---

name: sast-fileupload description: >- Detect insecure file upload vulnerabilities in a codebase using a three-phase approach: discovery (find all upload sites), batched verify (check extension bypass and related issues in parallel subagents, 3 sites each), and merge (consolidate batch results). Requires sast/architecture.md (run sast-analysis first). Outputs findings to sast/fileupload-results.md. Use when asked to find file upload, unrestricted upload, or extension bypass bugs.

Insecure File Upload Detection

You are performing a focused security assessment to find insecure file upload vulnerabilities in a codebase. This skill uses a three-phase approach with subagents: discovery (find all places where uploaded files are received and stored), batched verify (check bypass vectors in parallel batches of up to 3 upload sites each), and merge (consolidate batch reports into one results file).

Prerequisites: sast/architecture.md must exist. Run the analysis skill first if it doesn't.

---

What is an Insecure File Upload

Insecure file upload occurs when an application accepts files from users without properly validating or restricting what can be uploaded, allowing an attacker to upload executable or malicious files. The most critical outcome is Remote Code Execution (RCE): an attacker uploads a web shell (e.g., a .php file) and the server executes it when accessed via a direct URL.

The core pattern: a user-supplied file reaches a storage location without adequate extension validation, and the stored file is accessible or executable.

What Insecure File Upload IS

• Accepting any file type with no extension or content check: file.save(upload_path) with no validation
• Content-Type-only validation: checking Content-Type: image/png without verifying the actual extension or file content — trivially bypassed by setting the header manually
• Extension blocklist with gaps: .php is blocked but .php3, .php4, .php5, .phtml, .phar, .shtml are not
• Case-insensitive bypass: blocking .php but allowing .PHP, .Php, .pHp
• Double extension bypass: shell.php.jpg — code extracts the last .jpg and considers it safe, but the server (Apache) serves it as PHP
• Path traversal in filenames: ../../webroot/shell.php stored via an unsanitized filename
• Incomplete filename sanitization: only stripping ../ but not encoded variants %2e%2e%2f
• Serving uploaded files from a web-executable directory without disabling execution

What Insecure File Upload is NOT

Do not flag these as file upload vulnerabilities:

• Stored XSS via SVG: uploading an SVG with embedded <script> that is reflected back — that's XSS, not an upload execution issue
• SSRF via file content: uploading an XML or SVG that triggers an outbound request — that's XXE/SSRF, not a file upload execution issue
• DoS via large files: missing file size limits — a separate availability issue
• IDOR on download: accessing another user's uploaded file without authorization — that's IDOR
• Secure uploads: files stored outside the web root, or served through a controlled download endpoint that sets Content-Disposition: attachment, or stored in an object storage bucket with no public execution capability

Patterns That Prevent Insecure File Upload

When you see these patterns together, the code is likely not vulnerable:

1. Allowlist of safe extensions (most important)

ALLOWED_EXTENSIONS = {'png', 'jpg', 'jpeg', 'gif', 'pdf'}
ext = filename.rsplit('.', 1)[-1].lower()
if ext not in ALLOWED_EXTENSIONS:
    abort(400)

2. Magic byte / file content validation (defense in depth)

import magic
mime = magic.from_buffer(file.read(2048), mime=True)
ALLOWED_MIMES = {'image/png', 'image/jpeg', 'image/gif'}
if mime not in ALLOWED_MIMES:
    abort(400)

3. Filename sanitization using a trusted library

from werkzeug.utils import secure_filename
filename = secure_filename(file.filename)  # strips path separators and dangerous chars

4. Storing uploads outside the web root

/var/uploads/  ← not served by the web server
/var/www/html/ ← web root (do NOT store uploads here)

5. Serving uploads through a controlled endpoint with Content-Disposition

@app.route('/download/<filename>')
def download(filename):
    return send_from_directory(UPLOAD_FOLDER, filename,
                               as_attachment=True)  # forces download, prevents execution

