Fix SSRF (Server Side Request Forgery) in Tornado
SSRF in Tornado occurs when the AsyncHTTPClient fetches a user-controlled URL without validation, allowing attackers to pivot into your internal network, hit cloud metadata services (169.254.169.254), or bypass firewalls. To secure this, you must implement strict protocol enforcement, DNS resolution checks, and IP blacklisting of private/reserved ranges.
The Vulnerable Pattern
import tornado.web from tornado.httpclient import AsyncHTTPClient
class VulnerableHandler(tornado.web.RequestHandler): async def get(self): # CRITICAL: User input is passed directly to fetch() target_url = self.get_argument(‘url’) client = AsyncHTTPClient() response = await client.fetch(target_url) self.write(response.body)
The Secure Implementation
The secure implementation introduces a validation layer before the request is dispatched. First, it restricts the scheme to HTTP/HTTPS to prevent protocol smuggling (e.g., file://, gopher://). Second, it resolves the target hostname to an IP address. Third, it uses the ipaddress module to verify that the IP is not part of a private range (RFC 1918), loopback, or link-local (Cloud Metadata) address. This prevents the backend from being used as a proxy to scan internal infrastructure.
import socket
import ipaddress
from urllib.parse import urlparse
import tornado.web
from tornado.httpclient import AsyncHTTPClient
ALLOWED_SCHEMES = {‘http’, ‘https’}
def validate_url(url):
parsed = urlparse(url)
if parsed.scheme not in ALLOWED_SCHEMES:
return False
try:
# Resolve hostname to IP to prevent DNS Rebinding and check ranges
resolved_ip = socket.gethostbyname(parsed.hostname)
ip_obj = ipaddress.ip_address(resolved_ip)
if ip_obj.is_private or ip_obj.is_loopback or ip_obj.is_link_local:
return False
return True
except Exception:
return False
class SecureHandler(tornado.web.RequestHandler):
async def get(self):
target_url = self.get_argument(‘url’)
if not validate_url(target_url):
raise tornado.web.HTTPError(403, ‘Forbidden: Invalid Target’)
client = AsyncHTTPClient()
response = await client.fetch(target_url)
self.write(response.body)</code></pre>
Your Tornado API
might be exposed to SSRF (Server Side Request Forgery)
74% of Tornado apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.
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