How API Abuse Nearly Bankrupted a Developer Tools Startup

On Monday morning, the founder of a small developer tools company opened an email from OpenAI with the subject line "Usage Alert - Action Required." The number inside made him physically ill.

The Horrifying Discovery

The company's typical OpenAI bill was $500/month. That weekend had generated nearly 100x that amount. Someone had found the AI endpoint and used it as their personal GPT-4 server.

How They Found the Endpoint

The company had built an AI-powered code documentation tool. The frontend sent requests to /api/generate which proxied to OpenAI. Simple, clean, and completely unprotected.

// The original (terrible) endpoint
app.post('/api/generate', async (req, res) => {
  const { prompt } = req.body;

  // No auth check. No rate limit. Just vibes.
  const completion = await openai.chat.completions.create({
    model: "gpt-4",
    messages: [{ role: "user", content: prompt }]
  });

  res.json(completion);
});

The attacker probably found the endpoint through:

• Scanning JavaScript bundles for API endpoints
• Checking network requests in browser dev tools
• Automated scanners looking for unprotected AI endpoints

Once they found a working endpoint, they set up a script to pump through requests 24/7.

The Recovery

The founder immediately rotated the API key to stop the bleeding. Then began the painful process of dealing with the bill.

OpenAI support was understanding but needed assurance this wouldn't happen again. The team documented everything:

• Proof that requests came from unknown IPs, not their users
• The new authentication and rate limiting implementation
• Spending alerts configured at multiple thresholds
• Monthly spending caps enabled

After two weeks of back-and-forth, OpenAI forgave $42,000 of the charges. The company still paid $5,000, which was painful but survivable.

The Fix

// The secure version
import { rateLimit } from 'express-rate-limit';
import { authMiddleware } from './auth';

const aiLimiter = rateLimit({
  windowMs: 60 * 1000, // 1 minute
  max: 10, // 10 requests per minute per user
  message: 'Too many requests, please slow down'
});

app.post('/api/generate',
  authMiddleware,        // Require login
  aiLimiter,             // Rate limit
  usageTracker,          // Track per-user usage
  async (req, res) => {
    const user = req.user;

    // Check user's remaining quota
    if (user.aiCredits <= 0) {
      return res.status(403).json({
        error: 'Usage limit reached'
      });
    }

    // Process request...
    await decrementUserCredits(user.id);
});

The team also set up proper OpenAI spending controls:

• Hard spending cap: $1,000/month maximum
• Email alerts: At $100, $300, $500, $800
• Daily monitoring: Automated checks for unusual spikes
• Per-user limits: Maximum requests per user per day