Author Archives: mcontino

Enforcing Device Trust with Certificate‑Based Client Authentication – A Practical Guide for Modern Identity Providers

Posted on by

Introduction

In today’s zero‑trust world, granting access solely based on a user’s password or even an OTP is no longer sufficient. Threat actors are increasingly targeting compromised credentials, and the security community has responded with identity provider empowered device trust, a model that ensures only verified devices can obtain tokens from an identity provider (IdP).

When you combine device trust with TLS client‑certificate authentication, you create a powerful barrier: before any session token is issued for a sensitive application or service, the IdP validates a device‑bound certificate presented by the client. This approach works equally well for on‑premises workloads, cloud native services, and containerized applications.

In this post we’ll walk through how to configure popular IdPs like Keycloak, Authentik, Okta, OneLogin, and Auth0 to require Certificate ClientAuth using device trust certificates. We’ll also discuss best‑practice considerations for certificate issuance, rotation, and revocation.

Why Certificate‑Based Device Trust?

BenefitExplanation
Strong mutual TLS (mTLS)The client proves possession of a private key bound to a trusted device, eliminating credential stuffing attacks.
Device bindingCertificates are issued per device (or per container/compute) and can be tied to hardware TPMs or secure enclaves, making them hard to steal.
Zero‑trust enforcementEven if a user’s password is compromised, an attacker cannot obtain a token without the correct device certificate.
Fine‑grained policy controlIdPs can apply separate authentication policies based on certificate attributes (e.g., OU=Mobile, OU=Workstation).
Auditable provenanceEvery successful login includes the certificate fingerprint, simplifying forensic investigations.

Device Trust Certificate Lifecycle

  1. Enrollment – During first‑boot or via automated provisioning, the device presents its hardware-bound public certificate to an enrollment endpoint (often part of the IdP or Third-party service like ACME Attestation service).
  2. Provisioning – A trusted CA (internal PKI or managed service) issues a short‑lived X.509 certificate to each device. The private key is bound by the vTPM, HSM, or container runtime.
  3. Authentication – When a user attempts to access a protected app, the client initiates TLS with client‑certificate request. The presented certificate is validated against Device Trust certificate chain.
  4. Renewal / Rotation – Before expiry (typically 30–90 days), the device automatically requests a new cert via the provisioner. Old certificates are revoked or marked as expired in the CRL database.

Identify Provider Empowered Device Trust Authentication Flow

identity provider empowered with device trust Session flow diagram

The diagram illustrates that no token is ever issued unless the device presents a valid certificate that matches the IdP’s trusted CA list. This is the essence of an identity provider empowered device trust.

Preparing Your PKI

All IdPs discussed support the use of external CA integration. The steps are similar across platforms:

  1. Create a dedicated CA hierarchy – Root CA (offline) → Intermediate “Device‑Trust” CA (online).
  2. Define certificate profile – Include extensions such as subjectAltName for device ID, extendedKeyUsage = clientAuth, and optionally certificatePolicies to tag the trust domain.
  3. Deploy an automated provisioning service – For example, cert‑manager in Kubernetes, ACME with device attestation, or a simple SCEP service.

Note: Alternatively you can use smallstep step-ca as outlined in my practical cloud-native guide.

Sample OpenSSL config for the Device‑Trust CA:

[ req ]
distinguished_name = req_distinguished_name
prompt = no

[ req_distinguished_name ]
C  = US
ST = WA
L  = Seattle
O  = AcmeCorp
OU = DeviceTrustCA
CN = device-trust.acme.local

[ v3_intermediate_ca ]
basicConstraints = critical, CA:true, pathlen:0
keyUsage = critical, digitalSignature, cRLSign, keyCertSign
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always,issuer

Use this to generate the intermediate CA and then sign device CSRs with -extensions client_auth:

openssl req -new -nodes -newkey rsa:2048 \
  -keyout device.key -out device.csr \
  -subj "/C=US/ST=WA/L=Seattle/O=AcmeCorp/OU=Workstation/CN=device01"

openssl ca -config openssl.cnf -extensions client_auth \
  -days 30 -notext -md sha256 \
  -in device.csr -out device.crt

Configuring Identity Providers

Below we present the minimal configuration needed to enable Certificate ClientAuth for each IdP. The examples assume you already have a running instance of the IdP and that your PKI’s intermediate certificate is available as device-trust-ca.pem.