6. Renaming the file to a server-generated UUID

import uuid
stored_name = str(uuid.uuid4()) + '.jpg'  # extension is server-controlled, not user-controlled

---

Vulnerable vs. Secure Examples

Python — Flask

# VULNERABLE: no extension check, file stored in web-accessible directory
@app.route('/upload', methods=['POST'])
def upload():
    f = request.files['file']
    f.save(os.path.join('static/uploads', f.filename))
    return 'uploaded'

# VULNERABLE: content-type only check (trivially bypassed with curl -H)
@app.route('/upload', methods=['POST'])
def upload():
    f = request.files['file']
    if f.content_type not in ['image/png', 'image/jpeg']:
        abort(400)
    f.save(os.path.join('static/uploads', f.filename))
    return 'uploaded'

# VULNERABLE: blocklist — .phtml/.phar/.php5 not covered
BLOCKED = {'.php', '.sh', '.exe'}
@app.route('/upload', methods=['POST'])
def upload():
    f = request.files['file']
    ext = os.path.splitext(f.filename)[1].lower()
    if ext in BLOCKED:
        abort(400)
    f.save(os.path.join('static/uploads', f.filename))
    return 'uploaded'

# SECURE: allowlist + sanitized filename + outside web root
ALLOWED = {'png', 'jpg', 'jpeg', 'gif'}
UPLOAD_FOLDER = '/var/uploads'  # outside web root

@app.route('/upload', methods=['POST'])
def upload():
    f = request.files['file']
    filename = secure_filename(f.filename)
    ext = filename.rsplit('.', 1)[-1].lower()
    if ext not in ALLOWED:
        abort(400)
    f.save(os.path.join(UPLOAD_FOLDER, filename))
    return 'uploaded'

Python — Django

# VULNERABLE: no validation on FileField
class DocumentForm(forms.ModelForm):
    class Meta:
        model = Document
        fields = ['upload']

# VULNERABLE: manual save with no extension check
def upload(request):
    f = request.FILES['file']
    with open(f'media/uploads/{f.name}', 'wb+') as dest:
        for chunk in f.chunks():
            dest.write(chunk)

# SECURE: custom validator on FileField
def validate_file_extension(value):
    ext = os.path.splitext(value.name)[1].lower()
    if ext not in ['.png', '.jpg', '.jpeg', '.gif']:
        raise ValidationError('Unsupported file extension.')

class DocumentForm(forms.ModelForm):
    upload = forms.FileField(validators=[validate_file_extension])

Node.js — Multer (Express)

// VULNERABLE: no file filter, stored in public directory
const upload = multer({ dest: 'public/uploads/' });
app.post('/upload', upload.single('file'), (req, res) => {
    res.send('uploaded');
});

// VULNERABLE: MIME type filter only (can be faked)
const upload = multer({
    dest: 'uploads/',
    fileFilter: (req, file, cb) => {
        if (!file.mimetype.startsWith('image/')) return cb(null, false);
        cb(null, true);
    }
});

// SECURE: allowlist of extensions + storage outside web root
const ALLOWED_EXT = ['.jpg', '.jpeg', '.png', '.gif'];
const storage = multer.diskStorage({
    destination: '/var/uploads',  // not served by Express
    filename: (req, file, cb) => {
        const ext = path.extname(file.originalname).toLowerCase();
        cb(null, `${uuidv4()}${ext}`);
    }
});
const upload = multer({
    storage,
    fileFilter: (req, file, cb) => {
        const ext = path.extname(file.originalname).toLowerCase();
        cb(null, ALLOWED_EXT.includes(ext));
    }
});

PHP

// VULNERABLE: no extension check, stored in web root
move_uploaded_file($_FILES['file']['tmp_name'], 'uploads/' . $_FILES['file']['name']);

// VULNERABLE: checking only content type header
if ($_FILES['file']['type'] !== 'image/jpeg') {
    die('Invalid file type');
}
move_uploaded_file($_FILES['file']['tmp_name'], 'uploads/' . $_FILES['file']['name']);

