Spring Security offers an authentication replacement feature, often referred to as Run-As, that can replace the current user's authentication (and thus permissions) during a single secured object invocation. Using this feature makes sense when a backend system invoked during request processing requires different privileges than the current application.
For example, an application might want to expose a financial transaction log to the currently logged in user, but the backend system that provides it only permits this action to the members of a special "auditor" role. The application can not simply assign this role to the user as that would potentially permit them to execute other restricted actions. Instead, the user can be given this right exclusively for viewing their transaction log.
Only two classes are used to implement this feature. Instances of
RunAsManager are tasked with producing the actual replacement authentication tokens. A sensible default implementation is already provided by Spring Security.
As with other types of authentication, it is also necessary to register an instance of an appropriate
AuthenticationProvider.
<bean id="runAsManager"
class="org.springframework.security.access.intercept.RunAsManagerImpl">
<property name="key" value="my_run_as_key"/>
</bean>
<bean id="runAsAuthenticationProvider"
class="org.springframework.security.access.intercept.RunAsImplAuthenticationProvider">
<property name="key" value="my_run_as_key"/>
</bean>
Tokens produced by
runAsManager are signed with the provided key (
my_run_as_key in the example above) and are later checked against the same key by
runAsAuthenticationProvider, in order to mitigate the risk of fake tokens being provided. These keys can have any value, but need to be the same in both objects. Otherwise,
runAsAuthenticationProvider will reject the produced tokens as invalid.
If an instance is registered, RunAsManager will be invoked by AbstractSecurityInterceptor for every intercepted object invocation for which the user has already been given access. If RunAsManager returns a token, this token will be used be used instead of the original one for the duration of the invocation, thus granting the user different privileges. There are two key points here. In order for the authentication replacement feature to do anything, the call has to actually be secured (and thus intercepted), and the user has to already have been granted access.
To register a
RunAsManager instance with the method security interceptor, something similar to the following is needed:
<global-method-security secured-annotations="enabled" run-as-manager-ref="runAsManager"/>
Now, all methods secured by the
@Secured annotation will be able to trigger
RunAsManager. One important point here is that
global-method-security will only work in the Spring context in which it is defined. In Spring MVC applications, there usually are two Spring contexts: the parent context, attached to
ContextLoaderListener, and the child context, attached to
DispatcherServlet.
To secure Controller methods in this way, global-method-security must be added to DispatcherServlet's context. To secure methods in beans not in this context,
global-method-security should also be added to
ContextLoaderListener's context.
Otherwise, security annotations will be ignored.
The default implementation of RunAsManager (RunAsManagerImpl) will inspect the secured object's configuration and if it finds any attributes prefixed with RUN_AS_, it will create a token identical to the original, with the addition of one new GrantedAuthorty per RUN_AS_ attribute found. The new GrantedAuthority will be a role (prefixed by ROLE_ by default) named like the found attribute without the RUN_AS_ prefix.
So, if a user with a role ROLE_REGISTERED_USER invokes a method annotated with @Secured({"ROLE_REGISTERED_USER","RUN_AS_AUDITOR"}), e.g.
@Controller
public class TransactionLogController {
@Secured({"ROLE_REGISTERED_USER","RUN_AS_AUDITOR"}) //Authorities needed for method access and authorities added by RunAsManager prefixed with RUN_AS_
@RequestMapping(value = "/transactions", method = RequestMethod.GET) //Spring MVC configuration. Not related to security
@ResponseBody //Spring MVC configuration. Not related to security
public List<Transaction> getTransactionLog(...) {
... //Invoke something in the backend requiring ROLE_AUDITOR
{
... //User does not have ROLE_AUDITOR here
}
the resulting token created by
RunAsManagerImpl with be granted
ROLE_REGISTERED_USER and
ROLE_AUDITOR. Thus, the user will also be allowed actions, normally reserved for
ROLE_AUDITOR members, during the current invocation, permitting them, in this case, to access the transaction log.
To enable
runAsAuthenticationProvider, register it as usual:
<authentication-manager alias="authenticationManager">
<authentication-provider ref="runAsAuthenticationProvider"/>
... other authentication-providers used by the application ...
</authentication-manager>
This is all that is necessary to have the default implementation activated.
Still, this setting will not work for methods secured by @PreAuthorize and @PostAuthorize annotations as their configuration attributes are differently evaluated (they are SpEL expressions and not a simple list or required authorities like with @Secured) and will not be recognized by RunAsManagerImpl.
For this scenario to work, a custom RunAsManager implementation is required, as, at least at the time of writing, no applicable implementation is provided by Spring.
A custom RunAsManager implementation for use with @PreAuthorize/@PostAuthorize
A convenient implementation relying on a custom annotation is provided below:
public class AnnotationDrivenRunAsManager extends RunAsManagerImpl {
@Override
public Authentication buildRunAs(Authentication authentication, Object object, Collection<ConfigAttribute> attributes) {
if(!(object instanceof ReflectiveMethodInvocation) || ((ReflectiveMethodInvocation)object).getMethod().getAnnotation(RunAsRole.class) == null) {
return super.buildRunAs(authentication, object, attributes);
}
String roleName = ((ReflectiveMethodInvocation)object).getMethod().getAnnotation(RunAsRole.class).value();
if (roleName == null || roleName.isEmpty()) {
return null;
}
GrantedAuthority runAsAuthority = new SimpleGrantedAuthority(roleName);
List<GrantedAuthority> newAuthorities = new ArrayList<GrantedAuthority>();
// Add existing authorities
newAuthorities.addAll(authentication.getAuthorities());
// Add the new run-as authority
newAuthorities.add(runAsAuthority);
return new RunAsUserToken(getKey(), authentication.getPrincipal(), authentication.getCredentials(),
newAuthorities, authentication.getClass());
}
}
This implementation will look for a custom
@RunAsRole annotation on a protected method (e.g.
@RunAsRole("ROLE_AUDITOR")) and, if found, will add the given authority (
ROLE_AUDITOR in this case) to the list of granted authorities.
RunAsRole itself is just a simple custom annotation:
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface RunAsRole {
String value();
}
This new implementation would be instantiated in the same way as before:
<bean id="runAsManager"
class="org.springframework.security.access.intercept.RunAsManagerImpl">
<property name="key" value="my_run_as_key"/>
</bean>
And registered in a similar fashion:
<global-method-security pre-post-annotations="enabled" run-as-manager-ref="runAsManager">
<expression-handler ref="expressionHandler"/>
</global-method-security>
The expression-handler is always required for
pre-post-annotations to work. It is a part of the standard Spring Security configuration, and not related to the topic described here.
Both
pre-post-annotations and
secured-annotations can be enabled at the same time, but
should never be used in the same class.
The protected controller method from above could now look like this:
@Controller
public class TransactionLogController {
@PreAuthorize("hasRole('ROLE_REGISTERED_USER')") //Authority needed to access the method
@RunAsRole("ROLE_AUDITOR") //Authority added by RunAsManager
@RequestMapping(value = "/transactions", method = RequestMethod.GET) //Spring MVC configuration. Not related to security
@ResponseBody //Spring MVC configuration. Not related to security
public List<Transaction> getTransactionLog(...) {
... //Invoke something in the backend requiring ROLE_AUDITOR
{
... //User does not have ROLE_AUDITOR here
}
