it's crucial to restrict the CPU registers and MSRs to which guests can write to. Each VM is associated with a different VMCS and are configured to trigger a VM exit when some instructions are executed by the guest. Cappsule implements a set of two VMCS templates, which allows the allocation and creation of different VMCS correctly configured for the trusted guest and the cappsules.

Even if virtualization is activated on every CPU, only one of them is available from a given cappsule. The reason is simple: every other CPU (but the current one) is offlined before snapshoting and put right back online after the snapshot is completed. This method allows several cappsules to run simultaneously on different CPUs. Having one CPU in a cappsule is way easier to handle and avoid a lot of race condition attacks.

user@ubuntu:~$ cat /sys/devices/system/cpu/online
user@ubuntu:~$ virt exec -p unrestricted -n bash
user@capsule-6:~$ cat /sys/devices/system/cpu/online

Memory Virtualization

Memory is virtualized with Intel EPT, which introduces a new set of page tables to convert guest physical addresses (GPA) to host physical addresses (HPA).This set of page tables is initialized with freshly allocated pages to share memory between the cappsule and the host.

Once the cappsule starts its execution, the algorithm to handle EPT violations is trivial: if the requested GPA is present in the snapshot, this page is given to the guest (copy-on-write), otherwise a new page is allocated. This algorithm guarantees that VMs can only access to a copy-on-write version of the kernel memory as seen during the snapshot. As a side effect, no DMA is possible. Since I/O instructions aren't allowed, there's no feasible hardware access.

VM Scheduling

A CPU is exclusively in one of these 2 modes: VMX root or VMX non-root. The hypervisor is executed in VMX root while the trusted guest and the cappsules are executed in VMX non-root.

VMX root and VMX non-root

A kernel thread (the shadow process) executed in the kernel of the trusted guest is associated to each VM and is responsible of its scheduling. The algorithm executed by kernel threads is straightforward:

while (!stop) {

The hypervisor stops the current cappsule when a VM exit occurs or when its time quantum is reached. VMs can also inform the hypervisor, thanks to a vmcall that they want to pause their execution when the idle thread is executed. Shadow processes, while being kernel threads, are allowed to receive 2 signals: SIGKILL and SIGTERM. If one of these signals is received, or the init process inside the cappsule terminates, the shadow processes exits and the associated cappsule is killed . The cappsule may also be killed during a VM exit if it does a forbidden operation (e.g. execution of an I/O instruction).

Cappsule's Kernel

A cappsule begins its execution in a kernel stub (in cappsule_guest.ko) which proceeds to various initializations (timer, date, shared memory, tty, etc.). The snapshot memory is slightly modified in order to hijack saved RIP in the kernel stack, and return in the ioctl code which triggered the snapshot creation. During the memory snapshot, almost no processes currently running will be allowed to continue their execution in the cappsules. For example, the scheduling of I/O threads isn't allowed. There are a few exceptions for necessary kernel threads and workqueues: watchdog, vmstat, vmstat_update, etc. Four userland processes are also allowed: the snapshot process (init), fsclient, guiclient and netclient.

A few hooks are necessary and the hypervisor is responsible for handling the VM-exits when breakpoints are hit. For example, vt_console_print() is hooked to export cappsule's dmesg to the host. Some APIC related functions are also patched to avoid hardware access which would cause the cappsule to be killed.


Since cappsules aren't allowed to access hardware, devices are mostly pointless. 4 userland devices emulate the following features: filesystem, net, console and GUI. These devices are divided in 2 parts: a host process and a guest process, communicating through shared memory (this mechanism is called xchan in Cappsule's source code). For instance, the net device is composed of a userland process called netclient running in the cappsule and netserver, a userland process running in the host and working as a filtering proxy.


The Linux kernel doesn't make any direct access to the filesystem except for crashdumps. Thanks to this assumption, the init process of the cappsule chroots into a FUSE filesystem. fsclient (the FUSE process running in the cappsule) communicates with fsserver (running in the host) through shared memory. fsserver applies rules to allow or forbid access to files and folders.

If allowed, this mechanism allows cappsules to access the host filesystem transparently. An interesting consequence is that software updates in the host are instantly available in running cappsules. Additionally, overlayfs is used to provide a copy-on-write filesystem; thus, the host filesystem is never modified.

Finally, fsserver is executed with the privileges of the user who launched the cappsule. Even if an issue were to be found in the rules, the whole host filesystem wouldn't be compromised.


The userland process netclient (inside the cappsule) creates a TUN interface. It communicates with the netserver host process through shared memory. netserver also creates a TUN interface and a set of iptables rules to filter packets coming from the cappsule according to cappsule's policy.


The GUI is the only component not developed from scratch. It's a fork of the graphical part of Qubes OS, which fits perfectly into our model.

In the cappsule, an X11 server instance is launched alongside a specific window manager (guiclient), forwarding each GUI operation to the host through shared memory. Since the protocol between guiclient and guiserver is custom, guiserver translates and forwards the requests to the host X11 server. Detailed information about the Qubes GUI protocol can be found here.

More Info

Excited? Check out Cappsule's website, read the doc, test it and try to break out from cappsules. If you're still curious about Cappsule's internals, feel free to join the IRC channel (##cappsule on Finally, the project is under GPL, don't hesitate to contribute! We're eager to get any feedback and new ideas.

Thanks to w1gz and jb for the reviewing of this blog post!


If you would like to learn more about our security audits and explore how we can help you, get in touch with us!