Mutual TLS Authentication

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Looking for the plugin's configuration parameters? You can find them in the Mutual TLS Authentication configuration reference doc.

Add mutual TLS authentication based on client-supplied or server-supplied certificate, and on the configured trusted CA list.

The mTLS plugin automatically maps certificates to consumers based on the common name field.

Usage

In order to authenticate the consumer, it must provide a valid certificate and complete mutual TLS handshake with Kong.

The plugin validates the certificate provided against the configured CA list based on the requested route or service.

If the certificate is not trusted or has expired, the response will be HTTP 401 TLS certificate failed verification.
If consumer did not present a valid certificate (this includes requests not sent to the HTTPS port), then the response will be HTTP 401 No required TLS certificate was sent. The exception is if the config.anonymous option was configured on the plugin, in which case the anonymous consumer is used and the request is allowed to proceed.

Client certificate request

Client certificates are requested in the ssl_certificate_by_lua phase where Kong does not have access to route and workspace information. Due to this information gap, Kong asks for the client certificate by default on every handshake if the mtls-auth plugin is configured on any route or service. In most cases, the failure of the client to present a client certificate is not going to affect subsequent proxying if that route or service does not have the mtls-auth plugin applied. The exception is where the client is a desktop browser, which prompts the end user to choose the client cert to send and lead to user experience issues rather than proxy behavior problems. To improve this situation, Kong builds an in-memory map of SNIs from the configured Kong routes that should present a client certificate. To limit client certificate requests during handshake while ensuring the client certificate is requested when needed, the in-memory map is dependent on the routes in Kong having the SNIs attribute set. When any routes do not have SNIs set, Kong must request the client certificate during every TLS handshake:

On every request irrespective of Workspace when the plugin is enabled in global Workspace scope.
On every request irrespective of Workspace when the plugin is applied at the service level and one or more of the routes do not have SNIs set.
On every request irrespective of Workspace when the plugin is applied at the route level and one or more routes do not have SNIs set.
On specific requests only when the plugin is applied at the route level and all routes have SNIs set.

SNIs must be set for all routes that mutual TLS authentication uses.

Adding certificate authorities

To use this plugin, you must add certificate authority (CA) certificates. These are stored in a separate ca_certificates store rather than the main certificates store because they do not require private keys. To add one, obtain a PEM-encoded copy of your CA certificate and POST it to /ca_certificates:

Kong Admin API

Konnect

curl -sX POST https://kong:8001/ca_certificates -F cert=@cert.pem
{
  "tags": null,
  "created_at": 1566597621,
  "cert": "-----BEGIN CERTIFICATE-----\FullPEMOmittedForBrevity==\n-----END CERTIFICATE-----\n",
  "id": "322dce96-d434-4e0d-9038-311b3520f0a3"
}

Go through the Gateway Manager:

In Konnect, click Gateway Manager.
Select the control plane you want to add the CA certificate to.
Click Certificates.
Select the CA Certificates tab.
Click + Add CA Certificate
Copy and paste your certificate information and click Save.

You can view your certificate listed in the Certificates tab.

To add a certificate via curl, you need:

Konnect ID
A generated access cookie

curl -X POST https:konnect.konghq.com/api/control_planes/[Konnect-ID]/ca_certificates -F cert=@testCACert.pem --cookie '[generated access cookie]'

The id value returned can now be used for mTLS plugin configurations or consumer mappings.

Sending the CA DNs during TLS handshake

By default, Kong Gateway won’t send the CA DN list during the TLS handshake. More specifically, the certificate_authorities in the CertificateRequest message is empty.

In some cases, the client may need this certificate_authorities to guide certificate selection. Setting config.send_ca_dn to true will add the CA certificates configured in the config.ca_certificate to the lists of the corresponding SNIs.

As mentioned in Client certificate request, due to the phase gap, Kong Gateway doesn’t know the route information in the ssl_certificate_by_lua phase, which is decided in the later access phase. Therefore Kong Gateway builds an in-memory map of SNIs. The CA DN list will eventually be associated with the SNIs. If multiple mtls-auth plugins with different config.ca_certificate are associated to the same SNI, the CA DNs will be merged. For example:

When the plugin is enabled in the global Workspace scope, the CA DNs are associated with a special SNI, “*”.
When the plugin is applied at the service level, the CA DNs are associated with every SNI of every route to this service. If a route has no SNIs set, then the CA DNs are associated with a special SNI, “*”.
When the plugin is applied at the route level, the CA DNs are associated with every SNI configured on this route. If the route has no SNIs set, then the CA DNs are associated with a special SNI, “*”.