Keycloak

Keycloak provides TLS client‑certificate authentication via the X509 Authentication flow.

# keycloak-realm.yaml – add a new authentication flow called "DeviceTrust"
realm: myrealm
authenticationFlows:
  - alias: DeviceTrust
    providerId: basic-flow
    topLevel: true
    builtIn: false
    authenticationExecutions:
      - authenticator: x509-browser-authenticator
        requirement: REQUIRED
        priority: 10
        config:
          # Truststore containing device‑trust CA
          trustStoreFile: /opt/keycloak/conf/device-trust-ca.p12
          trustStorePassword: changeit
          # Map certificate subject DN to user attribute "deviceId"
          principalAttribute: cn
      - authenticator: auth-cookie
        requirement: REQUIRED
        priority: 20
  1. Upload the CA – Convert device-trust-ca.pem into a PKCS‑12 keystore (keytool -importcert).
  2. Create the flow via the Admin Console or import the YAML above.
  3. Set the flow as default for the desired client (application) under Authentication → Flows.

Now, any request to /auth/realms/myrealm/protocol/openid-connect/auth will trigger a TLS handshake that expects a device‑trust certificate.

For More information about Keycloak clientAuth review the x509 documentation.

Authentik

Authentik’s Certificate provider can be combined with the Device stage.

# authentik.yaml – DeviceTrustProvider definition
providers:
  - name: device-trust-mtls
    kind: cert
    config:
      ca_file: /etc/authentik/certs/device-trust-ca.pem
      allowed_usages:
        - clientAuth
      map_subject_to_user_attribute: "device_id"
stages:
  - name: DeviceTrustStage
    kind: authentication
    flow: default-authentication-flow
    providers:
      - device-trust-mtls
  1. Place device-trust-ca.pem under /etc/authentik/certs.
  2. Restart Authentik; the stage will now reject any TLS handshake lacking a valid client cert.

For more information about configuring Authentik authentication review the stag flow documentation.

Okta

Okta’s Certificate Authentication is configured via API Access Management.

{
  "type": "CERTIFICATE",
  "name": "DeviceTrustPolicy",
  "settings": {
    "trustedCertificates": [
      { "x5c": ["MIID..."] }   // Base64‑encoded device‑trust CA cert
    ],
    "subjectMatchPattern": "CN=*.device.acme.com"
  }
}

Steps:

  1. In the Okta Admin Console, navigate to Security → Authenticators → Certificate and add a new authenticator with the above JSON.
  2. Attach this authenticator to an Authentication Policy that protects your sensitive app (e.g., “Sensitive‑App‑Policy”).

Okta will now enforce mTLS for any request hitting the OIDC /authorize endpoint of that app.

For more information on configuring certificate-based authentication for Okta review the official docs.

OneLogin

OneLogin supports X.509 certificate authentication through its MFA configuration.

<!-- one-login-mfa-config.xml -->
<CertificateAuthenticator>
    <TrustedCA>$CA_Chain_URL</TrustedCA>
    <SubjectRegex>CN=([a-z0-9\-]+)</SubjectRegex>
    <MapToUserAttribute>device_id</MapToUserAttribute>
</CertificateAuthenticator>

Upload the XML via Settings → Security → Multifactor Authentication → Certificate. Then enable this factor for the Security Policy that guards your high‑value applications.

For more information about how Onelogin handles third-party certificate authentication to validate a trusted device, review the official documentation.

Auth0

Auth0 uses Custom Database Connections with a pre‑login hook to verify client certificates.

