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Fix API Rate Limit Exhaustion in Tornado

API Rate Limit Exhaustion in Tornado allows attackers to orchestrate Denial of Service (DoS) or brute-force attacks by flooding the event loop with unthrottled requests. To secure the endpoint, we must implement a distributed rate-limiting mechanism, typically using a Token Bucket or Sliding Window algorithm backed by an atomic store like Redis.

The Vulnerable Pattern

import tornado.web
import tornado.ioloop

class VulnerableAPI(tornado.web.RequestHandler):
async def post(self):
# VULNERABILITY: No check on request frequency.
# An attacker can spam this endpoint to exhaust DB connections or CPU.
data = self.get_argument(‘data’)
self.write({‘status’: ‘processed’, ‘data’: data})


def make_app():
return tornado.web.Application([
(r’/api/data’, VulnerableAPI),
])

The Secure Implementation

The secure implementation introduces a custom decorator that intercepts the request before execution. It uses an asynchronous Redis client to track request counts per IP address. By using the INCR and EXPIRE commands, we create a fixed-window counter. If the count exceeds the defined 'limit' within the 'window' (in seconds), the handler immediately returns a 429 Too Many Requests status code, protecting downstream resources and the Tornado event loop from saturation.

import tornado.web
import redis.asyncio as redis
from functools import wraps

Initialize Redis for distributed state


r = redis.from_url(‘redis://localhost’)


def limit_requests(limit: int, window: int):
def decorator(method):
@wraps(method)
async def wrapper(self, *args, **kwargs):
ip = self.request.remote_ip
key = f’rate_limit:{ip}’


        # Atomic increment and TTL check
        count = await r.incr(key)
        if count == 1:
            await r.expire(key, window)
        
        if count > limit:
            self.set_status(429)
            self.write({'error': 'Too Many Requests'})
            return
        
        return await method(self, *args, **kwargs)
    return wrapper
return decorator


class SecureAPI(tornado.web.RequestHandler):
@limit_requests(limit=5, window=60)
async def post(self):
self.write({‘status’: ‘secure_processed’})

System Alert • ID: 1616
Target: Tornado API
Potential Vulnerability

Your Tornado API might be exposed to API Rate Limit Exhaustion

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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