During the mTLS handshake, if the client sends a SNI in the ClientHello message and the SNI is found in the in-memory map of SNIs, then the corresponding CA DN list is sent in CertificatRequest message.

If the client doesn’t send SNIs in the ClientHello message or the SNI sent is unknown to Kong Gateway, then the CA DN list associated with “*” is sent only when the client certificate is requested.

Create manual mappings between certificate and consumer object

Sometimes, you might not want to use automatic consumer lookup, or you have certificates that contain a field value not directly associated with consumer objects. In those situations, you may manually assign one or more subject names to the consumer object for identifying the correct consumer.

Note: Subject names refer to the certificate’s Subject Alternative Names (SAN) or Common Name (CN). CN is only used if the SAN extension does not exist.

Kong Admin API

Declarative (YAML)

Create a mapping:

curl -X POST http://kong:8001/consumers/{consumer}/mtls-auth \
    -d 'subject_name=test@example.com'

Where {consumer} is the id or username property of the consumer entity to associate the credentials to.

Once created, you’ll see a 201 success message:

HTTP/1.1 201 Created

{
    "consumer": { "id": "876bf719-8f18-4ce5-cc9f-5b5af6c36007" },
    "created_at": 1443371053000,
    "subject_name": "test@example.com"
}

To create a subject name mapping using declarative configuration, you must generate a UUID for each mtls_auth_credentials mapping. You can use any UUID generator to do this. Here are some common ones, depending on your OS:

After you have generated a UUID, add the following to your declarative configuration file:

consumers:
- custom_id: my-consumer
  username: {consumer}
  mtls_auth_credentials:
  - id: bda09448-3b10-4da7-a83b-2a8ba6021f0c
    subject_name: test@example.com

Parameters for manual mapping

Form Parameter	Default	Description
`id` required for declarative config	none	UUID of the consumer-mapping. Required if adding mapping using declarative configuration, otherwise generated automatically by Kong’s Admin API.
`subject_name` required	none	The Subject Alternative Name (SAN) or Common Name (CN) that should be mapped to `consumer` (in order of lookup).
`ca_certificate` optional	none	If using the Kong Admin API: UUID of the Certificate Authority (CA). If using declarative configuration: Full PEM-encoded CA certificate. The provided CA UUID or full certificate has to be verifiable by the issuing certificate authority for the mapping to succeed. This is to help distinguish multiple certificates with the same subject name that are issued under different CAs. If empty, the subject name matches certificates issued by any CA under the corresponding `config.ca_certificates`.

Matching behaviors

After a client certificate has been verified as valid, the consumer object is determined in the following order, unless skip_consumer_lookup is set to true:

Manual mappings with subject_name matching the certificate’s SAN or CN (in that order) and ca_certificate = <issuing authority of the client certificate>
Manual mappings with subject_name matching the certificate’s SAN or CN (in that order) and ca_certificate = NULL
If config.consumer_by is not null, consumer with username and/or id matching the certificate’s SAN or CN (in that order)
The config.anonymous consumer (if set)

Note: Matching stops as soon as the first successful match is found.

When a client has been authenticated, the plugin appends some headers to the request before proxying it to the upstream service, so that you can identify the consumer in your code:

X-Consumer-ID: The ID of the consumer in Kong.
X-Consumer-Custom-ID: The custom_id of the consumer (if set).
X-Consumer-Username: The username of the consumer (if set).
X-Credential-Identifier: The identifier of the credential (only if the consumer is not the anonymous consumer).
X-Anonymous-Consumer: Is set to true if authentication fails, and the anonymous consumer is set instead.

You can use this information on your side to implement additional logic. You can use the X-Consumer-ID value to query the Kong Admin API and retrieve more information about the consumer.

When skip_consumer_lookup is set to true, consumer lookup is skipped and instead of appending aforementioned headers, the plugin appends the following two headers

X-Client-Cert-Dn, distinguished name of the client certificate
X-Client-Cert-San, SAN of the client certificate

Once skip_consumer_lookup is applied, any client with a valid certificate can access the Service/API. To restrict usage to only some of the authenticated users, also add the ACL plugin (not covered here) and create allowed or denied groups of users using the same certificate property being set in authenticated_group_by.

Troubleshooting

When authentication fails, the client does not have access to any details that explain the failure. The security reason for this omission is to prevent malicious reconnaissance. Instead, the details are recorded inside Kong’s error logs under the [mtls-auth] filter.

Next Mutual TLS Authentication Configuration