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This guide is a primer for Spring Security, offering insight into the design and basic building blocks of the framework. We cover only the very basics of application security. However, in doing so, we can clear up some of the confusion experienced by developers who use Spring Security. To do this, we take a look at the way security is applied in web applications by using filters and, more generally, by using method annotations. Use this guide when you need a high-level understanding of how a secure application works, how it can be customized, or if you need to learn how to think about application security.
This guide is not intended as a manual or recipe for solving more than the most basic problems (there are other sources for those), but it could be useful for beginners and experts alike. Spring Boot is also often referenced, because it provides some default behavior for a secure application, and it can be useful to understand how that fits in with the overall architecture.
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Note
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All of the principles apply equally well to applications that do not use Spring Boot. |
Application security boils down to two more or less independent problems: authentication (who are you?) and authorization (what are you allowed to do?). Sometimes people say “access control” instead of "authorization", which can get confusing, but it can be helpful to think of it that way because “authorization” is overloaded in other places. Spring Security has an architecture that is designed to separate authentication from authorization and has strategies and extension points for both.
The main strategy interface for authentication is
AuthenticationManager, which has only one method:
public interface AuthenticationManager {
Authentication authenticate(Authentication authentication)
throws AuthenticationException;
}An AuthenticationManager
can do one of 3 things in its authenticate() method:
-
Return an
Authentication(normally withauthenticated=true) if it can verify that the input represents a valid principal. -
Throw an
AuthenticationExceptionif it believes that the input represents an invalid principal. -
Return
nullif it cannot decide.
AuthenticationException is a runtime exception. It is usually
handled by an application in a generic way, depending on the style or
purpose of the application. In other words, user code is not normally
expected to catch and handle it. For example, a web UI might render a
page that says that the authentication failed, and a backend HTTP
service might send a 401 response, with or without a WWW-Authenticate
header depending on the context.
The most commonly used implementation of AuthenticationManager is
ProviderManager, which delegates to a chain of
AuthenticationProvider instances. An AuthenticationProvider is a
bit like an AuthenticationManager, but it has an extra method to
allow the caller to query whether it supports a given Authentication
type:
public interface AuthenticationProvider {
Authentication authenticate(Authentication authentication)
throws AuthenticationException;
boolean supports(Class<?> authentication);
}The Class<?> argument in the supports() method is really Class<?
extends Authentication> (it is only ever asked if it supports
something that is passed into the authenticate() method). A
ProviderManager can support multiple different authentication
mechanisms in the same application by delegating to a chain of
AuthenticationProviders. If a ProviderManager does not recognize a
particular Authentication instance type, it is skipped.
A ProviderManager has an optional parent, which it can consult if
all providers return null. If the parent is not available, a
null Authentication results in an AuthenticationException.
Sometimes, an application has logical groups of protected resources
(for example, all web resources that match a path pattern, such as /api/**), and
each group can have its own dedicated AuthenticationManager. Often,
each of those is a ProviderManager, and they share a parent. The
parent is then a kind of “global” resource, acting as a fallback for
all providers.
AuthenticationManager hierarchy using ProviderManagerSpring Security provides some configuration helpers to quickly get
common authentication manager features set up in your application. The
most commonly used helper is the AuthenticationManagerBuilder, which
is great for setting up in-memory, JDBC, or LDAP user details or for
adding a custom UserDetailsService. The following example shows an
application that configures the global (parent) AuthenticationManager:
@Configuration
public class ApplicationSecurity extends WebSecurityConfigurerAdapter {
... // web stuff here
@Autowired
public void initialize(AuthenticationManagerBuilder builder, DataSource dataSource) {
builder.jdbcAuthentication().dataSource(dataSource).withUser("dave")
.password("secret").roles("USER");
}
}This example relates to a web application, but the usage of
AuthenticationManagerBuilder is more widely applicable (see Web Security
for more detail on how web application security is implemented). Note
that the AuthenticationManagerBuilder is @Autowired into a method
in a @Bean — that is what makes it build the global (parent)
AuthenticationManager. In contrast, consider the following example:
@Configuration
public class ApplicationSecurity extends WebSecurityConfigurerAdapter {
@Autowired
DataSource dataSource;
... // web stuff here
@Override
public void configure(AuthenticationManagerBuilder builder) {
builder.jdbcAuthentication().dataSource(dataSource).withUser("dave")
.password("secret").roles("USER");
}
}If we had used an @Override of a method in the configurer, the
AuthenticationManagerBuilder would be used only to build a “local”
AuthenticationManager, which would be a child of the global one. In a
Spring Boot application, you can @Autowired the global one into
another bean, but you cannot do that with the local one unless you
explicitly expose it yourself.
