sast-file-security-testing

name	sast-file-security-testing
description	Investigate file operation vulnerabilities including unrestricted file upload, path traversal in file operations, and insecure file handling. Use when threat model identifies CWE-434 (Unrestricted Upload), CWE-73 (External Control of File Path), CWE-427 (Uncontrolled Search Path), or file security concerns.
allowed-tools	Read, Grep, Glob

SAST File Security Testing Skill

Purpose

Investigate file operation vulnerabilities by analyzing:

• File upload handling (type, size, name validation)
• File path construction from user input
• File read/write operations with user-controlled paths
• File execution and dynamic loading

Vulnerability Types Covered

1. Unrestricted File Upload (CWE-434)

Uploading files without proper validation can lead to RCE.

Dangerous Patterns:

# No file type validation
@app.route('/upload', methods=['POST'])
def upload():
    file = request.files['file']
    file.save(os.path.join(UPLOAD_DIR, file.filename))
    return 'Uploaded'

# Weak validation (extension only)
if file.filename.endswith('.jpg'):
    file.save(path)  # Bypass: shell.jpg.php, shell.php.jpg

# Content-Type from client (can be forged)
if file.content_type == 'image/jpeg':
    file.save(path)

Safe Patterns:

from werkzeug.utils import secure_filename
import magic

ALLOWED_EXTENSIONS = {'png', 'jpg', 'jpeg', 'gif'}
MAX_FILE_SIZE = 5 * 1024 * 1024  # 5MB

def allowed_file(filename):
    return '.' in filename and \
           filename.rsplit('.', 1)[1].lower() in ALLOWED_EXTENSIONS

@app.route('/upload', methods=['POST'])
def upload():
    file = request.files['file']
    
    # Check extension
    if not allowed_file(file.filename):
        return 'Invalid file type', 400
    
    # Check actual content (magic bytes)
    mime = magic.from_buffer(file.read(1024), mime=True)
    file.seek(0)
    if mime not in ['image/png', 'image/jpeg', 'image/gif']:
        return 'Invalid file content', 400
    
    # Sanitize filename
    filename = secure_filename(file.filename)
    
    # Generate unique name
    filename = 
    
    file.save(os.path.join(UPLOAD_DIR, filename))

2. Path Traversal in File Operations (CWE-73, CWE-22)

User input in file paths allows accessing arbitrary files.

Dangerous Patterns:

# Direct path concatenation
@app.route('/download/<filename>')
def download(filename):
    return send_file(os.path.join(FILES_DIR, filename))
    # Attack: /download/../../../etc/passwd

# User-controlled directory
directory = request.args.get('dir')
files = os.listdir(os.path.join(BASE, directory))

# Template/config loading
template = request.args.get('template')
content = open(f"templates/{template}").read()

Safe Patterns:

import os

@app.route('/download/<filename>')
def download(filename):
    # Sanitize filename
    filename = secure_filename(filename)
    
    # Resolve and validate path
    safe_path = os.path.realpath(os.path.join(FILES_DIR, filename))
    
    # Ensure path is within allowed directory
    if not safe_path.startswith(os.path.realpath(FILES_DIR)):
        return 'Access denied', 403
    
    if not os.path.exists(safe_path):
        return 'Not found', 404
    
    return send_file(safe_path)

3. Uncontrolled Search Path (CWE-427)

Loading executables or libraries from user-controlled paths.

Dangerous Patterns:

# Dynamic import from user input
module_name = request.args.get('module')
module = __import__(module_name)

# Subprocess with user-controlled command
cmd = request.args.get('tool')
subprocess.run([cmd, '-v'])

# Loading plugins from user path
plugin_path = user_config['plugin_dir']
exec(open(os.path.join(plugin_path, 'plugin.py')).read())

Safe Patterns:

# Allowlist for dynamic imports
ALLOWED_MODULES = {'math', 'json', 'datetime'}
module_name = request.args.get('module')
if module_name not in ALLOWED_MODULES:
    raise ValueError("Module not allowed")

# Absolute paths for executables
TOOL_PATH = '/usr/local/bin/approved-tool'
subprocess.run([TOOL_PATH, '-v'])

4. Insecure Temporary Files (CWE-377)

Predictable or insecure temporary file creation.