// auth0-pre-login.js – Deploy as an Action (Pre‑Login)
exports.onExecutePostLogin = async (event, api) => {
  const certHeader = event.request.headers['x-client-cert'];
  if (!certHeader) {
    return api.access.deny('client_certificate_missing');
  }

  // Decode PEM and verify against trusted CA
  const forge = require('node-forge');
  const pki = forge.pki;
  const caPem = `-----BEGIN CERTIFICATE-----
MIID...
-----END CERTIFICATE-----`;
  const caCert = pki.certificateFromPem(caPem);
  const clientCert = pki.certificateFromPem(certHeader);

  // Basic chain validation
  const verified = pki.verifyCertificateChain(pki.createCaStore([caCert]), [clientCert]);
  if (!verified) {
    return api.access.deny('invalid_device_certificate');
  }

  // Optional: map CN to user metadata
  const deviceId = clientCert.subject.getField('CN').value;
  event.user.app_metadata = { ...event.user.app_metadata, device_id: deviceId };
};

Deploy this Action and enable TLS termination with client‑certificate forwarding on your reverse proxy (e.g., Nginx proxy_set_header X-Client-Cert $ssl_client_cert;). Auth0 will reject any login that lacks a valid device‑trust certificate.

For more information about how to use Auth0’s mTLS authentication flow to validate device trust certificates, review their official mTLS docs.

Note: For services that may not directly support your Identity provider or may not be exposed externally, you can utilize a simple nginx proxy to validate a device trust certificate before allowing users to login.

Best Practices for Production Deployments

AreaRecommendation
Certificate LifetimeUse short lifetimes (30 days) and automate renewal via a provisioner like ACME or SCEP
Key ProtectionStore private keys in credential manager, KMS, Key Vault, or secrets store. Never write them to disk unencrypted.
RevocationPublish CRLs or use OCSP stapling; IdPs should query the revocation endpoint on each login.
Logging & AuditingInclude tls.client.subject_dn and certificate fingerprint in SIEM logs. Enable audit‑log retention for at least 90 days.
Fail‑Open vs Fail‑CloseDefault to fail‑close: if the client cert cannot be validated, deny access.
Device InventoryKeep a synchronized inventory service (e.g., CMDB) that tracks active device fingerprints; automate de‑provisioning when devices leave the fleet.

Testing & Validation

  1. OpenSSL verification – From a client machine with the device cert:
   openssl s_client -connect idp.acme.local:443 \
     -cert device.crt -key device.key -CAfile device-trust-ca.pem

You should see Verify return code: 0 (ok) and the TLS handshake succeed.

  1. Token request – Use curl with the client cert:
   curl -k https://idp.acme.local/auth/realms/myrealm/protocol/openid-connect/token \
     -E device.crt --key device.key \
     -d "grant_type=client_credentials&client_id=myapp"

If the certificate is invalid or missing, the response will be 401 Unauthorized.

  1. Audit log check – In Keycloak’s admin console go to Events → Config and enable Login events. Verify that each successful login entry contains client_certificate_fingerprint.

Common Pitfalls & How to Avoid Them

SymptomRoot CauseFix
Handshake fails with “unknown ca”Device‑trust CA not added to truststore or wrong file format (PEM vs PKCS12)Convert to PKCS12 and import correctly.
Token issued despite missing certReverse proxy terminates TLS before forwarding to IdP, losing client‑cert headerEnable proxy_ssl_verify and forward X-Client-Cert or use end‑to‑end mTLS (no TLS termination at proxy).
Frequent revocation failuresCRL/OCSP endpoint unreachable from IdP and user deviceHost a OCSP responder or cache CRLs; ensure network connectivity.
Certificate renewal breaks sessionsApplications cache the old cert and do not reload new filesUse cronjob or sidecar containers that check the certificate and automate renewal

Conclusion

By integrating device‑bound certificates with your identity provider, you transform authentication from a just a 2 factor authentication model and combine it into a robust multi factor with verifiable trust paradigm. The configurations shown for Keycloak, Authentik, Okta, OneLogin, and Auth0 prove that enabling Certificate ClientAuth is straightforward, often just a few lines of YAML or JSON plus the import of a trusted CA.

