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Expression-Based Access Control

Overview

Spring Security uses SpEL for expression support and you should look at how that works if you are interested in understanding the topic in more depth. Expressions are evaluated with a “root object” as part of the evaluation context. Spring Security uses specific classes for web and method security as the root object to provide built-in expressions and access to values, such as the current principal.

Common Built-In Expressions

The base class for expression root objects is SecurityExpressionRoot. This provides some common expressions that are available in both web and method security:

Table 1. Common built-in expressions
Expression	Description
`hasRole(String role)`	Returns `true` if the current principal has the specified role. Example: `hasRole('admin')` By default, if the supplied role does not start with `ROLE_`, it is added. You can customize this behavior by modifying the `defaultRolePrefix` on `DefaultWebSecurityExpressionHandler`.
`hasAnyRole(String… roles)`	Returns `true` if the current principal has any of the supplied roles (given as a comma-separated list of strings). Example: `hasAnyRole('admin', 'user')`. By default, if the supplied role does not start with `ROLE_`, it is added. You can customize this behavior by modifying the `defaultRolePrefix` on `DefaultWebSecurityExpressionHandler`.
`hasAuthority(String authority)`	Returns `true` if the current principal has the specified authority. Example: `hasAuthority('read')`
`hasAnyAuthority(String… authorities)`	Returns `true` if the current principal has any of the supplied authorities (given as a comma-separated list of strings). Example: `hasAnyAuthority('read', 'write')`.
`principal`	Allows direct access to the principal object that represents the current user.
`authentication`	Allows direct access to the current `Authentication` object obtained from the `SecurityContext`.
`permitAll`	Always evaluates to `true`.
`denyAll`	Always evaluates to `false`.
`isAnonymous()`	Returns `true` if the current principal is an anonymous user.
`isRememberMe()`	Returns `true` if the current principal is a remember-me user.
`isAuthenticated()`	Returns `true` if the user is not anonymous.
`isFullyAuthenticated()`	Returns `true` if the user is not an anonymous and is not a remember-me user.
`hasPermission(Object target, Object permission)`	Returns `true` if the user has access to the provided target for the given permission. Example, `hasPermission(domainObject, 'read')`.
`hasPermission(Object targetId, String targetType, Object permission)`	Returns `true` if the user has access to the provided target for the given permission. Example, `hasPermission(1, 'com.example.domain.Message', 'read')`.

Web Security Expressions

To use expressions to secure individual URLs, you first need to set the use-expressions attribute in the <http> element to true. Spring Security then expects the access attributes of the <intercept-url> elements to contain SpEL expressions. Each expression should evaluate to a Boolean, defining whether access should be allowed or not. The following listing shows an example:

<http>
	<intercept-url pattern="/admin*"
		access="hasRole('admin') and hasIpAddress('192.168.1.0/24')"/>
	...
</http>

Here, we have defined that the admin area of an application (defined by the URL pattern) should be available only to users who have the granted authority (admin) and whose IP address matches a local subnet. We have already seen the built-in hasRole expression in the previous section. The hasIpAddress expression is an additional built-in expression that is specific to web security. It is defined by the WebSecurityExpressionRoot class, an instance of which is used as the expression root object when evaluating web-access expressions. This object also directly exposed the HttpServletRequest object under the name request so that you can invoke the request directly in an expression. If expressions are being used, a WebExpressionVoter is added to the AccessDecisionManager that is used by the namespace. So, if you do not use the namespace and want to use expressions, you have to add one of these to your configuration.

Referring to Beans in Web Security Expressions

If you wish to extend the expressions that are available, you can easily refer to any Spring Bean you expose. For example, you could use the following, assuming you have a Bean with the name of webSecurity that contains the following method signature:

Java
Kotlin

public class WebSecurity {
		public boolean check(Authentication authentication, HttpServletRequest request) {
				...
		}
}

class WebSecurity {
    fun check(authentication: Authentication?, request: HttpServletRequest?): Boolean {
        // ...
    }
}

You could then refer to the method as follows:

Refer to method

Java
XML
Kotlin

http
    .authorizeHttpRequests(authorize -> authorize
        .requestMatchers("/user/**").access(new WebExpressionAuthorizationManager("@webSecurity.check(authentication,request)"))
        ...
    )

<http>
	<intercept-url pattern="/user/**"
		access="@webSecurity.check(authentication,request)"/>
	...
</http>

http {
    authorizeRequests {
        authorize("/user/**", "@webSecurity.check(authentication,request)")
    }
}

Path Variables in Web Security Expressions

At times, it is nice to be able to refer to path variables within a URL. For example, consider a RESTful application that looks up a user by ID from a URL path in a format of /user/{userId}.

