Description

LXD has arbitrary file read+write on host via rootfs/ symlink in malicious image Reported from https://github.com/lxc/incus/security/advisories/GHSA-2q3f-q5pq-g8wv

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

Incus validates an image as soon as it sees a normal metadata.yaml and a rootfs/ entry, but full extraction can later process a duplicate top-level rootfs symlink. Later, the stopped-container file API opens d.RootfsPath() and passes that file descriptor to forkfile, which chroots to it.

metadata.yaml
rootfs/
rootfs -> /

In practice, this allows a malicious actor to access the host's filesystem with root privileges.

PoC

Below, we map the container's rootfs to / on the host, but it can be mapped anywhere. We then retrieve the host's /etc/shadow file and create a file in /.

#!/bin/sh
set -eu

tmpdir=$(mktemp -d)
cleanup() {
    rm -rf "${tmpdir}"
}
trap cleanup EXIT INT QUIT TERM HUP...
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Impact

Arbitrary file read and write on the host via unsanitized symlink; possibly leading to command execution.

Mitigation

Aliases

References