Securely Handle File Uploads and Cloud Storage

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.