When you adopt this pattern:

  • Security posture improves dramatically – compromised passwords no longer grant access.
  • Compliance becomes easier – many regulations (e.g., NIST 800‑63B, PCI DSS) encourage strong mutual authentication.
  • Operational overhead stays low – automated short‑lived cert issuance and rotation eliminate manual key management.

If you’re looking to future‑proof your applications against credential‑theft attacks, make the shift today: empower your identity provider with device trust and let mTLS do the heavy lifting.

Creating a Simple Device Trust Gateway Using Device Certificates

Posted on by

In the evolving world of cybersecurity, identity-based access alone is no longer sufficient. The modern Zero Trust model mandates that access decisions consider not just the user but also the device. A user might be who they claim to be, but what if they’re logging in from a compromised machine or a jailbroken phone?

That’s where a device trust gateway comes in—a simple, scalable method to enforce access controls based on both user identity and device posture. Surprisingly, this doesn’t require complex architecture. In fact, with just a few lines of configuration in common web proxies like NGINX, you can create a robust checkpoint to validate device certificates before allowing application access.

In this post, we’ll explore how to build a simple yet effective device trust gateway using web proxy configurations, why it matters, and how it enhances your Zero Trust posture.

What Is a Device Trust Gateway?

A device trust gateway is a proxy layer that sits in front of applications and checks whether the connecting device presents a valid, cryptographically signed certificate. This certificate—typically issued by a corporate Certificate Authority (CA)—acts as a machine identity, verifying that the device is registered, managed, and secure.

By validating the certificate before allowing a user session to proceed, organizations can enforce stronger controls such as:

  • Allowing access only from corporate-managed endpoints
  • Blocking jailbroken or unmanaged devices
  • Issuing short-lived access tokens only after successful posture checks

This approach complements MFA and SSO. Even if credentials are phished or stolen, an attacker can’t authenticate without access to a trusted device.

How It Works

  1. Device Enrollment: Devices are provisioned with client certificates from an internal CA.
  2. Proxy Enforcement: A reverse proxy (like NGINX or Apache) is configured to validate client certificates.
  3. Access Control: Only clients presenting valid certificates can reach upstream applications or IdPs (Identity Providers).
  4. Logging and Auditing: All device certificate checks are logged for forensics and compliance.

Why This Matters

In many organizations, devices are a weak link. Remote work, BYOD, and cloud-native services increase the risk of unmanaged or misconfigured endpoints.

By enabling device trust enforcement at the proxy level, you:

  • Avoid re-architecting your identity system
  • Add a powerful security control with minimal code changes
  • Stop attackers who steal credentials but don’t have trusted hardware

The best part? You likely already have the infrastructure to make it happen.

NGINX: Enforcing Client Certificate Validation

NGINX makes it straightforward to enable cleintAuth and client certificate validation.

server {
    listen 443 ssl;
    server_name secure.mycompany.com;

    ssl_certificate /etc/nginx/certs/server.crt;
    ssl_certificate_key /etc/nginx/certs/server.key;
    ssl_client_certificate /etc/nginx/certs/ca.crt; # Your CA Chain
    ssl_verify_client on;# <‑ key line

    location / {
        proxy_pass http://internal-app;
        proxy_set_header X-Client-Cert $ssl_client_cert;
        proxy_set_header X-Client-DN  $ssl_client_s_dn;
    }
}

In this snippet:

  • ssl_client_certificate points to the CA that signed your device certificates
  • ssl_verify_client on enforces certificate presentation
  • The subject DN is passed upstream for audit or additional policy checks

If a device doesn’t present a valid certificate, NGINX terminates the connection.

Note: The client cert can be passed to through the proxy to other backend services using the nginx variable $ssl_client_cert which contains the entire URL encoded client certificate in PEM format.