// VULNERABLE: blocklist missing phtml/phar
$ext = strtolower(pathinfo($_FILES['file']['name'], PATHINFO_EXTENSION));
$blocked = ['php', 'sh', 'py'];
if (in_array($ext, $blocked)) die('Blocked');
move_uploaded_file($_FILES['file']['tmp_name'], 'uploads/' . $_FILES['file']['name']);

// SECURE: allowlist + rename to UUID + outside web root
$allowed = ['jpg', 'jpeg', 'png', 'gif'];
$ext = strtolower(pathinfo($_FILES['file']['name'], PATHINFO_EXTENSION));
if (!in_array($ext, $allowed)) die('Invalid extension');
$stored = '/var/uploads/' . bin2hex(random_bytes(16)) . '.' . $ext;
move_uploaded_file($_FILES['file']['tmp_name'], $stored);

Java — Spring Boot (MultipartFile)

// VULNERABLE: no validation, stored in web-accessible path
@PostMapping("/upload")
public String upload(@RequestParam("file") MultipartFile file) throws IOException {
    Path path = Paths.get("src/main/resources/static/uploads/" + file.getOriginalFilename());
    Files.write(path, file.getBytes());
    return "uploaded";
}

// VULNERABLE: content type header only
@PostMapping("/upload")
public String upload(@RequestParam("file") MultipartFile file) throws IOException {
    if (!file.getContentType().startsWith("image/")) throw new BadRequestException();
    Files.write(Paths.get("uploads/" + file.getOriginalFilename()), file.getBytes());
    return "uploaded";
}

// SECURE: allowlist + UUID rename + path outside web root
private static final Set<String> ALLOWED = Set.of("jpg", "jpeg", "png", "gif");

@PostMapping("/upload")
public String upload(@RequestParam("file") MultipartFile file) throws IOException {
    String original = StringUtils.cleanPath(file.getOriginalFilename());
    String ext = FilenameUtils.getExtension(original).toLowerCase();
    if (!ALLOWED.contains(ext)) throw new BadRequestException("Invalid extension");
    String stored = UUID.randomUUID() + "." + ext;
    Files.write(Paths.get("/var/uploads/" + stored), file.getBytes());
    return "uploaded";
}

Go

// VULNERABLE: no extension check, stored in static directory
func uploadHandler(w http.ResponseWriter, r *http.Request) {
    file, header, _ := r.FormFile("file")
    defer file.Close()
    dst, _ := os.Create("static/uploads/" + header.Filename)
    defer dst.Close()
    io.Copy(dst, file)
}

// SECURE: allowlist extension + UUID rename + outside web root
var allowed = map[string]bool{"jpg": true, "jpeg": true, "png": true, "gif": true}

func uploadHandler(w http.ResponseWriter, r *http.Request) {
    file, header, _ := r.FormFile("file")
    defer file.Close()
    ext := strings.ToLower(filepath.Ext(header.Filename))
    if ext == "" || !allowed[ext[1:]] {
        http.Error(w, "invalid extension", http.StatusBadRequest)
        return
    }
    stored := "/var/uploads/" + uuid.New().String() + ext
    dst, _ := os.Create(stored)
    defer dst.Close()
    io.Copy(dst, file)
}

Ruby on Rails

# VULNERABLE: no content type or extension validation
def upload
  file = params[:file]
  File.open(Rails.root.join('public', 'uploads', file.original_filename), 'wb') do |f|
    f.write(file.read)
  end
end

# SECURE: ActiveStorage with content type allowlist (Rails 6+)
has_one_attached :avatar
validates :avatar, content_type: ['image/png', 'image/jpg', 'image/jpeg']
# Note: still validate extension too — content_type is user-supplied in some configurations

# SECURE: CarrierWave with extension and content type allowlist
class AvatarUploader < CarrierWave::Uploader::Base
  def extension_allowlist
    %w[jpg jpeg png gif]
  end

  def content_type_allowlist
    /image\//
  end
end

C# — ASP.NET Core

// VULNERABLE: no extension c