How to use custom expressions in Spring Security @PreAuthorize/@PostAuthorize annotations

Question

Is there a way to create more expressive statements in @Preauthorize blocks? Here\'s an example of something I find myself repeating, because the @Preauthorize is not terrib

Answer 1

1) First you have to reimplement MethodSecurityExpressionRoot which contains extra method-specific functionality. The original Spring Security implementation is package private and hence it is not possible to just extend it. I suggest checking the source code for the given class.

public class CustomMethodSecurityExpressionRoot extends SecurityExpressionRoot implements MethodSecurityExpressionOperations {

    // copy everything from the original Spring Security MethodSecurityExpressionRoot

    // add your custom methods

    public boolean isAdmin() {
        // do whatever you need to do, e.g. delegate to other components

        // hint: you can here directly access Authentication object 
        // via inherited authentication field
    }

    public boolean isOwner(Long id) {
        // do whatever you need to do, e.g. delegate to other components
    }
}

2) Next you have to implement custom MethodSecurityExpressionHandler that will use the above defined CustomMethodSecurityExpressionRoot.

public class CustomMethodSecurityExpressionHandler extends DefaultMethodSecurityExpressionHandler {

    private final AuthenticationTrustResolver trustResolver = new AuthenticationTrustResolverImpl();

    @Override
    public void setReturnObject(Object returnObject, EvaluationContext ctx) {
        ((MethodSecurityExpressionRoot) ctx.getRootObject().getValue()).setReturnObject(returnObject);
    }

    @Override
    protected MethodSecurityExpressionOperations createSecurityExpressionRoot(Authentication authentication,
        MethodInvocation invocation) {
        final CustomMethodSecurityExpressionRoot root = new CustomMethodSecurityExpressionRoot(authentication);
        root.setThis(invocation.getThis());
        root.setPermissionEvaluator(getPermissionEvaluator());
        root.setTrustResolver(this.trustResolver);
        root.setRoleHierarchy(getRoleHierarchy());

        return root;
    }
}

3) Define expression handler bean in your context, e.g. via XML you can do it as follows

<bean id="methodSecurityExpressionHandler"
    class="my.package.CustomMethodSecurityExpressionHandler">
    <property name="roleHierarchy" ref="roleHierarchy" />
    <property name="permissionEvaluator" ref="permissionEvaluator" />
</bean>

4) Register the above defined handler

<security:global-method-security pre-post-annotations="enabled">
    <security:expression-handler ref="methodSecurityExpressionHandler"/>
</security:global-method-security>

5) Then just use the defined expressions in your @PreAuthorize and/or @PostAuthorize annotations

@PreAuthorize("isAdmin() or isOwner(#id)")
public void deleteGame(@PathVariable int id, @ModelAttribute currentGame) {
    // do whatever needed
}

And one more thing. It is not very common to use method level security to secure controller methods but rather to secure methods with business logic (a.k.a. your service layer methods). Then you could use something like the below.

public interface GameService {

    // rest omitted

    @PreAuthorize("principal.admin or #game.owner = principal.username")
    public void delete(@P("game") Game game);
}

But keep in mind that this is just an example. It expects that the actual principal has isAdmin() method and that the game has getOwner() method returning username of the owner.

Answer 2

Since @PreAuthorize evaluates SpEl-expressions, the easiest way is just to point to a bean:

    @PreAuthorize("@mySecurityService.someFunction()")

MySecurityService.someFunction should have return type boolean.

Spring-security will automatically provide a variable named authentication if you want to pass the Authentication-object. You can also use any valid SpEl-expressions to access any arguments passed to your secure method, evaluate regular expressions, call static methods, etc. E.g:

    @PreAuthorize("@mySecurityService.someFunction(authentication, #someParam)")

Answer 3

You could write your annotation something like:

@PreAuthorize("hasRole('ROLE_ADMIN') and hasPermission(#id, 'Game', 'DELETE')")

To get the hasPermission part working you need to implement PermissionEvaluator interface.

Then define an expression handler bean:

@Autowired
private PermissionEvaluator permissionEvaluator;

@Bean
public DefaultMethodSecurityExpressionHandler expressionHandler()
{
    DefaultMethodSecurityExpressionHandler handler = new DefaultMethodSecurityExpressionHandler();
    handler.setPermissionEvaluator(permissionEvaluator);
    return handler;
}

And inject in your security config:

<global-method-security pre-post-annotations="enabled">
  <expression-handler ref="expressionHandler" />
</global-method-security>