Optional: Enforce Device Policies

If you want to go beyond “certificate is valid” and enforce per‑device rules, leverage OpenSSL extensions or X.509 Subject Alternative Names (SAN). For example:

# Add a custom extension in the CSR:
openssl req -new -key device-01.key.pem \
    -subj "/CN=device-01.acme.com/O=Acme Devices/C=US" \
    -addext "subjectAltName = @alt_names" \
    -config <(cat /etc/ssl/openssl.cnf <(printf "[alt_names]\nrole=admin\n"))

Then in nginx you can inspect $ssl_client_s_dn or $ssl_client_cert and use map directives to block or allow based on the role.

Apache HTTPD: A Similar ClientAuth Approach

Apache’s mod_ssl module can perform the same function.

<VirtualHost *:443>
    ServerName secure.mycompany.com

    SSLEngine on
    SSLCertificateFile /etc/httpd/certs/server.crt
    SSLCertificateKeyFile /etc/httpd/certs/server.key
    SSLCACertificateFile /etc/httpd/certs/ca.crt
    SSLVerifyClient require

    <Location />
        ProxyPass http://internal-app/
        ProxyPassReverse http://internal-app/
    </Location>
</VirtualHost>

Apache enforces client cert verification with SSLVerifyClient require, ensuring only trusted devices make it through.

Monitoring & Logging

Nginx logs each handshake, including whether client cert verification succeeded. Add a custom log format:

log_format devicelog '$remote_addr - $remote_user [$time_local] '
                     '"$request" $status $body_bytes_sent '
                     'client_cert="$ssl_client_verify" '
                     'cn="$ssl_client_s_dn"';
access_log /var/log/nginx/device_access.log devicelog;

Now you can audit which devices accessed the gateway, detect expired certs, or spot anomalies.

Testing the Gateway

Valid Device – On a client machine, install device-01.cert.pem and device-01.key.pem. Or use curl:

curl -k --cert device-01.cert.pem \
     --key  device-01.key.pem \
     https://proxy.acme.com/

You should get the backend response.

  • Invalid Device – Remove or rename the cert/key and try again; you’ll receive a 403.
  • Expired Certificate – Tamper with device-01.cert.pem’s validity period or use openssl x509 -in device-01.cert.pem -noout -dates to verify expiration. The gateway will reject it automatically.

Device Trust Gateway Flow

Device Trust Gateway Authentication work flow

Steps:

  1. Device connects to proxy and presents client certificate
  2. Proxy checks cert against trusted CA
  3. If valid, forwards request to application
  4. If invalid, terminates connection

Implementation Tips

  • Use short-lived device certificates (e.g., 24 hours)
  • Automate provisioning with MDM scripts and/or SCEP
  • Use headers like X-Client-Cert to enrich identity at the application layer
  • Monitor failed certificate handshakes as potential threats

Conclusion

  • Fast Implementation – Adding just two lines (ssl_verify_client on + ssl_client_certificate) turns any TLS‑enabled proxy into a device trust gateway.
  • Zero‑Trust Foundation – Every device must prove its identity before accessing sensitive resources.
  • Scalable – The same CAs can issue thousands of certificates; you can automate provisioning via scripts or PKI tools like step-ca.

Final Thoughts

You don’t need to overhaul your infrastructure to implement device trust. Adding a few lines of proxy configuration can provide a powerful gateway that ensures only secure, trusted devices can access your applications.

In a Zero Trust world, identity is not enough. Trust must be earned—and verified—by the devices themselves.

Device Attestation: A Critical Pillar of Device Trust

Posted on by

In an increasingly cloud-native, remote-access-driven world, enterprises are embracing zero trust architecture (ZTA) to protect digital assets. While identity-based authentication has matured with strong multi-factor authentication (MFA) and identity federation, one crucial element of Zero Trust often remains under-addressed: device attestation.

As a security engineer, I see firsthand the rising importance of securing the endpoints that interact with sensitive applications and data. Device attestation, as a foundational component of device trust, ensures that devices are not only authenticated but also verifiably in a known good state before accessing protected resources.

In this post, we explore what device attestation is, why it matters in modern architectures, how emerging standards like the IETF ACME device-attest-01 draft support scalable adoption, and how you can begin implementing it in real environments.

