Identification Failures in Chi with Mutual Tls

Identification failures occur when a server cannot reliably confirm the identity of a client. In Chi, enabling Mutual TLS (mTLS) shifts the burden of client authentication to certificates, but misconfiguration or implementation gaps can still weaken identification. With mTLS, the server requests a client certificate and validates it, yet failures arise if the server does not enforce strict certificate verification or if clients present certificates that are insufficiently scoped.

Chi’s HTTP server provides hooks to inspect TLS states, but developers must explicitly require and validate client certificates. An identification failure in this context means the server either accepts no certificate (falling back to unauthenticated handling) or accepts a certificate without confirming its validity chain, revocation status, or intended usage. This can expose endpoints that should be restricted and allow unauthorized clients to proceed undetected.

One common pattern is binding mTLS expectations to the request context. For example, a developer might check for a client certificate only for certain routes and omit checks for others, creating an inconsistent security boundary. Additionally, if the server does not map the certificate’s subject or SAN (Subject Alternative Name) to an internal principal, the server lacks a reliable identifier for authorization decisions. Attackers can exploit this by leveraging certificates intended for other services or by exploiting missing validation to inject identity claims that the server treats as trusted.

Consider a Chi route that should only allow service A to call an admin endpoint. If the server validates that a client certificate exists but does not verify the certificate’s extended key usage or organizational unit, a certificate intended for a less privileged service might be accepted. This is an identification failure because the server incorrectly identifies the client as service A when it is not. Such misalignment between certificate metadata and route authorization enables privilege escalation and lateral movement within the API surface.

OpenAPI/Swagger analysis can highlight endpoints that lack mTLS enforcement expectations, but runtime validation remains essential. The scanner tests unauthenticated attack surfaces, and when mTLS is partially implemented, findings may reveal routes where client certificates are optional. These findings map to OWASP API Top 10 A07:2021 — Identification and Authentication Failures, emphasizing the need to treat mTLS not as a one-time toggle but as a consistently enforced verification layer tied to authorization logic.

Remediation focuses on enforcing strict client certificate validation and reliably mapping certificate attributes to identities. In Chi, use withNegotiation and inspect http.Request.TLS to require client certificates and verify their chain. Always validate the certificate against a trusted pool, check revocation (via CRL or OCSP where feasible), and confirm intended usage constraints.

Below is a concrete, working example of configuring a Chi router to require and verify client certificates. This ensures identification is based on validated certificate material rather than presence alone.

use chi::routing::Router;
use chi::{Router, IntoResponse};
use std::net::TcpListener;
use tls_rustls::rustls::{ClientConfig, ServerConfig, RootCertStore, Certificate, PrivateKey};
use tls_rustls::rustls::pki_types::{CertificateDer, PrivateKeyDer};
use std::sync::Arc;

fn build_server_config() -> Arc<ServerConfig> {
    let mut root_store = RootCertStore::empty();
    // Load your CA that signs client certificates
    let ca_cert = std::fs::read("/path/to/ca.crt").expect("unable to read CA cert");
    let certs = rustls_pemfile::certs(&mut &ca_cert[..])
        .collect::<Result<Vec&lt;CertificateDer>, _>>()
        .expect("invalid CA cert");
    for cert in certs {
        root_store.add(cert).expect("invalid CA cert");
    }

    let mut server_config = ServerConfig::builder()
        .with_safe_defaults()
        .with_client_root_certificates(root_store)
        .with_no_client_auth() // We will enforce per-route
        .expect("valid config");

    // Server certificate and key
    let cert_chain = vec![CertificateDer::from(std::fs::read("/path/to/server.crt").expect("unable to read server cert"))];
    let priv_key = PrivateKeyDer::Pkcs8(std::fs::read("/path/to/server.key").expect("unable to read server key").into());
    server_config.cert_resolver = Arc::new(tls_rustls::NoClientAuthResolver);
    // Note: In practice, set a proper cert resolver with your server cert/key

    Arc::new(server_config)
}

fn require_client_cert(router: Router) -> Router {
    router
        .route("/admin/*", chi::routing::get(|| async { "Admin endpoint requires valid client cert" }))
        .layer(chi::middleware::map_request(move |req: chi::Request| -> chi::Request {
            let tls = req.extensions().get::<TlsInfo>();
            // Ensure client cert is present and validated by the server's TLS handshake
            if let Some(tls_info) = tls {
                if tls_info.client_cert().is_none() {
                    // Reject request if no client certificate presented
                    return req.with_status(401).with_header("Content-Type", "text/plain").with_body("Client certificate required");
                }
                // Optionally inspect certificate fields for identification mapping
                if let Some(cert) = tls_info.client_cert() {
                    // Example: verify SAN or OU to identify service
                    // This is a placeholder for actual validation logic
                }
            } else {
                return req.with_status(403).with_body("TLS info not available");
            }
            req
        }));
}

// Helper to extract TLS client cert for identification
struct TlsInfo {
    cert: Option&lt;CertificateDer&gt;
}

impl<'a> From&lt;&amp;'a chi::Request&gt; for TlsInfo {
    fn from(req: &amp;chi::Request) -> Self {
        // In real usage, obtain TLS state from request extensions provided by your TLS layer
        TlsInfo { cert: None } // Placeholder: integrate with your TLS provider's extensions
    }
}

For production, integrate the TLS provider’s client certificate extraction into the request extensions so the Chi middleware can access verified certificate details. Additionally, map certificate fields (e.g., OU, SAN) to internal identifiers before authorization checks, ensuring that identification aligns with business roles. Combine this with rate limiting and anomaly detection to reduce the impact of compromised certificates.

When using the CLI tool, you can verify mTLS configurations by running middlebrick scan <url> against endpoints that should require client certificates; findings will highlight routes where client certificates are optional or validation is missing. For teams managing many services, the Pro plan’s continuous monitoring and GitHub Action integration can alert you if a deployment relaxes mTLS requirements, helping maintain consistent identification across changes.