Spring Boot provides a default global AuthenticationManager (with
only one user) unless you pre-empt it by providing your own bean of
type AuthenticationManager. The default is secure enough on its own
for you not to have to worry about it much, unless you actively need a
custom global AuthenticationManager. If you do any configuration
that builds an AuthenticationManager, you can often do it locally to
the resources that you are protecting and not worry about the global
default.
Once authentication is successful, we can move on to authorization,
and the core strategy here is AccessDecisionManager. There are three
implementations provided by the framework and all three delegate to a
chain of AccessDecisionVoter instances, a bit like the ProviderManager
delegates to AuthenticationProviders.
An AccessDecisionVoter considers an Authentication (representing a
principal) and a secure Object, which has been decorated with
ConfigAttributes:
boolean supports(ConfigAttribute attribute);
boolean supports(Class<?> clazz);
int vote(Authentication authentication, S object,
Collection<ConfigAttribute> attributes);The Object is completely generic in the signatures of the
AccessDecisionManager and AccessDecisionVoter. It represents
anything that a user might want to access (a web resource or a method
in a Java class are the two most common cases). The ConfigAttributes
are also fairly generic, representing a decoration of the secure
Object with some metadata that determines the level of permission
required to access it. ConfigAttribute is an interface. It
has only one method (which is quite generic and returns a String), so these
strings encode in some way the intention of the owner of the resource,
expressing rules about who is allowed to access it. A typical
ConfigAttribute is the name of a user role (like ROLE_ADMIN or
ROLE_AUDIT), and they often have special formats (like the ROLE_
prefix) or represent expressions that need to be evaluated.
Most people use the default AccessDecisionManager, which is
AffirmativeBased (if any voters return affirmatively, access is granted). Any
customization tends to happen in the voters, either by adding new ones
or modifying the way that the existing ones work.
It is very common to use ConfigAttributes that are Spring Expression
Language (SpEL) expressions — for example, isFullyAuthenticated() &&
hasRole('user'). This is supported by an AccessDecisionVoter that
can handle the expressions and create a context for them. To extend
the range of expressions that can be handled requires a custom
implementation of SecurityExpressionRoot and sometimes also
SecurityExpressionHandler.
Spring Security in the web tier (for UIs and HTTP back ends) is based
on Servlet Filters, so it is helpful to first look at the role of
Filters generally. The following picture shows the typical
layering of the handlers for a single HTTP request.
The client sends a request to the application, and the container decides which
filters and which servlet apply to it based on the path of the request
URI. At most, one servlet can handle a single request, but filters form
a chain, so they are ordered. In fact, a filter can veto the rest
of the chain if it wants to handle the request itself. A filter can
also modify the request or the response used in the downstream
filters and servlet. The order of the filter chain is very important,
and Spring Boot manages it through two mechanisms: @Beans
of type Filter can have an @Order or implement Ordered, and
they can be part of a FilterRegistrationBean that
itself has an order as part of its API. Some off-the-shelf filters
define their own constants to help signal what order they like to be
in relative to each other (for example, the SessionRepositoryFilter from
Spring Session has a DEFAULT_ORDER of Integer.MIN_VALUE + 50,
which tells us it likes to be early in the chain, but it does not rule
out other filters coming before it).
Spring Security is installed as a single Filter in the chain, and
its concrete type is FilterChainProxy, for reasons that we cover
soon. In a Spring Boot application, the security filter is a @Bean
in the ApplicationContext, and it is installed by default so that it
is applied to every request. It is installed at a position defined by
SecurityProperties.DEFAULT_FILTER_ORDER, which in turn is anchored
by FilterRegistrationBean.REQUEST_WRAPPER_FILTER_MAX_ORDER (the
maximum order that a Spring Boot application expects filters to have if they
wrap the request, modifying its behavior). There is more to it than
that, though: From the point of view of the container, Spring Security
is a single filter, but, inside of it, there are additional filters, each
playing a special role. The following image shows this relationship:
Filter but delegates processing to a chain of internal filtersIn fact, there is even one more layer of indirection in the security
filter: It is usually installed in the container as a
DelegatingFilterProxy, which does not have to be a Spring
@Bean. The proxy delegates to a FilterChainProxy, which is always a
@Bean, usually with a fixed name of springSecurityFilterChain. It
is the FilterChainProxy that contains all the security logic
arranged internally as a chain (or chains) of filters. All the filters
have the same API (they all implement the Filter interface from the
Servlet specification), and they all have the opportunity to veto the rest of
the chain.
