Identification Failures in Axum with Mutual Tls

Identification failures occur when an API cannot correctly determine the identity of a peer. In Axum, combining the framework’s native routing and extractor patterns with mutual TLS (mTLS) can expose subtle identification gaps. mTLS requires both client and server to present valid certificates during the TLS handshake, and Axum typically relies on runtime extractors to access certificate details for authorization decisions. If these extractors are missing, optional, or incorrectly composed, the request may proceed with an unverified or default identity, leading to authorization bypass or misattribution of the caller.

One common pattern is relying solely on the presence of a client certificate without validating the certificate chain against a trusted CA or checking specific fields such as the Subject Common Name (CN) or Organization. In Axum, developers might extract peer certificates using Extension<Vec> or by inspecting the request extensions populated by the TLS layer. If the extractor returns None and the handler does not guard against this, the request appears as unauthenticated at the application layer, even though mTLS was configured at the transport layer. This mismatch creates an identification failure where the system assumes a known identity when none is reliably verified.

Another issue arises when certificate information is used to derive user roles or permissions without normalization and strict validation. For example, extracting a certificate’s subject and directly mapping it to a role without checking revocation status or ensuring a canonical form can lead to privilege escalation or misclassification. An attacker with a valid certificate issued by a trusted CA but intended for a low-privilege service account could be misidentified as a higher-privilege entity if the mapping logic does not enforce least privilege and explicit identity-to-role bindings.

Additionally, in distributed setups where Axum services sit behind load balancers or API gateways that terminate mTLS, the framework may receive requests over plain HTTP. In such cases, the absence of forwarded certificate information in headers (e.g., a custom header carrying the client certificate fingerprint) can cause the application to lose identity context. Axum extractors expecting TLS-derived data will then operate on incomplete inputs, resulting in identification failures. This scenario highlights the importance of ensuring that identity propagation mechanisms are explicitly handled and validated within the application logic, rather than assumed from transport-layer properties.

To detect these issues, scanning an Axum endpoint with mTLS configuration using middleBrick’s 12 security checks can surface missing authentication guarantees and improper identity handling. middleBrick validates whether runtime observations align with the intended access control model, providing findings mapped to frameworks such as OWASP API Top 10 and SOC2. The CLI tool allows you to scan from the terminal with middlebrick scan <url>, while the GitHub Action can enforce security gates in CI/CD pipelines, and the MCP Server enables scanning directly from AI coding assistants in your IDE.

Remediation centers on rigorous validation of client certificates and consistent identity extraction within Axum handlers and extractors. You should configure your TLS layer to request and verify client certificates, then enforce strict checks in your application code before using certificate data for identification or authorization. Below are concrete code examples demonstrating how to implement mTLS-aware extractors and handlers in Axum.

1. Configure TLS with client certificate verification

Ensure your HTTPS listener requests and verifies client certificates. This is typically done at the server or reverse proxy layer, but your Axum service should expect valid certificates in the request extensions.

// Example using rustls programmatically (conceptual)
use rustls::{ServerConfig, NoClientAuth, RootCertStore, Certificate, PrivateKey};
use std::sync::Arc;

let mut root_store = RootCertStore::empty();
// Load trusted CA certificates
root_store.add(&Certificate(ca_cert_der.to_vec())).expect("Invalid CA");

let config = ServerConfig::builder()
    .with_safe_defaults()
    .with_no_client_auth() // We'll handle verification via extractors or middleware
    .with_single_cert(server_cert, server_key)
    .expect("Invalid server certificate");

// In practice, client verification can be enforced via require_client_auth in proxy/load balancer
// and certificate presence checked in Axum via extensions

2. Define an extractor for validated client identity

Create a robust extractor that retrieves the peer certificate, validates it, and returns a normalized identity. This extractor can then be used across your routes.

use axum::async_trait;
use axum::extract::{Extension, FromRequest};
use axum::http::Request;
use std::convert::Infallible;
use openssl::x509::X509;

struct AuthenticatedClient {
    pub subject: String,
    pub issuer: String,
}

#[async_trait]
impl FromRequest<S> for AuthenticatedClient
where
    S: Send + Sync,
{
    type Rejection = (axum::http::StatusCode, String);

    async fn from_request(req: Request<()>, _state: &S) -> Result {
        // Extract certificate list from request extensions (populated by TLS layer)
        let certs = req.extensions()
            .get::<Vec<openssl::x509::X509>>()
            .ok_or((axum::http::StatusCode::FORBIDDEN, "Missing client certificate".to_string()))?;

        let client_cert = certs.first().ok_or(
            (axum::http::StatusCode::FORBIDDEN, "No client certificate found".to_string())
        )?;

        // Validate certificate usage, e.g., extended key usage or custom OIDs
        // This is a simplified example; in production, validate chain and revocation
        let subject = client_cert.subject_name().to_string();
        let issuer = client_cert.issuer_name().to_string();

        Ok(AuthenticatedClient { subject, issuer })
    }
}

use axum::{routing::get, Router};

async fn profile_handler(client: AuthenticatedClient) -> String {
    // Now you can safely use client.subject and client.issuer
    format!("Profile for subject: {}", client.subject)
}

fn app() -> Router {
    Router::new()
        .route("/profile", get(profile_handler))
}

use axum::extract::Extension;
use axum::http::Request;
use axum::middleware::{self, Next};
use std::convert::Infallible;

async fn mtls_validation_middleware(
    request: Request<()>,
    next: Next<()>
) -> Result<impl IntoResponse, Infallible> {
    let certs = request.extensions().get::<Vec<X509>>().cloned();
    match certs.and_then(|certs| certs.first().map(|c| c.clone())) {
        Some(cert) => {
            // Normalize identity, e.g., map subject to internal user ID
            let identity = cert.subject_name().to_string();
            request.extensions_mut().insert(identity);
            next.run(request).await
        }
        None => Err((StatusCode::FORBIDDEN, "Client certificate required".into()).into()),
    }
}

// In your app builder:
// let app = Router::new()
//     .layer(middleware::from_fn(mtls_validation_middleware))
//     .route(...);