= OAuth 2.0 Bearer Tokens

[[oauth2resourceserver-bearertoken-resolver]]
== Bearer Token Resolution

By default, Resource Server looks for a bearer token in the `Authorization` header.
This, however, can be customized in a handful of ways.

=== Reading the Bearer Token from a Custom Header

For example, you may have a need to read the bearer token from a custom header.
To achieve this, you can expose a `DefaultBearerTokenResolver` as a bean, or wire an instance into the DSL, as you can see in the following example:

.Custom Bearer Token Header
====
.Java
[source,java,role="primary"]
----
@Bean
BearerTokenResolver bearerTokenResolver() {
    DefaultBearerTokenResolver bearerTokenResolver = new DefaultBearerTokenResolver();
    bearerTokenResolver.setBearerTokenHeaderName(HttpHeaders.PROXY_AUTHORIZATION);
    return bearerTokenResolver;
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun bearerTokenResolver(): BearerTokenResolver {
    val bearerTokenResolver = DefaultBearerTokenResolver()
    bearerTokenResolver.setBearerTokenHeaderName(HttpHeaders.PROXY_AUTHORIZATION)
    return bearerTokenResolver
}
----

.Xml
[source,xml,role="secondary"]
----
<http>
    <oauth2-resource-server bearer-token-resolver-ref="bearerTokenResolver"/>
</http>

<bean id="bearerTokenResolver"
        class="org.springframework.security.oauth2.server.resource.web.DefaultBearerTokenResolver">
    <property name="bearerTokenHeaderName" value="Proxy-Authorization"/>
</bean>
----
====

Or, in circumstances where a provider is using both a custom header and value, you can use `HeaderBearerTokenResolver` instead.

=== Reading the Bearer Token from a Form Parameter

Or, you may wish to read the token from a form parameter, which you can do by configuring the `DefaultBearerTokenResolver`, as you can see below:

.Form Parameter Bearer Token
====
.Java
[source,java,role="primary"]
----
DefaultBearerTokenResolver resolver = new DefaultBearerTokenResolver();
resolver.setAllowFormEncodedBodyParameter(true);
http
    .oauth2ResourceServer(oauth2 -> oauth2
        .bearerTokenResolver(resolver)
    );
----

.Kotlin
[source,kotlin,role="secondary"]
----
val resolver = DefaultBearerTokenResolver()
resolver.setAllowFormEncodedBodyParameter(true)
http {
    oauth2ResourceServer {
        bearerTokenResolver = resolver
    }
}
----

.Xml
[source,xml,role="secondary"]
----
<http>
    <oauth2-resource-server bearer-token-resolver-ref="bearerTokenResolver"/>
</http>

<bean id="bearerTokenResolver"
        class="org.springframework.security.oauth2.server.resource.web.HeaderBearerTokenResolver">
    <property name="allowFormEncodedBodyParameter" value="true"/>
</bean>
----
====

== Bearer Token Propagation

Now that you're resource server has validated the token, it might be handy to pass it to downstream services.
This is quite simple with `{security-api-url}org/springframework/security/oauth2/server/resource/web/reactive/function/client/ServletBearerExchangeFilterFunction.html[ServletBearerExchangeFilterFunction]`, which you can see in the following example:

====
.Java
[source,java,role="primary"]
----
@Bean
public WebClient rest() {
    return WebClient.builder()
            .filter(new ServletBearerExchangeFilterFunction())
            .build();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun rest(): WebClient {
    return WebClient.builder()
            .filter(ServletBearerExchangeFilterFunction())
            .build()
}
----
====

When the above `WebClient` is used to perform requests, Spring Security will look up the current `Authentication` and extract any `{security-api-url}org/springframework/security/oauth2/core/AbstractOAuth2Token.html[AbstractOAuth2Token]` credential.
Then, it will propagate that token in the `Authorization` header.

For example:

====
.Java
[source,java,role="primary"]
----
this.rest.get()
        .uri("https://other-service.example.com/endpoint")
        .retrieve()
        .bodyToMono(String.class)
        .block()
----

.Kotlin
[source,kotlin,role="secondary"]
----
this.rest.get()
        .uri("https://other-service.example.com/endpoint")
        .retrieve()
        .bodyToMono<String>()
        .block()
----
====

Will invoke the `https://other-service.example.com/endpoint`, adding the bearer token `Authorization` header for you.

