GitLab as an OAuth2 provider

This document covers using the OAuth2 protocol to allow other services to access GitLab resources on user's behalf.

If you want GitLab to be an OAuth authentication service provider to sign into other services, see the OAuth2 authentication service provider documentation. This functionality is based on the doorkeeper Ruby gem.

Supported OAuth2 flows

GitLab currently supports the following authorization flows:

• Authorization code with Proof Key for Code Exchange (PKCE): Most secure. Without PKCE, you'd have to include client secrets on mobile clients, and is recommended for both client and server apps.
• Authorization code: Secure and common flow. Recommended option for secure server-side apps.
• Implicit grant: Originally designed for user-agent only apps, such as single page web apps running on GitLab Pages). The IETF recommends against Implicit grant flow.
• Resource owner password credentials: To be used only for securely hosted, first-party services. GitLab recommends against use of this flow.

The draft specification for OAuth 2.1 specifically omits both the Implicit grant and Resource Owner Password Credentials flows. it will be deprecated in the next OAuth specification version.

Refer to the OAuth RFC to find out how all those flows work and pick the right one for your use case.

Both authorization code (with or without PKCE) and implicit grant flows require application to be registered first via the /profile/applications page in your user's account. During registration, by enabling proper scopes, you can limit the range of resources which the application can access. Upon creation, you obtain the application credentials: Application ID and Client Secret - keep them secure.

Prevent CSRF attacks

To protect redirect-based flows, the OAuth specification recommends the use of "One-time use CSRF tokens carried in the state parameter, which are securely bound to the user agent", with each request to the /oauth/authorize endpoint. This can prevent CSRF attacks.

Use HTTPS in production

For production, please use HTTPS for your redirect_uri. For development, GitLab allows insecure HTTP redirect URIs.

As OAuth2 bases its security entirely on the transport layer, you should not use unprotected URIs. For more information, see the OAuth 2.0 RFC and the OAuth 2.0 Threat Model RFC. These factors are particularly important when using the Implicit grant flow, where actual credentials are included in the redirect_uri.

In the following sections you can find detailed instructions on how to obtain authorization with each flow.

Authorization code with Proof Key for Code Exchange (PKCE)

The PKCE RFC includes a detailed flow description, from authorization request through access token. The following steps describe our implementation of the flow.

The Authorization code with PKCE flow, PKCE for short, makes it possible to securely perform the OAuth exchange of client credentials for access tokens on public clients.

Before starting the flow, generate the STATE, the CODE_VERIFIER and the CODE_CHALLENGE.

• The STATE a value that can't be predicted used by the client to maintain state between the request and callback. It should also be used as a CSRF token.
• The CODE_VERIFIER is a random string, between 43 and 128 characters in length, which use the characters A-Z, a-z, 0-9, -, ., _, and ~.
• The CODE_CHALLENGE is an URL-safe base64-encoded string of the SHA256 hash of the CODE_VERIFIER
  • In Ruby, you can set that up with Base64.urlsafe_encode64(Digest::SHA256.digest(CODE_VERIFIER)).

1. Request authorization code. To do that, you should redirect the user to the /oauth/authorize page with the following query parameters:

   https://gitlab.example.com/oauth/authorize?client_id=APP_ID&redirect_uri=REDIRECT_URI&response_type=code&state=STATE&scope=REQUESTED_SCOPES&code_challenge=CODE_CHALLENGE&code_challenge_method=S256

   This page asks the user to approve the request from the app to access their account based on the scopes specified in REQUESTED_SCOPES. The user is then redirected back to the specified REDIRECT_URI. The scope parameter is a space separated list of scopes associated with the user. For example,scope=read_user+profile requests the read_user and profile scopes. The redirect includes the authorization code, for example:

   https://example.com/oauth/redirect?code=1234567890&state=STATE

2. With the authorization code returned from the previous request (denoted as RETURNED_CODE in the following example), you can request an access_token, with any HTTP client. The following example uses Ruby's rest-client:

   parameters = 'client_id=APP_ID&client_secret=APP_SECRET&code=RETURNED_CODE&grant_type=authorization_code&redirect_uri=REDIRECT_URI&code_verifier=CODE_VERIFIER'
   RestClient.post 'https://gitlab.example.com/oauth/token', parameters