Dangerous Patterns:

# Predictable temp file name
temp_file = f"/tmp/app_{user_id}.txt"
with open(temp_file, 'w') as f:
    f.write(data)

# Race condition in temp file
if not os.path.exists(temp_path):
    with open(temp_path, 'w') as f:
        f.write(data)

Safe Patterns:

import tempfile

# Secure temp file
with tempfile.NamedTemporaryFile(delete=False) as f:
    f.write(data)
    temp_path = f.name

# Or with context manager
with tempfile.TemporaryDirectory() as tmpdir:
    # Files auto-deleted when context exits

5. Download Without Integrity Check (CWE-494)

Downloading and executing code without verification.

Dangerous Patterns:

# Download and execute
url = config['update_url']
script = requests.get(url).text
exec(script)

# Install package from URL
os.system(f"pip install {url}")

Safe Patterns:

import hashlib

# Verify checksum
expected_hash = "sha256:abc123..."
content = requests.get(url).content
actual_hash = hashlib.sha256(content).hexdigest()
if f"sha256:{actual_hash}" != expected_hash:
    raise ValueError("Integrity check failed")

Investigation Methodology

Step 1: Find File Upload Endpoints

Search for: request.files, multipart, upload
           save(, write(, wb, file.save
           multer, formidable, busboy

Step 2: Check Upload Validation

For each upload handler:

1. Is file extension validated?
2. Is content type checked (magic bytes, not just header)?
3. Is filename sanitized?
4. Is file size limited?
5. Where is the file stored (web-accessible?)

Step 3: Find File Path Operations

Search for: open(, os.path.join, send_file
           read_file, write_file, shutil
           Path(, pathlib

Step 4: Trace Path Sources

For each file operation:

1. Where does the path come from?
2. Is any part user-controllable?
3. Is path normalized and validated?

Step 5: Check for Execution Risks

Search for: exec(, eval(, __import__
           subprocess, os.system, popen
           require(, import(, load(

Classification Criteria

TRUE_POSITIVE:

• File upload with no content validation
• User input directly in file path without sanitization
• Uploaded files stored in web-accessible directory
• No path traversal protection (realpath check)

FALSE_POSITIVE:

• Files stored outside web root with random names
• Proper content-type validation via magic bytes
• Path canonicalization with directory containment check
• Allowlist-based filename/path validation

UNVALIDATED:

• Validation exists but may be bypassable
• Storage location unclear
• Framework-level protections may apply

Output Format

### Verdict
- **verdict**: TRUE_POSITIVE or FALSE_POSITIVE
- **confidence_score**: 1-10
- **risk_level**: LOW, MEDIUM, HIGH, or CRITICAL

### Vulnerability Type
- **Category**: Upload / Path Traversal / Execution
- **CWE**: CWE-XXX

### Evidence
- **Location**: file:line
- **Vulnerable Code**: [Code snippet]
- **User Input Source**: [How attacker provides input]

### Attack Scenario
1. [How attacker exploits this]
2. [What they can achieve: RCE, data theft, etc.]

### Recommendations
- [Specific fix with code example]

CWE Mapping

File Upload

• CWE-434: Unrestricted Upload of File with Dangerous Type
• CWE-646: Reliance on File Name or Extension
• CWE-351: Insufficient Type Distinction

Path Operations

• CWE-73: External Control of File Name or Path
• CWE-22: Path Traversal (also in injection-testing)
• CWE-36: Absolute Path Traversal
• CWE-427: Uncontrolled Search Path Element

File Integrity

• CWE-494: Download of Code Without Integrity Check
• CWE-377: Insecure Temporary File
• CWE-379: Creation of Temporary File in Directory with Insecure Permissions

Cross-Skill Dependencies

File security investigations may need:

• sast-injection-testing: Path traversal overlaps with injection
• sast-authorization-testing: Who can access uploaded files?
• sast-authentication-testing: Are upload endpoints authenticated?

Common Frameworks & Patterns

Python/Flask

request.files['file']
send_file(), send_from_directory()
secure_filename()

Python/Django

request.FILES['file']
FileField, ImageField
MEDIA_ROOT, MEDIA_URL

Node.js/Express

multer({ dest: 'uploads/' })
req.file, req.files
fs.readFile(path)
path.join(base, userInput)

Java/Spring

@RequestParam("file") MultipartFile
Files.copy(file.getInputStream(), path)
new File(userPath)

Safety Rules

• Only analyze code in the repository provided
• Do not attempt to upload files or access file systems
• Consider the full request lifecycle (upload → storage → retrieval)
• Check both direct file operations and framework abstractions