Brute Force Attack in Axum with Mutual Tls

A brute force attack against an Axum service that uses mutual TLS (mTLS) focuses on exhausting authentication or rate-limiting controls rather than breaking the cryptographic handshake. mTLS ensures that both client and server present valid certificates during the TLS handshake, which strongly authenticates the client identity before HTTP request processing begins. However, this does not automatically prevent credential-based or token-based brute forcing at the application layer.

In Axum, if routes rely on extracting a secondary credential (e.g., an API key, JWT, or username/password) after mTLS client certificate validation, an attacker who can present a valid client certificate can still attempt many requests to guess or iterate through secrets. The presence of mTLS may inadvertently reduce surface area for some network-level attacks but can also create a false sense of security. For example, an API might accept mTLS and then check an X-API-Key header; without rate limiting on that header, an attacker can brute force keys efficiently.

The attack surface is shaped by how Axum routes are composed. If a route chains mTLS verification with permissive authorization logic, such as skipping per-request authorization checks or using a single shared secret across many identities, the attacker’s workload is reduced. Additionally, if TLS session resumption is enabled without strict limits, an attacker can reuse handshakes to reduce overhead and sustain higher request rates during a brute force attempt.

Consider an endpoint that expects a client certificate plus a bearer token or API key. Without rate limiting or account lockout, each request can probe a new token value. Since mTLS has already verified the client’s possession of a certificate, the backend may treat the request as trusted and only validate the token, making it a target for online guessing. Logging and monitoring gaps can further allow attackers to test credentials without triggering alerts, enabling iterative refinement of guesses.

Real attack patterns include iterating over predictable API key formats or abusing token enumeration to discover valid identities. These map to OWASP API Top 10 controls such as excessive data exposure and broken authentication. Although mTLS binds identity to a certificate, it does not replace application-level protections like rate limiting, which must be enforced explicitly in Axum to mitigate brute force risks.

To secure an Axum service using mutual TLS and reduce brute force risks, enforce strict per-identity rate limiting, tie authentication checks to the certificate subject, and avoid single shared secrets across many identities. The following code examples illustrate how to implement mTLS in Axum and layer application-level protections.

First, configure TLS with client certificate verification using rustls. This ensures only clients with valid certificates can reach your Axum routes.

use axum::Router;
use std::net::SocketAddr;
use tokio_rustls::rustls::{ServerConfig, NoClientAuth, Certificate, PrivateKey};
use tokio_rustls::TlsAcceptor;
use std::sync::Arc;
use tokio::net::TcpListener;

async fn build_tls_config() -> Arc {
    // Load server certificate and key
    let certs = vec![Certificate(/* DER bytes */)];
    let key = PrivateKey(/* DER bytes */);
    let mut server_config = ServerConfig::builder()
        .with_safe_defaults()
        .with_no_client_auth() // We will customize this
        .with_single_cert(certs, key)
        .expect("invalid server cert or key");

    // Require and verify client certificates
    let client_auth = rustls::server::ClientAuthMode::Required(Arc::new(|certs, _| {
        // Validate client cert chain here, e.g., check issuer, CN, revocation
        !certs.is_empty()
    }));
    server_config.client_auth = client_auth;
    Arc::new(server_config)
}

#[tokio::main]
async fn main() {
    let tls_config = build_tls_config().await;
    let tls_acceptor = TlsAcceptor::from(tls_config);
    let listener = TcpListener::bind("0.0.0.0:8443").await.unwrap();

    let app = Router::new()
        .route("/secure", axum::routing::get(secure_handler));

    loop {
        let (stream, _) = listener.accept().await.unwrap();
        let tls = tls_acceptor.accept(stream).await.unwrap();
        let service = app.clone().into_make_service();
        tokio::task::spawn(async move {
            service.call(tls).await.unwrap();
        });
    }
}

Next, implement per-identity rate limiting using the certificate’s subject or a mapped identifier. This prevents an attacker who presents a valid certificate from hammering endpoints.

use axum::{routing::get, Router, extract::Extension, http::HeaderMap};
use std::collections::HashMap;
use std::sync::{Arc, Mutex};
use std::time::{Duration, Instant};

struct RateLimiter {
    limits: HashMap>,
    max_requests: usize,
    window: Duration,
}

impl RateLimiter {
    fn new(max_requests: usize, window: Duration) -> Self {
        Self {
            limits: HashMap::new(),
            max_requests,
            window,
        }
    }

    fn allow(&mut self, key: &str) -> bool {
        let now = Instant::now();
        let requests = self.limits.entry(key.to_string()).or_default();
        requests.retain(|t| now.duration_since(*t) < self.window);
        if requests.len() < self.max_requests {
            requests.push(now);
            true
        } else {
            false
        }
    }
}

async fn secure_handler(
    Extension(limiter): Extension<Arc<Mutex<RateLimiter>>>,
    headers: HeaderMap,
) -> String {
    // Extract identity from client certificate (e.g., subject or SAN)
    let identity = headers.get("x-client-identity").map(|v| v.to_str().unwrap_or("unknown")).unwrap_or("unknown");
    let mut limiter = limiter.lock().unwrap();
    if limiter.allow(identity) {
        format!("Hello {}", identity)
    } else {
        (http::StatusCode::TOO_MANY_REQUESTS, "Rate limit exceeded").into_response().into_body()
    }
}

Finally, prefer per-identity secrets or scoped tokens rather than a single shared secret. If your backend issues API keys after mTLS authentication, bind each key to a specific certificate subject and enforce strict rate limits per key. This approach aligns with compliance frameworks by ensuring that authentication and authorization are explicit and auditable.