What is Device Attestation?

Device attestation is the process of verifying the identity and integrity of a device using cryptographic techniques. The attestation typically comes from a hardware-based root of trust—like a Trusted Platform Module (TPM), Secure Enclave, or other secure hardware—which signs data about the system state (e.g., firmware versions, serial numbers, secure boot status, software measurements).

A verified attestation ensures that:

  • The device is the one it claims to be
  • It hasn’t been tampered with or rooted
  • It is running known, validated software

This concept is central to building certificate-based device trust, where devices are issued X.509 certificates only if they pass attestation checks. Without this safeguard, attackers could spoof or clone devices to bypass access controls.

Why Device Attestation is Vital to Zero Trust

Zero trust isn’t just about verifying users—it’s about verifying everything, including the devices users use to access systems. When organizations fail to verify the integrity of endpoints, they invite risk into their ecosystems, even if identities are protected with MFA.

Without attestation:

  • A compromised device could still access sensitive data
  • Rooted phones or unmanaged laptops could bypass posture checks
  • Malware could exploit weaknesses at the hardware or boot level

By incorporating device attestation:

  • Enterprises can make dynamic access decisions based on device state
  • Access can be denied to jailbroken or policy-violating endpoints
  • Certificates for authentication can be gated behind strong attestation proof

ACME Device Attestation: IETF’s Draft Standard

The IETF ACME device-attest-01 draft proposes a way to bind device attestation to the Automatic Certificate Management Environment (ACME) protocol, which is widely used to automate certificate issuance (e.g., Let’s Encrypt).

This draft defines:

  • New challenge types to convey device attestation evidence
  • Integration with TPM and other hardware-based identity sources
  • A method to bind device identifiers to issued certificates

This standard enables:

  • Scalable issuance of certificates to attested devices
  • Secure bootstrapping of IoT, cloud, and edge workloads
  • Easy integration into existing ACME-compatible certificate authorities

This is an important step forward in codifying device trust into modern PKI workflows, making device attestation more accessible across a range of enterprise and industrial use cases.

Real-World Example: Attesting Kubernetes Worker Nodes

In cloud-native environments like Kubernetes, securing nodes is paramount. Let’s say you operate a cluster with autoscaling worker nodes. Traditionally, you might authenticate these nodes with IAM roles or bootstrap tokens.

But what if someone were able to launch a rogue instance with the same token?

Using device attestation, you can:

  • Provision a hardware-backed key in the worker node’s TPM or vTPM
  • Generate a cryptographic attestation of the node’s state (e.g., kernel version, secure boot)
  • Submit that evidence to a certificate authority via ACME
  • If the attestation passes, issue a short-lived certificate

Now, kubelet instances must present a valid certificate bound to an attested node identity to join the cluster.

Summary Diagram: Device Attestation Workflow

  1. Device boots securely and measures state (BIOS, OS, etc.)
  2. TPM signs quote including measurements and nonce
  3. Client sends quote to attestation service / ACME server
  4. Server verifies TPM signature, nonce, and measurement policy
  5. Certificate issued only if attestation passes

Getting Started with Device Attestation

If you’re ready to implement or pilot device attestation:

  1. Enable (v)TPMs or secure elements in your fleet (laptops, cloud VMs, IoT)
  2. Use attestation frameworks like Keylime, Azure Device Attestation, or Apple Managed Device Attestation
  3. Integrate ACME workflows using Letsencrypt Boulder or Smallstep step-ca with device-attest extensions
  4. Map attestation results to access policies in systems like Okta, Istio, or AWS IAM

Final Thoughts

Device attestation is no longer optional—it is becoming a necessity in a world where devices are the weakest link in many attack chains. With standards like IETF’s ACME device-attest-01 and open-source tooling, organizations now have a clear path to implement scalable, cryptographically verifiable device trust.

Whether you’re managing laptops, cloud instances, IoT fleets, or container nodes, attestation can ensure that only known, safe, and secure devices touch your critical infrastructure.