File upload endpoints have size and type restrictions

Unrestricted file uploads are the intersection of CWE-434 (unrestricted upload of dangerous file type) and CWE-770 (resource exhaustion). Accepting unbounded file sizes lets attackers fill your storage quota and exhaust memory parsing the upload body. Validating file type only by extension or the caller-controlled Content-Type header is trivially bypassed: renaming exploit.php to exploit.png and setting Content-Type: image/png defeats both checks. OWASP API 2023 flags this under API4 (unrestricted resource consumption) and API8 (security misconfiguration).

Harden every upload endpoint in app/api/ with three independent controls. First, limit size at the parser level. Second, validate MIME type from actual file bytes using the file-type package — Content-Type headers are attacker-controlled and cannot be trusted. Third, enforce an allowlist:

import { fileTypeFromBuffer } from 'file-type'

const MAX_SIZE = 5 * 1024 * 1024 // 5MB
const ALLOWED_TYPES = new Set(['image/jpeg', 'image/png', 'image/webp', 'application/pdf'])

const arrayBuffer = await file.arrayBuffer()
if (arrayBuffer.byteLength > MAX_SIZE) return apiError('FILE_TOO_LARGE', '...', 413)

const detected = await fileTypeFromBuffer(Buffer.from(arrayBuffer))
if (!detected || !ALLOWED_TYPES.has(detected.mime)) {
  return apiError('INVALID_FILE_TYPE', '...', 415)
}

Store the file in object storage (S3, Supabase Storage, R2) under a random UUID key — never the user-supplied filename.

• ID: saas-api-design.api-security.file-upload-restrictions
• Severity: high
• What to look for: Enumerate all API routes that handle file uploads (look for multipart/form-data handling, FormData, multer, formidable, busboy, direct file buffer handling, or Next.js formData() calls). For each upload endpoint, check: (1) Is there a maximum file size limit enforced? (2) Is there a file type allowlist (MIME type check and/or file extension check)? (3) Is the MIME type validated from the actual file content, not just the filename or Content-Type header (which can be spoofed)?
• Pass criteria: At least 100% of file upload endpoints enforce: an explicit maximum file size of no more than 10MB, a file type allowlist based on MIME type inspection (not just extension or Content-Type header alone), and rejection of disallowed types with a clear error response.
• Fail criteria: Any upload endpoint without a size limit; or type validation based only on file extension or Content-Type header (easily spoofed); or no type validation at all.
• Skip (N/A) when: No file upload functionality exists in the project. Signal: no multipart/form-data handling, no multer/formidable/busboy dependencies, no FormData processing in any route handler.
• Detail on fail: Example: POST /api/upload has no file size limit. Identify which upload routes lack restrictions (e.g., "POST /api/upload has no file size limit; type is validated only by extension, not MIME content"). Max 500 chars.
• Remediation: Harden every file upload endpoint in app/api/ with three controls. First, enforce a maximum file size at the parser level (e.g., multer({ limits: { fileSize: 5 * 1024 * 1024 } }) for 5MB). Second, validate the MIME type from the actual file content — use the file-type npm package which reads the file's magic bytes rather than trusting the declared Content-Type. Third, maintain an explicit allowlist of permitted types (e.g., ['image/jpeg', 'image/png', 'image/webp', 'application/pdf']) and reject anything not on it. Store uploaded files in cloud object storage (S3, Supabase Storage, Cloudflare R2) rather than the local filesystem, and give them random names (not user-supplied names) to prevent path traversal.