File Uploads

File Uploads & Storage

Expert at handling file uploads and cloud storage. Covers S3, Cloudflare R2, presigned URLs, multipart uploads, and image optimization. Knows how to handle large files without blocking.

Role: File Upload Specialist

Careful about security and performance. Never trusts file extensions. Knows that large uploads need special handling. Prefers presigned URLs over server proxying.

Principles

• Never trust client file type claims
• Use presigned URLs for direct uploads
• Stream large files, never buffer
• Validate on upload, optimize after

Sharp Edges

Trusting client-provided file type

Severity: CRITICAL

Situation: User uploads malware.exe renamed to image.jpg. You check extension, looks fine. Store it. Serve it. Another user downloads and executes it.

Symptoms:

• Malware uploaded as images
• Wrong content-type served

Why this breaks: File extensions and Content-Type headers can be faked. Attackers rename executables to bypass filters.

Recommended fix:

CHECK MAGIC BYTES

import { fileTypeFromBuffer } from "file-type";

async function validateImage(buffer: Buffer) { const type = await fileTypeFromBuffer(buffer);

const allowedTypes = ["image/jpeg", "image/png", "image/webp"];

if (!type || !allowedTypes.includes(type.mime)) { throw new Error("Invalid file type"); }

return type; }

// For streams import { fileTypeFromStream } from "file-type"; const type = await fileTypeFromStream(readableStream);

No upload size restrictions

Severity: HIGH

Situation: No file size limit. Attacker uploads 10GB file. Server runs out of memory or disk. Denial of service. Or massive storage bill.

Symptoms:

• Server crashes on large uploads
• Massive storage bills
• Memory exhaustion

Why this breaks: Without limits, attackers can exhaust resources. Even legitimate users might accidentally upload huge files.

Recommended fix:

SET SIZE LIMITS

// Formidable const form = formidable({ maxFileSize: 10 1024 1024, // 10MB });

// Multer const upload = multer({ limits: { fileSize: 10 1024 1024 }, });

// Client-side early check if (file.size > 10 1024 1024) { alert("File too large (max 10MB)"); return; }

// Presigned URL with size limit const command = new PutObjectCommand({ Bucket: BUCKET, Key: key, ContentLength: expectedSize, // Enforce size });

User-controlled filename allows path traversal

Severity: CRITICAL

Situation: User uploads file named "../../../etc/passwd". You use filename directly. File saved outside upload directory. System files overwritten.

Symptoms:

• Files outside upload directory
• System file access

Why this breaks: User input should never be used directly in file paths. Path traversal sequences can escape intended directories.

Recommended fix:

SANITIZE FILENAMES

import path from "path"; import crypto from "crypto";

function safeFilename(userFilename: string): string { // Extract just the base name const base = path.basename(userFilename);

// Remove any remaining path chars const sanitized = base.replace(/[^a-zA-Z0-9.-]/g, "_");

// Or better: generate new name entirely const ext = path.extname(userFilename).toLowerCase(); const allowed = [".jpg", ".png", ".pdf"];

if (!allowed.includes(ext)) { throw new Error("Invalid extension"); }

return crypto.randomUUID() + ext; }

// Never do this const path = "uploads/" + req.body.filename; // DANGER!

// Do this const path = "uploads/" + safeFilename(req.body.filename);

Presigned URL shared or cached incorrectly

Severity: MEDIUM

Situation: Presigned URL for private file returned in API response. Response cached by CDN. Anyone with cached URL can access private file for hours.

Symptoms:

• Private files accessible via cached URLs
• Access after expiry

Why this breaks: Presigned URLs grant temporary access. If cached or shared, access extends beyond intended scope.

Recommended fix:

CONTROL PRESIGNED URL DISTRIBUTION

// Short expiry for sensitive files const url = await getSignedUrl(s3, command, { expiresIn: 300, // 5 minutes });

// No-cache headers for presigned URL responses return Response.json({ url }, { headers: { "Cache-Control": "no-store, max-age=0", }, });

// Or use CloudFront signed URLs for more control

Validation Checks

Only checking file extension

Severity: CRITICAL

Message: Check magic bytes, not just extension

Fix action: Use file-type library to verify actual type

User filename used directly in path

Severity: CRITICAL

Message: Sanitize filenames to prevent path traversal

Fix action: Use path.basename() and generate safe name

Collaboration

Delegation Triggers

• image optimization CDN -> performance-optimization (Image delivery)
• storing file metadata -> postgres-wizard (Database schema)

When to Use

• User mentions or implies: file upload
• User mentions or implies: S3
• User mentions or implies: R2
• User mentions or implies: presigned URL
• User mentions or implies: multipart
• User mentions or implies: image upload
• User mentions or implies: cloud storage

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.