Api Rate Abuse in Axum with Api Keys

Rate abuse in Axum when using API keys occurs when an API key is accepted but no per-key rate-limiting enforcement exists at the application or gateway layer. Axum does not provide built-in rate limiting, so developers must add it explicitly. If a route accepts an API key for authentication but does not also enforce a request cap per key, an attacker who obtains or guesses a valid key can issue a high volume of requests. This exposes the endpoint to denial-of-service via resource exhaustion, unfair usage, or brute-force attacks against other controls.

In a black-box scan, middleBrick tests unauthenticated attack surfaces and checks whether rate limiting is applied consistently across authenticated paths. When API keys are used without corresponding rate limits, findings related to Rate Limiting and Authentication may be surfaced. For example, a route that expects a header like X-API-Key but lacks a middleware layer to track and restrict key usage allows unbounded request bursts. Attack patterns include credential stuffing with a known key, or simply overwhelming backend services when keys are shared among clients. Because API keys are often treated as lightweight secrets, they can be leaked in logs or client-side code, increasing the likelihood of abuse if rate controls are missing.

Consider an endpoint that queries a database or invokes downstream services. Without per-key throttling, a single compromised key can generate expensive operations, leading to inflated compute costs or contention for shared state. In OpenAPI/Swagger specs, if rate-limiting parameters are described only at the global level but not enforced per key in the implementation, runtime behavior may diverge from documentation. middleBrick’s checks for Rate Limiting cross-reference spec definitions with observed behavior to highlight gaps where authentication and rate control are not properly integrated.

To remediate rate abuse with API keys in Axum, enforce per-key rate limits using middleware that inspects the key before allowing the request to proceed. Store keys with associated metadata such as allowed requests per time window, and apply throttling consistently across all routes that require authentication.

Example: define a simple in-memory rate limiter using std::sync::Arc, tokio::sync::RwLock, and a hash map keyed by the API key string. Track request timestamps and reject requests that exceed the quota.

use axum::{routing::get, Router, extract::State, http::HeaderMap};
use std::collections::VecDeque;
use std::sync::Arc;
use tokio::sync::RwLock;

struct RateLimiter {
    limits: std::collections::HashMap)>,
}

impl RateLimiter {
    fn new() -> Self {
        Self { limits: std::collections::HashMap::new() }
    }

    fn allow(&mut self, key: &str) -> bool {
        let (max_requests, window, ref mut timestamps) = self.limits.entry(key.to_string()).or_insert((5, std::time::Duration::from_secs(60), VecDeque::new()));
        let now = std::time::Instant::now();
        while let Some(oldest) = timestamps.front() {
            if now.duration_since(*oldest) > *window {
                timestamps.pop_front();
            } else {
                break;
            }
        }
        if timestamps.len() < *max_requests {
            timestamps.push_back(now);
            true
        } else {
            false
        }
    }
}

type SharedLimiter = Arc<RwLock<RateLimiter>>;

async fn handler(
    State(limiter): State<SharedLimiter>,
    headers: HeaderMap,
) -> (axum::http::StatusCode, String) {
    let api_key = match headers.get("X-API-Key") {
        Some(v) => v.to_str().unwrap_or(""),
        None => return (axum::http::StatusCode::UNAUTHORIZED, "missing key".into()),
    };
    let mut limiter = limiter.write().await;
    if limiter.allow(api_key) {
        (axum::http::StatusCode::OK, "ok".into())
    } else {
        (axum::http::StatusCode::TOO_MANY_REQUESTS, "rate limit exceeded".into())
    }
}

#[tokio::main]
async fn main() {
    let limiter = Arc::new(RwLock::new(RateLimiter::new()));
    let app = Router::new()
        .route("/data", get(handler))
        .with_state(limiter);
    axum::Server::bind("0.0.0.0:3000".parse().unwrap())
        .serve(app.into_make_service())
        .await
        .unwrap();
}

In production, replace the in-memory store with a distributed solution such as Redis to coordinate limits across multiple instances. Ensure the API key is passed via a dedicated header and is validated for format and existence before the rate limiter is consulted. middleBrick’s scans can verify that rate-limiting checks are applied to authenticated paths and that the implementation aligns with the published OpenAPI contract.