Mutual TLS

Mutual TLS (mTLS) authentication ↗ ensures that traffic is both secure and trusted in both directions between a client and server. It allows requests that do not log in with an identity provider (like IoT devices) to demonstrate that they can reach a given resource. Client certificate authentication is also a second layer of security for team members who both log in with an identity provider (IdP) and present a valid client certificate.

With a root certificate authority (CA) in place, Access only allows requests from devices with a corresponding client certificate. When a request reaches the application, Access responds with a request for the client to present a certificate. If the device fails to present the certificate, the request is not allowed to proceed. If the client does have a certificate, Access completes a key exchange to verify.

Important

The mTLS certificate is used only to verify the client certificate. It does not control the SSL certificate presented during the server hello ↗.

Enforce mTLS authentication

Prerequisites

• An Access application for the hostname that you would like to secure with mTLS.
• A CA that issues client certificates for your devices.

  • The CA certificate can be from a publicly trusted CA or self-signed.

  • In the certificate Basic Constraints, the attribute CA must be set to TRUE.

  • The certificate must use one of the signature algorithms listed below:

    Allowed signature algorithms

    x509.SHA1WithRSA

    x509.SHA256WithRSA

    x509.SHA384WithRSA

    x509.SHA512WithRSA

    x509.ECDSAWithSHA1

    x509.ECDSAWithSHA256

    x509.ECDSAWithSHA384

    x509.ECDSAWithSHA512

Add mTLS to your Access application

1. In Cloudflare One ↗, go to Access controls > Service credentials > Mutual TLS.

2. Select Add mTLS Certificate.

3. Enter any name for the root CA.

4. In Certificate content, paste the contents of your root CA.

   If the client certificate is directly signed by the root CA, you only need to upload the root. If the client certificate is signed by an intermediate certificate, you must upload the entire CA chain (intermediate and root). For example:

   -----BEGIN CERTIFICATE-----
   <intermediate.pem>
   -----END CERTIFICATE-----
   -----BEGIN CERTIFICATE-----
   <rootCA.pem>
   -----END CERTIFICATE-----

   Do not include any SSL/TLS server certificates; Access only uses the CA chain to verify the connection between the user's device and Cloudflare.

5. In Associated hostnames, enter the fully-qualified domain names (FQDN) that will use this certificate.

   These FQDNs will be the hostnames used for the resources being protected in the Access policy. You must associate the Root CA with the FQDN that the application being protected uses.

6. Save the policy.

7. Go to Access controls > Policies.

8. Create an Access policy using one of the following selectors:

   • Valid Certificate: Any client certificate that can authenticate with the Root CA will be allowed to proceed.
   • Common Name: Only client certificates with a specific common name will be allowed to proceed.

9. If this is for a client who does not need to log in through an IdP, set the policy Action to Service Auth.

   Example mTLS policy

   Action	Rule type	Selector	Value
   Service Auth	Include	Common Name	John Doe

10. Save the policy, then go to Access controls > Applications.

11. Select the application you would like to enforce mTLS on and select Configure. The application must be included in the Associated hostnames list from Step 5.

12. In the Policies tab, add your mTLS policy.

13. Save the application.

You can now authenticate to the application using a client certificate. For instructions on how to present a client certificate, refer to Test mTLS.

Test mTLS

Test using cURL

To test the application protected by an mTLS policy:

1. First, attempt to curl the site without a client certificate. This curl command example is for the site example.com that has an Access application and policy set for https://auth.example.com:

   curl -sv https://auth.example.com

   Without a client certificate in the request, a 403 forbidden response displays and the site cannot be accessed.

2. Now, add your client certificate and key to the request:

   curl -sv https://auth.example.com --cert example.pem --key key.pem

When the authentication process completes successfully, a CF_Authorization Set-Cookie header returns in the response.

Warning

Cloudflare Gateway cannot inspect traffic to mTLS-protected domains. If a device has the Cloudflare One Client turned on and passes HTTP requests through Gateway, access will be blocked unless you bypass HTTP inspection for the domain.

Test in a browser

To access an mTLS-protected application in a browser, the client certificate must be imported into your browser's certificate manager. Instructions vary depending on the browser. Your browser may use the operating system's root store or its own internal trust store.

The following example demonstrates how to add a client certificate to the macOS system keychain:

Important

The command adds the client certificate to the trusted store on your device. Only proceed if you are comfortable doing so and intend to keep these testing certificates safeguarded.

1. Navigate to the directory containing the client certificate and key.
   2. Open the client.pem file in Keychain Access. If prompted, enter your local password.
   3. In Keychain, choose the access option that suits your needs and select Add.
   4. In the list of certificates, locate the newly installed certificate. Keychain Access will mark this certificate as not trusted. Right-click the certificate and select Get Info.
   5. Select Trust. Under When using this certificate, select Always Trust.

