Fix API Rate Limit Exhaustion in Helidon

API Rate Limit Exhaustion (CWE-770) is a classic DoS vector where an attacker floods endpoints to consume thread pools or database connections. In Helidon's reactive model, while efficient, an unprotected route will still lead to resource exhaustion or downstream service failure. To mitigate this, we must implement a Token Bucket or Leaky Bucket filter to intercept requests before they hit the business logic.

The Vulnerable Pattern

Routing.builder()
    .register("/api", (req, res) -> {
        // Vulnerable: No throttling mechanism. 
        // An attacker can script thousands of concurrent requests here.
        String data = service.getExpensiveData();
        res.send(data);
    })
    .build();

The Secure Implementation

The vulnerable code lacks any gatekeeping, allowing unbounded execution of 'expensive' logic. The secure implementation utilizes the Bucket4j library to implement a Token Bucket strategy within a Helidon Handler. We define a policy (100 requests per minute) and check the bucket before calling `req.next()`. If the limit is hit, we return a 429 status code, effectively short-circuiting the request and protecting the underlying resources. In a distributed environment, use a Redis-backed bucket to synchronize state across nodes.

Bucket bucket = Bucket4j.builder()
    .addLimit(Bandwidth.classic(100, Refill.greedy(100, Duration.ofMinutes(1))))
    .build();
Routing.builder()
.any(“/api/*”, (req, res) -> {
// Secure: Intercepting request to check token availability
if (bucket.tryConsume(1)) {
req.next();
} else {
res.status(Http.Status.TOO_MANY_REQUESTS_429)
.send(“Rate limit exceeded. Try again later.”);
}
})
.get(“/api/data”, (req, res) -> {
res.send(service.getExpensiveData());
})
.build();