identity/oidc: adds client_secret_post token endpoint authentication method

changelog/16598.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+identity/oidc: Adds the `client_secret_post` token endpoint authentication method.
+```

vault/identity_store_oidc_provider.go

@@ -510,8 +510,8 @@ func oidcProviderPaths(i *IdentityStore) []*framework.Path {
 					Description: "The code verifier associated with the authorization code.",
 				},
 				// For confidential clients, the client_id and client_secret are provided to
-				// the token endpoint via the 'client_secret_basic' authentication method, which
-				// uses the HTTP Basic authentication scheme. See the OIDC spec for details at:
+				// the token endpoint via the 'client_secret_basic' or 'client_secret_post'
+				// authentication methods. See the OIDC spec for details at:
 				// https://openid.net/specs/openid-connect-core-1_0.html#ClientAuthentication
 
 				// For public clients, the client_id is required and a client_secret does
@@ -522,6 +522,10 @@ func oidcProviderPaths(i *IdentityStore) []*framework.Path {
 					Type:        framework.TypeString,
 					Description: "The ID of the requesting client.",
 				},
+				"client_secret": {
+					Type:        framework.TypeString,
+					Description: "The secret of the requesting client.",
+				},
 			},
 			Operations: map[logical.Operation]framework.OperationHandler{
 				logical.UpdateOperation: &framework.PathOperation{
@@ -1483,6 +1487,7 @@ func (i *IdentityStore) pathOIDCProviderDiscovery(ctx context.Context, req *logi
 			// PKCE is required for auth method "none"
 			"none",
 			"client_secret_basic",
+			"client_secret_post",
 		},
 	}
 
@@ -1856,13 +1861,15 @@ func (i *IdentityStore) pathOIDCToken(ctx context.Context, req *logical.Request,
 		return tokenResponse(nil, ErrTokenInvalidRequest, "provider not found")
 	}
 
-	// Get the client ID
+	// client_secret_basic - Check for client credentials in the Authorization header
 	clientID, clientSecret, okBasicAuth := basicAuth(req)
 	if !okBasicAuth {
+		// client_secret_post - Check for client credentials in the request body
 		clientID = d.Get("client_id").(string)
 		if clientID == "" {
 			return tokenResponse(nil, ErrTokenInvalidRequest, "client_id parameter is required")
 		}
+		clientSecret = d.Get("client_secret").(string)
 	}
 	client, err := i.clientByID(ctx, req.Storage, clientID)
 	if err != nil {
@@ -1873,8 +1880,7 @@ func (i *IdentityStore) pathOIDCToken(ctx context.Context, req *logical.Request,
 		return tokenResponse(nil, ErrTokenInvalidClient, "client failed to authenticate")
 	}
 
-	// Authenticate the client using the client_secret_basic authentication method if it's a
-	// confidential client. The authentication method uses the HTTP Basic authentication scheme.
+	// Authenticate the client if it's a confidential client type.
 	// Details at https://openid.net/specs/openid-connect-core-1_0.html#ClientAuthentication
 	if client.Type == confidential &&
 		subtle.ConstantTimeCompare([]byte(client.ClientSecret), []byte(clientSecret)) == 0 {

vault/identity_store_oidc_provider_test.go

@@ -433,6 +433,22 @@ func TestOIDC_Path_OIDC_Token(t *testing.T) {
 				tokenReq: testTokenReq(s, "", clientID, clientSecret),
 			},
 		},
+		{
+			name: "valid token request with client_secret_post client authentication method",
+			args: args{
+				clientReq:     testClientReq(s),
+				providerReq:   testProviderReq(s, clientID),
+				assignmentReq: testAssignmentReq(s, entityID, groupID),
+				authorizeReq:  testAuthorizeReq(s, clientID),
+				tokenReq: func() *logical.Request {
+					req := testTokenReq(s, "", clientID, clientSecret)
+					req.Headers = nil
+					req.Data["client_id"] = clientID
+					req.Data["client_secret"] = clientSecret
+					return req
+				}(),
+			},
+		},
 		{
 			name: "valid token request",
 			args: args{
@@ -3613,7 +3629,7 @@ func TestOIDC_Path_OpenIDProviderConfig(t *testing.T) {
 		TokenEndpoint:         basePath + "/token",
 		UserinfoEndpoint:      basePath + "/userinfo",
 		GrantTypes:            []string{"authorization_code"},
-		AuthMethods:           []string{"none", "client_secret_basic"},
+		AuthMethods:           []string{"none", "client_secret_basic", "client_secret_post"},
 		RequestParameter:      false,
 		RequestURIParameter:   false,
 	}
@@ -3668,7 +3684,7 @@ func TestOIDC_Path_OpenIDProviderConfig(t *testing.T) {
 		TokenEndpoint:         basePath + "/token",
 		UserinfoEndpoint:      basePath + "/userinfo",
 		GrantTypes:            []string{"authorization_code"},
-		AuthMethods:           []string{"none", "client_secret_basic"},
+		AuthMethods:           []string{"none", "client_secret_basic", "client_secret_post"},
 		RequestParameter:      false,
 		RequestURIParameter:   false,
 	}