   Example response:

   {
    "access_token": "de6780bc506a0446309bd9362820ba8aed28aa506c71eedbe1c5c4f9dd350e54",
    "token_type": "bearer",
    "expires_in": 7200,
    "refresh_token": "8257e65c97202ed1726cf9571600918f3bffb2544b26e00a61df9897668c33a1",
    "created_at": 1607635748
   }

NOTE: The redirect_uri must match the redirect_uri used in the original authorization request.

You can now make requests to the API with the access token.

Authorization code flow

NOTE: Check the RFC spec for a detailed flow description.

The authorization code flow is essentially the same as authorization code flow with PKCE,

Before starting the flow, generate the STATE. It is a value that can't be predicted used by the client to maintain state between the request and callback. It should also be used as a CSRF token.

1. Request authorization code. To do that, you should redirect the user to the /oauth/authorize page with the following query parameters:

   https://gitlab.example.com/oauth/authorize?client_id=APP_ID&redirect_uri=REDIRECT_URI&response_type=code&state=STATE&scope=REQUESTED_SCOPES

   This page asks the user to approve the request from the app to access their account based on the scopes specified in REQUESTED_SCOPES. The user is then redirected back to the specified REDIRECT_URI. The scope parameter is a space separated list of scopes associated with the user. For example,scope=read_user+profile requests the read_user and profile scopes. The redirect includes the authorization code, for example:

   https://example.com/oauth/redirect?code=1234567890&state=STATE

2. With the authorization code returned from the previous request (shown as RETURNED_CODE in the following example), you can request an access_token, with any HTTP client. The following example uses Ruby's rest-client:

   parameters = 'client_id=APP_ID&client_secret=APP_SECRET&code=RETURNED_CODE&grant_type=authorization_code&redirect_uri=REDIRECT_URI'
   RestClient.post 'https://gitlab.example.com/oauth/token', parameters

   Example response:

   {
    "access_token": "de6780bc506a0446309bd9362820ba8aed28aa506c71eedbe1c5c4f9dd350e54",
    "token_type": "bearer",
    "expires_in": 7200,
    "refresh_token": "8257e65c97202ed1726cf9571600918f3bffb2544b26e00a61df9897668c33a1",
    "created_at": 1607635748
   }

NOTE: The redirect_uri must match the redirect_uri used in the original authorization request.

You can now make requests to the API with the access token returned.

Implicit grant flow

NOTE: For a detailed flow diagram, see the RFC specification.

WARNING: Implicit grant flow is inherently insecure and the IETF has removed it in OAuth 2.1. For this reason, support for it is deprecated. In GitLab 14.0, new applications can't be created using it. In GitLab 14.4, support for it is scheduled to be removed for existing applications.

We recommend that you use Authorization code with PKCE instead. If you choose to use Implicit flow, be sure to verify the application id (or client_id) associated with the access token before granting access to the data, as described in Retrieving the token information).

Unlike the authorization code flow, the client receives an access token immediately as a result of the authorization request. The flow does not use the client secret or the authorization code because all of the application code and storage is easily accessible on client browsers and mobile devices.

To request the access token, you should redirect the user to the /oauth/authorize endpoint using token response type:

https://gitlab.example.com/oauth/authorize?client_id=APP_ID&redirect_uri=REDIRECT_URI&response_type=token&state=YOUR_UNIQUE_STATE_HASH&scope=REQUESTED_SCOPES

This prompts the user to approve the applications access to their account based on the scopes specified in REQUESTED_SCOPES and then redirect back to the REDIRECT_URI you provided. The scope parameter is a space separated list of scopes you want to have access to (e.g. scope=read_user+profile would request read_user and profile scopes). The redirect includes a fragment with access_token as well as token details in GET parameters, for example:

https://example.com/oauth/redirect#access_token=ABCDExyz123&state=YOUR_UNIQUE_STATE_HASH&token_type=bearer&expires_in=3600

Resource owner password credentials flow

NOTE: Check the RFC spec for a detailed flow description.