Assuming your browser uses the macOS system store, you can now connect to the mTLS application through the browser.

Generate mTLS certificates

You can use open source private key infrastructure (PKI) tools to generate certificates to test the mTLS feature in Cloudflare Access.

OpenSSL

This section covers how to use OpenSSL ↗ to generate a root and intermediate certificate, and then issue client certificates that can authenticate against the CA chain.

Generate the root CA

1. Generate the root CA private key:

    openssl genrsa -aes256 -out rootCA.key 4096

   When prompted, enter a password to use with rootCA.key.

2. Create a self-signed root certificate called rootCA.pem:

   openssl req -x509 -new -nodes -key rootCA.key -sha256 -days 3650 -out rootCA.pem

   You will be prompted to enter your private key password and fill in some optional fields. For testing purposes, you can leave the optional fields blank.

Generate an intermediate certificate

1. Generate the intermediate CA private key:

    openssl genrsa -aes256 -out intermediate.key 4096

   When prompted, enter a password to use with intermediate.key.

2. Create a certificate signing request (CSR) for the intermediate certificate:

   openssl req -new -sha256 -key intermediate.key -out intermediate.csr

   You will be prompted to enter your private key password and fill in some optional fields. For testing purposes, you can leave the optional fields blank.

3. Create a CA Extension file called v3_intermediate_ca.ext. For example,

   subjectKeyIdentifier = hash
   authorityKeyIdentifier = keyid:always,issuer
   basicConstraints = critical, CA:true
   keyUsage = critical, cRLSign, keyCertSign

   Make sure that basicConstraints includes the CA:true property. This property allows the intermediate certificate to act as a CA and sign client certificates.

4. Sign the intermediate certificate with the root CA:

    openssl x509 -req -in intermediate.csr -CA rootCA.pem -CAkey rootCA.key -CAcreateserial -out intermediate.pem -days 1825 -sha256 -extfile v3_intermediate_ca.ext

Create a CA chain file

1. Combine the intermediate and root certificates into a single file:

   cat intermediate.pem rootCA.pem > ca-chain.pem

   The intermediate certificate should be at the top of the file, followed by its signing certificate.

2. Upload the contents of ca-chain.pem to Cloudflare Access. For instructions, refer to Add mTLS to your Access application.

Generate a client certificate

1. Generate a private key for the client:

    openssl genrsa -out client.key 2048

2. Create a CSR for the client certificate:

   openssl req -new -key client.key -out client.csr

   You will be prompted to fill in some optional fields. For testing purposes, you can set Common Name to something like John Doe.

3. Sign the client certificate with the intermediate certificate:

    openssl x509 -req -in client.csr -CA intermediate.pem -CAkey intermediate.key -CAcreateserial -out client.pem -days 365 -sha256

4. Validate the client certificate against the certificate chain:

   openssl verify -CAfile ca-chain.pem client.pem

   client.pem: OK

You can now use the client certificate (client.pem) and its key (client.key) to test mTLS.

Cloudflare PKI

This guide uses Cloudflare's PKI toolkit ↗ to generate a root CA and client certificates from JSON files.

1. Install dependencies

The process requires two packages from Cloudflare's PKI toolkit:

• cf-ssl
• cfssljson

You can install these packages from the Cloudflare SSL GitHub repository ↗. You will need a working installation of Go, version 1.12 or later. Alternatively, you can download the packages ↗ directly. Use the instructions under Installation to install the toolkit, and ensure that you install all of the utility programs in the toolkit.

2. Generate the root CA

1. Create a new directory to store the root CA.

2. Within that directory, create two new files:

   • CSR. Create a file named ca-csr.json and add the following JSON blob, then save the file.

     {
       "CN": "Access Testing CA",
       "key": {
         "algo": "rsa",
         "size": 4096
       },
       "names": [
         {
           "C": "US",
           "L": "Austin",
           "O": "Access Testing",
           "OU": "TX",
           "ST": "Texas"
         }
       ]
     }

   • config. Create a file named ca-config.json and add the following JSON blob, then save the file.