website/content/api-docs/secret/identity/oidc-provider.mdx

@@ -288,7 +288,7 @@ This endpoint creates or updates a client.
   - `confidential`
     - Capable of maintaining the confidentiality of its credentials
     - Has a client secret
-    - Uses the `client_secret_basic` [client authentication method](https://openid.net/specs/openid-connect-core-1_0.html#ClientAuthentication)
+    - Uses the `client_secret_basic` or `client_secret_post` [client authentication method](https://openid.net/specs/openid-connect-core-1_0.html#ClientAuthentication)
     - May use Proof Key for Code Exchange ([PKCE](https://datatracker.ietf.org/doc/html/rfc7636))
       for the authorization code flow
   - `public`
@@ -602,6 +602,7 @@ $ curl \
   ],
   "token_endpoint_auth_methods_supported": [
     "client_secret_basic",
+    "client_secret_post",
     "none"
   ]}
 ```
@@ -737,19 +738,24 @@ for an OIDC provider.
   authorization request was sent. This must match the `redirect_uri` used when the
   original authorization code was generated.
 
-- `client_id` `(string: <required>)` - The ID of the requesting client. This parameter
-  is only required for `public` clients which do not have a client secret. `confidential`
-  clients should not use this parameter.
+- `client_id` `(string: <optional>)` - The ID of the requesting client. This parameter
+  is required for `public` clients which do not have a client secret or `confidential`
+  clients using the `client_secret_post` client authentication method.
+
+- `client_secret` `(string: <optional>)` - The secret of the requesting client. This
+  parameter is required for `confidential` clients using the `client_secret_post` client
+  authentication method.
 
 - `code_verifier` `(string: <optional>)` - The code verifier associated with the given
   `code`. Required for authorization codes that were granted using [PKCE](https://datatracker.ietf.org/doc/html/rfc7636).
   Required for `public` clients.
 
 ### Headers
 
-- `Authorization: Basic` `(string: <required>)` - An HTTP Basic authentication scheme header
+- `Authorization: Basic` `(string: <optional>)` - An HTTP Basic authentication scheme header
   including the `client_id` and `client_secret` as described in the [client_secret_basic](https://openid.net/specs/openid-connect-core-1_0.html#ClientAuthentication)
-  authentication method. This header is only required for `confidential` clients.
+  authentication method. This header is only required for `confidential` clients using
+  the `client_secret_basic` client authentication method.
 
 ### Sample Request
 

website/content/docs/concepts/oidc-provider.mdx

@@ -138,7 +138,7 @@ Confidential clients may use Proof Key for Code Exchange ([PKCE](https://datatra
 during the authorization code flow.
 
 Confidential clients must authenticate to the token endpoint using the
-`client_secret_basic` [client authentication method](https://openid.net/specs/openid-connect-core-1_0.html#ClientAuthentication).
+`client_secret_basic` or `client_secret_post` [client authentication method](https://openid.net/specs/openid-connect-core-1_0.html#ClientAuthentication).
 
 ##### Public
 

website/content/docs/secrets/identity/oidc-provider.mdx

@@ -125,7 +125,8 @@ Any Vault auth method may be used within the OIDC flow. For simplicity, enable t
      ],
      "token_endpoint_auth_methods_supported": [
        "none",
-       "client_secret_basic"
+       "client_secret_basic",
+       "client_secret_post"
      ]
    }
    ```