Downscope a Token

Downscoping is a way to exchange an existing Access Token for a new one that is more restricted.

Reasons to downscope

An application might need to share the Access Token with an environment that it does not fully control. A common example of this would be when using Box UI Elements in a web browser.

When an application needs to pass an Access Token to the browser there is a potential security risk that needs to be resolved. In order to limit this risk the Access Token can be exchanged for a new token with much stricter permissions.

High-level overview

A downscoped token is a token that has fewer permissions (scopes) than the original token, as well as the optional additional restriction to only allow access to a specific file.

The new token takes the permissions of the original token and restricts them to the tokens passed in, as well as the resource provided.

Downscoping in practice

To downscope a token, pass the POST /oauth2/token endpoint an existing Access Token, a list of scopes, as well as an optional file URL to restrict the token to.

curl -X POST https://api.box.com/oauth2/token \
     -H 'Content-Type: application/x-www-form-urlencoded' \
     -d 'subject_token=[ACCESS_TOKEN]' \
     -d 'subject_token_type=urn:ietf:params:oauth:token-type:access_token' \
     -d 'scope=item_upload item_preview base_explorer' \
     -d 'resource=https://api.box.com/2.0/folders/123456' \
     -d 'grant_type=urn:ietf:params:oauth:grant-type:token-exchange'

var exchanger = new TokenExchange(client.Auth.Session.AccessToken, "item_preview");
exchanger.SetResource("https://api.box.com/2.0/files/123456789");
string downscopedToken = exchanger.Exchange();

BoxAPIConnection api = new BoxAPIConnection("YOUR-ACCESS-TOKEN");

String resource = "https://api.box.com/2.0/files/RESOURCE-ID";
List<String> scopes = new ArrayList<String>();
scopes.add("item_preview");
scopes.add("item_content_upload");

ScopedToken token = api.getLowerScopedToken(scopes, resource);

target_file = client.file(file_id='FILE_ID_HERE')
token_info = client.downscope_token(['item_preview'], target_file)
print('Got downscoped access token: {0}'.format(token_info.access_token))

client.exchangeToken('item_preview', 'https://api.box.com/2.0/files/123456789')
	.then(tokenInfo => {
		// tokenInfo.accessToken contains the new downscoped access token
	});

Downscoped Access Token Object

A downscoped Access Token returned by the POST /oauth2/token endpoint contains extra information on the specific restrictions.

{
    "access_token": "1!DgsZ6V9kMWZu2StrxwQDF5BudQNen-xUmU2cfcVKArE....",
    "expires_in": 4175,
    "token_type": "bearer",
    "restricted_to": [
        {
            "scope": "item_preview",
            "object": {
                "type": "folder",
                "id": "1234567890",
                "sequence_id": "0",
                "etag": "0",
                "name": "Test"
            }
        }
    ],
    "issued_token_type": "urn:ietf:params:oauth:token-type:access_token"
}

Most importantly here is the list of restricted_to entries that will contain each combination of object and scope that the new token has the permissions for.