     {
       "signing": {
         "default": {
           "expiry": "8760h"
         },
         "profiles": {
           "server": {
             "usages": ["signing", "key encipherment", "server auth"],
             "expiry": "8760h"
           },
           "client": {
             "usages": ["signing", "key encipherment", "client auth"],
             "expiry": "8760h"
           }
         }
       }
     }

3. Now, run the following command to generate the root CA with those files.

   cfssl gencert -initca ca-csr.json | cfssljson -bare ca

4. The command will output a root certificate (ca.pem) and its key (ca-key.pem).

   ls

   ca-config.json ca-csr.json ca-key.pem ca.csr  ca.pem

5. Upload the contents of ca.pem to Cloudflare Access. For instructions, refer to Add mTLS to your Access application.

3. Generate a client certificate

To generate a client certificate that will authenticate against the uploaded root CA:

1. Create a file named client-csr.json and add the following JSON blob:

   {
     "CN": "James Royal",
     "hosts": [""],
     "key": {
       "algo": "rsa",
       "size": 4096
     },
     "names": [
       {
         "C": "US",
         "L": "Austin",
         "O": "Access",
         "OU": "Access Admins",
         "ST": "Texas"
       }
     ]
   }

2. Now, use the following command to generate a client certificate with the Cloudflare PKI toolkit:

   cfssl gencert -ca=ca.pem -ca-key=ca-key.pem  -config=ca-config.json -profile=client client-csr.json | cfssljson -bare client

The command will output a client certificate file (client.pem) and its key (client-key.pem). You can now use these files to test mTLS.

Create a certificate revocation list

You can use the Cloudflare PKI toolkit to generate a certificate revocation list (CRL), as well. This list will contain client certificates that are revoked.

1. Get the serial number from the client certificate generated earlier. Add that serial number, or any others you intend to revoke, in hex format in a text file. This example uses a file named serials.txt.

2. Create the CRL with the following command.

   cfssl gencrl serials.txt ../mtls-test/ca.pem ../mtls-test/ca-key.pem | base64 -D > ca.crl

You will need to add the CRL to your server or enforce the revocation in a Cloudflare Worker. An example Worker Script can be found on the Cloudflare GitHub repository ↗.

Add Client-Cert and Client-Cert-Chain headers (RFC 9440)

RFC 9440 ↗ defines the Client-Cert and Client-Cert-Chain HTTP header fields for passing client certificate information to origin servers. You can construct these headers using request header modification rules with the following Ruleset Engine fields:

• cf.tls_client_auth.cert_rfc9440 — The client leaf certificate encoded in RFC 9440 formatting (see reference).
• cf.tls_client_auth.cert_chain_rfc9440 — The certificate chain (excluding the leaf certificate) encoded in RFC 9440 formatting (see reference).

As indicated in field definitions, the fields may be set to either an empty string or a valid RFC 9440 encoding. Proper usage depends on a couple of factors discussed in the following sections.

Security considerations

Important

Before constructing Client-Cert or Client-Cert-Chain headers, you must address the following security concerns. Failing to do so can expose your origin server to forged or unverified certificate data.

The cert_rfc9440 and cert_chain_rfc9440 fields are populated regardless of the certificate validation result. This means a client can present an invalid, expired, or self-signed certificate, and the fields will still contain the encoded certificate data. Always check the following fields before trusting the values:

• cf.tls_client_auth.cert_verified — Returns true when the client certificate is valid.
• cf.tls_client_auth.cert_revoked — Returns true when the client certificate has been revoked.

A client can also include its own Client-Cert or Client-Cert-Chain headers on a request to inject arbitrary values. As described in the RFC 9440 security considerations ↗, you must unconditionally remove any existing Client-Cert and Client-Cert-Chain headers from incoming requests, regardless of certificate validity. This prevents a client from injecting forged certificate data that your origin would trust.

See Enable mTLS for details on how to configure mTLS and certificate validation.

Size limits

The encoded leaf certificate is limited to 10 KiB and the encoded chain is limited to 16 KiB. If the encoded value exceeds the limit, the corresponding field contains an empty string. Use the following fields to check for this condition:

• cf.tls_client_auth.cert_rfc9440_too_large — Returns true when the encoded certificate exceeds 10 KiB.
• cf.tls_client_auth.cert_chain_rfc9440_too_large — Returns true when the encoded chain exceeds 16 KiB.

Example Transform Rules

Here we provide an example on how to securely use these fields to construct trusted Client-Cert and Client-Cert-Chain headers to be forwarded to your origin. The origin can then rely on the presence of the headers to be certain the client presented a valid certificate. Note: the Client-Cert-Chain header may be omitted when the client did not present any intermediates (only a leaf certificate).

You need to create the following request header modification rules. The Remove rules must be placed before the Set dynamic rules so that client-injected headers are stripped on every request before the validated values are set.

Rule 1 — Remove Client-Cert header

This rule unconditionally removes any Client-Cert header sent by the client.

Text in Expression Editor:

true

Selected operation under Modify request header: Remove

Header name: Client-Cert

Rule 2 — Remove Client-Cert-Chain header

This rule unconditionally removes any Client-Cert-Chain header sent by the client.