You can easily refer to the path variable by placing it in the pattern. For example, you could use the following if you had a Bean with the name of webSecurity that contains the following method signature:

Java
Kotlin

public class WebSecurity {
		public boolean checkUserId(Authentication authentication, int id) {
				...
		}
}

class WebSecurity {
    fun checkUserId(authentication: Authentication?, id: Int): Boolean {
        // ...
    }
}

You could then refer to the method as follows:

Path Variables

Java
XML
Kotlin

http
	.authorizeHttpRequests(authorize -> authorize
		.requestMatchers("/user/{userId}/**").access(new WebExpressionAuthorizationManager("@webSecurity.checkUserId(authentication,#userId)"))
		...
	);

<http>
	<intercept-url pattern="/user/{userId}/**"
		access="@webSecurity.checkUserId(authentication,#userId)"/>
	...
</http>

http {
    authorizeRequests {
        authorize("/user/{userId}/**", "@webSecurity.checkUserId(authentication,#userId)")
    }
}

In this configuration, URLs that match would pass in the path variable (and convert it) into the checkUserId method. For example, if the URL were /user/123/resource, the ID passed in would be 123.

Method Security Expressions

Method security is a bit more complicated than a simple allow or deny rule. Spring Security 3.0 introduced some new annotations to allow comprehensive support for the use of expressions.

@Pre and @Post Annotations

There are four annotations that support expression attributes to allow pre and post-invocation authorization checks and also to support filtering of submitted collection arguments or return values. They are @PreAuthorize, @PreFilter, @PostAuthorize, and @PostFilter. Their use is enabled through the global-method-security namespace element:

<global-method-security pre-post-annotations="enabled"/>

Access Control using @PreAuthorize and @PostAuthorize

The most obviously useful annotation is @PreAuthorize, which decides whether a method can actually be invoked or not. The following example (from the "Contacts" sample application) uses the @PreAuthorize annotation:

Java
Kotlin

@PreAuthorize("hasRole('USER')")
public void create(Contact contact);

@PreAuthorize("hasRole('USER')")
fun create(contact: Contact?)

This means that access is allowed only for users with the ROLE_USER role. Obviously, the same thing could easily be achieved by using a traditional configuration and a simple configuration attribute for the required role. However, consider the following example:

Java
Kotlin

@PreAuthorize("hasPermission(#contact, 'admin')")
public void deletePermission(Contact contact, Sid recipient, Permission permission);

@PreAuthorize("hasPermission(#contact, 'admin')")
fun deletePermission(contact: Contact?, recipient: Sid?, permission: Permission?)

Here, we actually use a method argument as part of the expression to decide whether the current user has the admin permission for the given contact. The built-in hasPermission() expression is linked into the Spring Security ACL module through the application context, as we see later in this section. You can access any of the method arguments by name as expression variables.

Spring Security can resolve the method arguments in a number of ways. Spring Security uses DefaultSecurityParameterNameDiscoverer to discover the parameter names. By default, the following options are tried for a method.

If Spring Security’s @P annotation is present on a single argument to the method, the value is used. This is useful for interfaces compiled with a JDK prior to JDK 8 (which do not contain any information about the parameter names). The following example uses the @P annotation:
- Java
- Kotlin
import org.springframework.security.access.method.P; ... @PreAuthorize("#c.name == authentication.name") public void doSomething(@P("c") Contact contact);
import org.springframework.security.access.method.P ... @PreAuthorize("#c.name == authentication.name") fun doSomething(@P("c") contact: Contact?)
Behind the scenes, this is implemented by using AnnotationParameterNameDiscoverer, which you can customize to support the value attribute of any specified annotation.
If Spring Data’s @Param annotation is present on at least one parameter for the method, the value is used. This is useful for interfaces compiled with a JDK prior to JDK 8 which do not contain any information about the parameter names. The following example uses the @Param annotation:
- Java
- Kotlin
import org.springframework.data.repository.query.Param; ... @PreAuthorize("#n == authentication.name") Contact findContactByName(@Param("n") String name);
import org.springframework.data.repository.query.Param ... @PreAuthorize("#n == authentication.name") fun findContactByName(@Param("n") name: String?): Contact?
Behind the scenes, this is implemented by using AnnotationParameterNameDiscoverer, which you can customize to support the value attribute of any specified annotation.
If JDK 8 was used to compile the source with the -parameters argument and Spring 4+ is being used, the standard JDK reflection API is used to discover the parameter names. This works on both classes and interfaces.
Finally, if the code was compiled with the debug symbols, the parameter names are discovered by using the debug symbols. This does not work for interfaces, since they do not have debug information about the parameter names. For interfaces, annotations or the JDK 8 approach must be used.

