Secure File Uploads in Next.js: Validation, Storage, and Virus Scanning

File uploads are a common attack vector. Here's how to handle them securely.

The Risks

• Malicious files: Executable uploads disguised as images
• Oversized uploads: DoS via huge files
• Path traversal: ../../etc/passwd in filenames
• MIME type spoofing: .jpg that's actually a PHP script
• Storage exhaustion: unlimited uploads filling your bucket

Client-Side Validation (First Layer)

function FileUpload() {
  const validateFile = (file: File): string | null => {
    const MAX_SIZE = 10 * 1024 * 1024  // 10MB
    const ALLOWED_TYPES = ['image/jpeg', 'image/png', 'image/webp', 'application/pdf']

    if (file.size > MAX_SIZE) return 'File too large (max 10MB)'
    if (!ALLOWED_TYPES.includes(file.type)) return 'File type not allowed'

    return null
  }

  const handleUpload = (e: React.ChangeEvent<HTMLInputElement>) => {
    const file = e.target.files?.[0]
    if (!file) return

    const error = validateFile(file)
    if (error) {
      toast.error(error)
      return
    }

    uploadFile(file)
  }

  return (
    <input
      type="file"
      accept="image/jpeg,image/png,image/webp,application/pdf"
      onChange={handleUpload}
    />
  )
}

Client-side validation is UX only. Always validate server-side too.

Server-Side Validation

// app/api/upload/route.ts
import { fileTypeFromBuffer } from 'file-type'

const ALLOWED_MIME_TYPES = new Set([
  'image/jpeg', 'image/png', 'image/webp', 'image/gif', 'application/pdf'
])

export async function POST(request: Request) {
  const formData = await request.formData()
  const file = formData.get('file') as File

  if (!file) return Response.json({ error: 'No file' }, { status: 400 })

  // Size check
  if (file.size > 10 * 1024 * 1024) {
    return Response.json({ error: 'File too large' }, { status: 413 })
  }

  const buffer = Buffer.from(await file.arrayBuffer())

  // Check ACTUAL file type from magic bytes (not client-reported MIME)
  const fileType = await fileTypeFromBuffer(buffer)
  if (!fileType || !ALLOWED_MIME_TYPES.has(fileType.mime)) {
    return Response.json({ error: 'Invalid file type' }, { status: 400 })
  }

  // Sanitize filename
  const safeFilename = sanitizeFilename(file.name)
  const key = `uploads/${crypto.randomUUID()}-${safeFilename}`

  // Upload to S3/R2
  await uploadToStorage(key, buffer, fileType.mime)

  return Response.json({ url: `https://cdn.example.com/${key}` })
}

function sanitizeFilename(name: string): string {
  return name
    .replace(/[^a-zA-Z0-9._-]/g, '_')  // remove dangerous chars
    .replace(/\.\./g, '_')              // prevent path traversal
    .slice(0, 100)                        // limit length
}

Using UploadThing (Managed)

npm install uploadthing @uploadthing/react

// server/uploadthing.ts
import { createUploadthing, type FileRouter } from 'uploadthing/next'
import { auth } from '@/auth'

const f = createUploadthing()

export const ourFileRouter = {
  imageUploader: f({ image: { maxFileSize: '4MB', maxFileCount: 4 } })
    .middleware(async () => {
      const session = await auth()
      if (!session) throw new Error('Unauthorized')
      return { userId: session.user.id }
    })
    .onUploadComplete(async ({ metadata, file }) => {
      await db.upload.create({
        data: { userId: metadata.userId, url: file.url, name: file.name },
      })
    }),
} satisfies FileRouter

export type OurFileRouter = typeof ourFileRouter

UploadThing handles chunking, resumable uploads, and CDN delivery.

Image Optimization After Upload

import sharp from 'sharp'

async function processImage(buffer: Buffer): Promise<Buffer> {
  return sharp(buffer)
    .resize(1200, 1200, { fit: 'inside', withoutEnlargement: true })
    .webp({ quality: 85 })
    .toBuffer()
}

Convert all uploads to WebP, resize to max dimensions. Reduces storage costs 60-80%.

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