Text in Expression Editor:

true

Selected operation under Modify request header: Remove

Header name: Client-Cert-Chain

Rule 3 — Set Client-Cert header

This rule sets the Client-Cert header only when the client presented a valid, non-revoked certificate that is within the size limit.

Text in Expression Editor:

cf.tls_client_auth.cert_verified
and not cf.tls_client_auth.cert_revoked
and not cf.tls_client_auth.cert_rfc9440_too_large

Selected operation under Modify request header: Set dynamic

Header name: Client-Cert

Value: cf.tls_client_auth.cert_rfc9440

Rule 4 — Set Client-Cert-Chain header

This rule sets the Client-Cert-Chain header only when the client presented a valid, non-revoked certificate and the chain is non-empty and within the size limit.

Text in Expression Editor:

cf.tls_client_auth.cert_verified
and not cf.tls_client_auth.cert_revoked
and cf.tls_client_auth.cert_chain_rfc9440 ne ""
and not cf.tls_client_auth.cert_chain_rfc9440_too_large

Selected operation under Modify request header: Set dynamic

Header name: Client-Cert-Chain

Value: cf.tls_client_auth.cert_chain_rfc9440

Cloudflare Workers

You can also construct RFC 9440 headers in a Cloudflare Worker using the tlsClientAuth properties on the incoming request.

The same security considerations mentioned above apply.

Forward a client certificate (legacy)

In addition to enforcing mTLS authentication for your host, you can also forward a client certificate to your origin server as an HTTP header. This setup is often helpful for server logging.

To avoid adding the certificate to every single request, the certificate is only forwarded on the first request of an mTLS connection.

Warning

This process is only available on accounts with Cloudflare Access.

Cloudflare API

The most common approach to forwarding a certificate is to use the Cloudflare API to update an mTLS certificate's hostname settings.

Required API token permissions

At least one of the following token permissions is required:

• Access: Mutual TLS Certificates Write

Update an mTLS certificate's hostname settings

curl "https://api.cloudflare.com/client/v4/zones/$ZONE_ID/access/certificates/settings" \
  --request PUT \
  --header "X-Auth-Email: $CLOUDFLARE_EMAIL" \
  --header "X-Auth-Key: $CLOUDFLARE_API_KEY" \
  --json '{
    "settings": [
        {
            "hostname": "<HOSTNAME>",
            "china_network": false,
            "client_certificate_forwarding": true
        }
    ]
  }'

Once client_certificate_forwarding is set to true, every request within an mTLS connection will now include the following headers:

• Cf-Client-Cert-Der-Base64
• Cf-Client-Cert-Sha256

Note

The Cf-Client-Cert-Der-Base64 and Cf-Client-Cert-Sha256 headers are a Cloudflare-proprietary mechanism. For a standardized approach, use RFC 9440 Client-Cert and Client-Cert-Chain headers.

Managed Transforms

You can also modify HTTP response headers using Managed Transforms to pass along TLS client auth headers.

Cloudflare Workers

Additionally, Workers can provide details around the client certificate.

JavaScript

const tlsHeaders = {
  "X-CERT-ISSUER-DN": request.cf.tlsClientAuth.certIssuerDN,
  "X-CERT-SUBJECT-DN": request.cf.tlsClientAuth.certSubjectDN,
  "X-CERT-ISSUER-DN-L": request.cf.tlsClientAuth.certIssuerDNLegacy,
  "X-CERT-SUBJECT-DN-L": request.cf.tlsClientAuth.certSubjectDNLegacy,
  "X-CERT-SERIAL": request.cf.tlsClientAuth.certSerial,
  "X-CERT-FINGER": request.cf.tlsClientAuth.certFingerprintSHA1,
  "X-CERT-VERIFY": request.cf.tlsClientAuth.certVerify,
  "X-CERT-NOTBE": request.cf.tlsClientAuth.certNotBefore,
  "X-CERT-NOTAF": request.cf.tlsClientAuth.certNotAfter,
};

Known limitations

mTLS does not currently work for:

• Cloudflare Pages site served on a custom domain
• Cloudflare R2 public bucket served on a custom domain

Notifications for mutual TLS certificates

Cloudflare will send the following notifications before your mutual TLS certificates expire:

Access mTLS Certificate Expiration Alert

Who is it for?

Access customers that use client certificates for mutual TLS authentication. This notification will be sent 30 and 14 days before the expiration of the certificate.

Other options / filters

None.

Included with

Purchase of Access and/or Cloudflare for SaaS.

What should you do if you receive one?

Upload a renewed certificate.