NOTE: The Resource Owner Password Credentials is disabled for users with two-factor authentication turned on. These users can access the API using personal access tokens instead.

In this flow, a token is requested in exchange for the resource owner credentials (username and password).

The credentials should only be used when:

• There is a high degree of trust between the resource owner and the client. For example, the client is part of the device operating system or a highly privileged application.
• Other authorization grant types are not available (such as an authorization code).

WARNING: Never store the user's credentials and only use this grant type when your client is deployed to a trusted environment, in 99% of cases personal access tokens are a better choice.

Even though this grant type requires direct client access to the resource owner credentials, the resource owner credentials are used for a single request and are exchanged for an access token. This grant type can eliminate the need for the client to store the resource owner credentials for future use, by exchanging the credentials with a long-lived access token or refresh token.

To request an access token, you must make a POST request to /oauth/token with the following parameters:

{
  "grant_type"    : "password",
  "username"      : "user@example.com",
  "password"      : "secret"
}

Example cURL request:

echo 'grant_type=password&username=<your_username>&password=<your_password>' > auth.txt
curl --data "@auth.txt" --request POST "https://gitlab.example.com/oauth/token"

You can also use this grant flow with registered OAuth applications, by using HTTP Basic Authentication with the application's client_id and client_secret:

echo 'grant_type=password&username=<your_username>&password=<your_password>' > auth.txt
curl --data "@auth.txt" --user client_id:client_secret \
     --request POST "https://gitlab.example.com/oauth/token"

Then, you receive a response containing the access token:

{
  "access_token": "1f0af717251950dbd4d73154fdf0a474a5c5119adad999683f5b450c460726aa",
  "token_type": "bearer",
  "expires_in": 7200
}

By default, the scope of the access token is api, which provides complete read/write access.

For testing, you can use the oauth2 Ruby gem:

client = OAuth2::Client.new('the_client_id', 'the_client_secret', :site => "https://example.com")
access_token = client.password.get_token('user@example.com', 'secret')
puts access_token.token

Access GitLab API with access token

The access token allows you to make requests to the API on behalf of a user. You can pass the token either as GET parameter:

GET https://gitlab.example.com/api/v4/user?access_token=OAUTH-TOKEN

or you can put the token to the Authorization header:

curl --header "Authorization: Bearer OAUTH-TOKEN" "https://gitlab.example.com/api/v4/user"

Retrieving the token information

To verify the details of a token, use the token/info endpoint provided by the Doorkeeper gem. For more information, see /oauth/token/info.

You must supply the access token, either:

• As a parameter:

  GET https://gitlab.example.com/oauth/token/info?access_token=<OAUTH-TOKEN>

• In the Authorization header:

  curl --header "Authorization: Bearer <OAUTH-TOKEN>" "https://gitlab.example.com/oauth/token/info"

The following is an example response:

{
    "resource_owner_id": 1,
    "scope": ["api"],
    "expires_in": null,
    "application": {"uid": "1cb242f495280beb4291e64bee2a17f330902e499882fe8e1e2aa875519cab33"},
    "created_at": 1575890427
}

Deprecated fields

The fields scopes and expires_in_seconds are included in the response.

These are aliases for scope and expires_in respectively, and have been included to prevent breaking changes introduced in doorkeeper 5.0.2.

Don't rely on these fields as they are slated for removal in a later release.

OAuth2 tokens and GitLab registries

Standard OAuth2 tokens support different degrees of access to GitLab registries, as they:

• Do not allow users to authenticate to:
  • The GitLab Container registry.
  • Packages listed in the GitLab Package registry.
• Allow users to get, list, and delete registries through the Container registry API.