This guide covers the use of authentication options with the MongoDB C Driver. Ensure that the MongoDB server is also properly configured for authentication before making a connection. For more information, see the MongoDB security documentation.

The MongoDB C driver supports several authentication mechanisms through the use of MongoDB connection URIs.

By default, if a username and password are provided as part of the connection string (and an optional authentication database), they are used to connect via the default authentication mechanism of the server.

To select a specific authentication mechanism other than the default, see the list of supported mechanism below.

mongoc_client_t *client = mongoc_client_new ("mongodb://user:password@localhost/?authSource=mydb");

Currently supported values for the authMechanism connection string option are:

Basic Authentication (SCRAM-SHA-1)

The default authentication mechanism when talking to MongoDB 3.0 and later is SCRAM-SHA-1 (RFC 5802). Using this authentication mechanism means that the password is never actually sent over the wire when authenticating, but rather a computed proof that the client password is the same as the password the server knows.

mongoc_client_t *client = mongoc_client_new ("mongodb://user:password@localhost/?authMechanism=SCRAM-SHA-1&authSource=mydb");


SCRAM-SHA-1 authenticates against the admin database by default. If the user is created in another database, then specifying the authSource is required.

Legacy Authentication (MONGODB-CR)

The MONGODB-CR authMechanism is deprecated and will no longer function in MongoDB 4.0. Instead, specify no authMechanism and the driver will use an authentication mechanism compatible with your server.

GSSAPI (Kerberos) Authentication


Kerberos support requires compiling the driver against cyrus-sasl on UNIX-like environments. On Windows, configure the driver to build against the Windows Native SSPI.

GSSAPI (Kerberos) authentication is available in the Enterprise Edition of MongoDB. To authenticate using GSSAPI, the MongoDB C driver must be installed with SASL support.

On UNIX-like environments, run the kinit command before using the following authentication methods:

$ kinit mongodbuser@EXAMPLE.COM
mongodbuser@EXAMPLE.COM's Password:
$ klistCredentials cache: FILE:/tmp/krb5cc_1000
        Principal: mongodbuser@EXAMPLE.COM

  Issued                Expires               Principal
Feb  9 13:48:51 2013  Feb  9 23:48:51 2013  krbtgt/EXAMPLE.COM@EXAMPLE.COM

Now authenticate using the MongoDB URI. GSSAPI authenticates against the $external virtual database, so a database does not need to be specified in the URI. Note that the Kerberos principal must be URL-encoded:

mongoc_client_t *client;

client = mongoc_client_new ("mongodb://");


GSSAPI authenticates against the $external database, so specifying the authSource database is not required.

The driver supports these GSSAPI properties:

  • CANONICALIZE_HOST_NAME: This might be required with Cyrus-SASL when the hosts report different hostnames than what is used in the Kerberos database. The default is “false”.
  • SERVICE_NAME: Use a different service name than the default, “mongodb”.

Set properties in the URL:

mongoc_client_t *client;

client = mongoc_client_new ("mongodb://"

If you encounter errors such as Invalid net address, check if the application is behind a NAT (Network Address Translation) firewall. If so, create a ticket that uses forwardable and addressless Kerberos tickets. This can be done by passing -f -A to kinit.

$ kinit -f -A mongodbuser@EXAMPLE.COM

SASL Plain Authentication


The MongoDB C Driver must be compiled with SASL support in order to use SASL PLAIN authentication.

MongoDB Enterprise Edition supports the SASL PLAIN authentication mechanism, initially intended for delegating authentication to an LDAP server. Using the SASL PLAIN mechanism is very similar to the challenge response mechanism with usernames and passwords. This authentication mechanism uses the $external virtual database for LDAP support:


SASL PLAIN is a clear-text authentication mechanism. It is strongly recommended to connect to MongoDB using SSL with certificate validation when using the PLAIN mechanism.

mongoc_client_t *client;

client = mongoc_client_new ("mongodb://");

PLAIN authenticates against the $external database, so specifying the authSource database is not required.

X.509 Certificate Authentication


The MongoDB C Driver must be compiled with SSL support for X.509 authentication support. Once this is done, start a server with the following options:

$ mongod --sslMode requireSSL --sslPEMKeyFile server.pem --sslCAFile ca.pem

The MONGODB-X509 mechanism authenticates a username derived from the distinguished subject name of the X.509 certificate presented by the driver during SSL negotiation. This authentication method requires the use of SSL connections with certificate validation.

mongoc_client_t *client;
mongoc_ssl_opt_t ssl_opts = { 0 };

ssl_opts.pem_file = "mycert.pem";
ssl_opts.pem_pwd = "mycertpassword";
ssl_opts.ca_file = "myca.pem";
ssl_opts.ca_dir = "trust_dir";
ssl_opts.weak_cert_validation = false;

client = mongoc_client_new ("mongodb://x509_derived_username@localhost/?authMechanism=MONGODB-X509");
mongoc_client_set_ssl_opts (client, &ssl_opts);

MONGODB-X509 authenticates against the $external database, so specifying the authSource database is not required. For more information on the x509_derived_username, see the MongoDB server x.509 tutorial.


The MongoDB C Driver will attempt to determine the x509 derived username when none is provided, and as of MongoDB 3.4 providing the username is not required at all.