In places where you need to override this behavior, it's a simple matter of supplying the header yourself, like so:

====
.Java
[source,java,role="primary"]
----
this.rest.get()
        .uri("https://other-service.example.com/endpoint")
        .headers(headers -> headers.setBearerAuth(overridingToken))
        .retrieve()
        .bodyToMono(String.class)
        .block()
----

.Kotlin
[source,kotlin,role="secondary"]
----
this.rest.get()
        .uri("https://other-service.example.com/endpoint")
        .headers{  headers -> headers.setBearerAuth(overridingToken)}
        .retrieve()
        .bodyToMono<String>()
        .block()
----
====

In this case, the filter will fall back and simply forward the request onto the rest of the web filter chain.

[NOTE]
Unlike the {security-api-url}org/springframework/security/oauth2/client/web/reactive/function/client/ServletOAuth2AuthorizedClientExchangeFilterFunction.html[OAuth 2.0 Client filter function], this filter function makes no attempt to renew the token, should it be expired.
To obtain this level of support, please use the OAuth 2.0 Client filter.

=== `RestTemplate` support

There is no `RestTemplate` equivalent for `ServletBearerExchangeFilterFunction` at the moment, but you can propagate the request's bearer token quite simply with your own interceptor:

====
.Java
[source,java,role="primary"]
----
@Bean
RestTemplate rest() {
	RestTemplate rest = new RestTemplate();
	rest.getInterceptors().add((request, body, execution) -> {
		Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
		if (authentication == null) {
			return execution.execute(request, body);
		}

		if (!(authentication.getCredentials() instanceof AbstractOAuth2Token)) {
			return execution.execute(request, body);
		}

		AbstractOAuth2Token token = (AbstractOAuth2Token) authentication.getCredentials();
	    request.getHeaders().setBearerAuth(token.getTokenValue());
	    return execution.execute(request, body);
	});
	return rest;
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun rest(): RestTemplate {
    val rest = RestTemplate()
    rest.interceptors.add(ClientHttpRequestInterceptor { request, body, execution ->
        val authentication: Authentication? = SecurityContextHolder.getContext().authentication
        if (authentication != null) {
            execution.execute(request, body)
        }

        if (authentication!!.credentials !is AbstractOAuth2Token) {
            execution.execute(request, body)
        }

        val token: AbstractOAuth2Token = authentication.credentials as AbstractOAuth2Token
        request.headers.setBearerAuth(token.tokenValue)
        execution.execute(request, body)
    })
    return rest
}
----
====


[NOTE]
Unlike the {security-api-url}org/springframework/security/oauth2/client/OAuth2AuthorizedClientManager.html[OAuth 2.0 Authorized Client Manager], this filter interceptor makes no attempt to renew the token, should it be expired.
To obtain this level of support, please create an interceptor using the xref:servlet/oauth2/client/index.adoc#oauth2client[OAuth 2.0 Authorized Client Manager].

[[oauth2resourceserver-bearertoken-failure]]
== Bearer Token Failure

A bearer token may be invalid for a number of reasons. For example, the token may no longer be active.

In these circumstances, Resource Server throws an `InvalidBearerTokenException`.
Like other exceptions, this results in an OAuth 2.0 Bearer Token error response:

[source,http request]
----
HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer error_code="invalid_token", error_description="Unsupported algorithm of none", error_uri="https://tools.ietf.org/html/rfc6750#section-3.1"
----

Additionally, it is published as an `AuthenticationFailureBadCredentialsEvent`, which you can xref:servlet/authentication/events.adoc#servlet-events[listen for in your application] like so:

====
.Java
[source,java,role="primary"]
----
@Component
public class FailureEvents {
	@EventListener
    public void onFailure(AuthenticationFailureBadCredentialsEvent badCredentials) {
		if (badCredentials.getAuthentication() instanceof BearerTokenAuthenticationToken) {
		    // ... handle
        }
    }
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Component
class FailureEvents {
    @EventListener
    fun onFailure(badCredentials: AuthenticationFailureBadCredentialsEvent) {
        if (badCredentials.authentication is BearerTokenAuthenticationToken) {
            // ... handle
        }
    }
}
----
====