Any SpEL functionality is available within the expression, so you can also access properties on the arguments. For example, if you wanted a particular method to allow access only to a user whose username matched that of the contact, you could write

Java
Kotlin

@PreAuthorize("#contact.name == authentication.name")
public void doSomething(Contact contact);

@PreAuthorize("#contact.name == authentication.name")
fun doSomething(contact: Contact?)

Here, we access another built-in expression, authentication, which is the Authentication stored in the security context. You can also access its principal property directly, by using the principal expression. The value is often a UserDetails instance, so you might use an expression such as principal.username or principal.enabled.

Filtering using @PreFilter and @PostFilter

Spring Security supports filtering of collections, arrays, maps, and streams by using expressions. This is most commonly performed on the return value of a method. The following example uses @PostFilter:

Java
Kotlin

@PreAuthorize("hasRole('USER')")
@PostFilter("hasPermission(filterObject, 'read') or hasPermission(filterObject, 'admin')")
public List<Contact> getAll();

@PreAuthorize("hasRole('USER')")
@PostFilter("hasPermission(filterObject, 'read') or hasPermission(filterObject, 'admin')")
fun getAll(): List<Contact?>

When using the @PostFilter annotation, Spring Security iterates through the returned collection or map and removes any elements for which the supplied expression is false. For an array, a new array instance that contains filtered elements is returned. filterObject refers to the current object in the collection. When a map is used, it refers to the current Map.Entry object, which lets you use filterObject.key or filterObject.value in the expression. You can also filter before the method call by using @PreFilter, though this is a less common requirement. The syntax is the same. However, if there is more than one argument that is a collection type, you have to select one by name using the filterTarget property of this annotation.

Note that filtering is obviously not a substitute for tuning your data retrieval queries. If you are filtering large collections and removing many of the entries, this is likely to be inefficient.

Built-In Expressions

There are some built-in expressions that are specific to method security, which we have already seen in use earlier. The filterTarget and returnValue values are simple enough, but the use of the hasPermission() expression warrants a closer look.

The PermissionEvaluator interface

hasPermission() expressions are delegated to an instance of PermissionEvaluator. It is intended to bridge between the expression system and Spring Security’s ACL system, letting you specify authorization constraints on domain objects, based on abstract permissions. It has no explicit dependencies on the ACL module, so you could swap that out for an alternative implementation if required. The interface has two methods:

boolean hasPermission(Authentication authentication, Object targetDomainObject,
							Object permission);

boolean hasPermission(Authentication authentication, Serializable targetId,
							String targetType, Object permission);

These methods map directly to the available versions of the expression, with the exception that the first argument (the Authentication object) is not supplied. The first is used in situations where the domain object, to which access is being controlled, is already loaded. Then the expression returns true if the current user has the given permission for that object. The second version is used in cases where the object is not loaded but its identifier is known. An abstract “type” specifier for the domain object is also required, letting the correct ACL permissions be loaded. This has traditionally been the Java class of the object but does not have to be, as long as it is consistent with how the permissions are loaded.

To use hasPermission() expressions, you have to explicitly configure a PermissionEvaluator in your application context. The following example shows how to do so:

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

<bean id="expressionHandler" class=
"org.springframework.security.access.expression.method.DefaultMethodSecurityExpressionHandler">
	<property name="permissionEvaluator" ref="myPermissionEvaluator"/>
</bean>

Where myPermissionEvaluator is the bean which implements PermissionEvaluator. Usually, this is the implementation from the ACL module, which is called AclPermissionEvaluator. See the Contacts sample application configuration for more details.

Method Security Meta Annotations

You can make use of meta annotations for method security to make your code more readable. This is especially convenient if you find that you repeat the same complex expression throughout your code base. For example, consider the following:

@PreAuthorize("#contact.name == authentication.name")

Instead of repeating this everywhere, you can create a meta annotation:

Java
Kotlin

@Retention(RetentionPolicy.RUNTIME)
@PreAuthorize("#contact.name == authentication.name")
public @interface ContactPermission {}

@Retention(AnnotationRetention.RUNTIME)
@PreAuthorize("#contact.name == authentication.name")
annotation class ContactPermission

You can use meta annotations for any of the Spring Security method security annotations. To remain compliant with the specification, JSR-250 annotations do not support meta annotations.