There can be multiple filter chains all managed by Spring Security in
the same top level FilterChainProxy and all are unknown to the
container. The Spring Security filter contains a list of filter
chains and dispatches a request to the first chain that matches
it. The following picture shows the dispatch happening based on matching
the request path (/foo/** matches before /**). This is very
common but not the only way to match a request. The most important
feature of this dispatch process is that only one chain ever handles a
request.
FilterChainProxy dispatches requests to the first chain that matches.A vanilla Spring Boot application with no custom security
configuration has a several (call it n) filter chains, where usually
n=6. The first (n-1) chains are there just to ignore static resource
patterns, like /css/** and /images/**, and the error view:
/error. (The paths can be controlled by the user with
security.ignored from the SecurityProperties configuration
bean.) The last chain matches the catch-all path (/**) and is more
active, containing logic for authentication, authorization, exception
handling, session handling, header writing, and so on. There are a total of
11 filters in this chain by default, but normally it is not necessary
for users to concern themselves with which filters are used and when.
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Note
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The fact that all filters internal to Spring Security are
unknown to the container is important, especially in a Spring Boot
application, where, by default, all @Beans of type Filter are registered
automatically with the container. So if you want to add a
custom filter to the security chain, you need to either not make it be a
@Bean or wrap it in a FilterRegistrationBean that explicitly
disables the container registration.
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The default fallback filter chain in a Spring Boot application (the one with
the /** request matcher) has a predefined order of
SecurityProperties.BASIC_AUTH_ORDER. You can switch it off
completely by setting security.basic.enabled=false, or you can use
it as a fallback and define other rules with a lower order. To do
the latter, add a @Bean of type WebSecurityConfigurerAdapter (or
WebSecurityConfigurer) and decorate the class with @Order, as follows:
@Configuration
@Order(SecurityProperties.BASIC_AUTH_ORDER - 10)
public class ApplicationConfigurerAdapter extends WebSecurityConfigurerAdapter {
@Override
protected void configure(HttpSecurity http) throws Exception {
http.antMatcher("/match1/**")
...;
}
}This bean causes Spring Security to add a new filter chain and order it before the fallback.
Many applications have completely different access rules for one set
of resources compared to another. For example, an application that
hosts a UI and a backing API might support cookie-based authentication
with a redirect to a login page for the UI parts and token-based
authentication with a 401 response to unauthenticated requests for the
API parts. Each set of resources has its own
WebSecurityConfigurerAdapter with a unique order and its own
request matcher. If the matching rules overlap, the earliest ordered
filter chain wins.
A security filter chain (or, equivalently, a
WebSecurityConfigurerAdapter) has a request matcher that is used to
decide whether to apply it to an HTTP request. Once the decision is
made to apply a particular filter chain, no others are applied. However,
within a filter chain, you can have more fine-grained control of
authorization by setting additional matchers in the HttpSecurity
configurer, as follows:
@Configuration
@Order(SecurityProperties.BASIC_AUTH_ORDER - 10)
public class ApplicationConfigurerAdapter extends WebSecurityConfigurerAdapter {
@Override
protected void configure(HttpSecurity http) throws Exception {
http.antMatcher("/match1/**")
.authorizeRequests()
.antMatchers("/match1/user").hasRole("USER")
.antMatchers("/match1/spam").hasRole("SPAM")
.anyRequest().isAuthenticated();
}
}One of the easiest mistakes to make when configuring Spring Security is to forget that these matchers apply to different processes. One is a request matcher for the whole filter chain, and the other is only to choose the access rule to apply.
If you use the Spring Boot Actuator for management endpoints,
you probably want them to be secure, and, by default, they are. In
fact, as soon as you add the Actuator to a secure application, you get
an additional filter chain that applies only to the actuator
endpoints. It is defined with a request matcher that matches only
actuator endpoints and it has an order of
ManagementServerProperties.BASIC_AUTH_ORDER, which is 5 fewer than
the default SecurityProperties fallback filter, so it is consulted
before the fallback.
If you want your application security rules to apply to the actuator
endpoints, you can add a filter chain that is ordered earlier than the actuator
one and that has a request matcher that includes all actuator
endpoints. If you prefer the default security settings for the
actuator endpoints, the easiest thing is to add your own filter
later than the actuator one, but earlier than the fallback
(for example, ManagementServerProperties.BASIC_AUTH_ORDER + 1), as follows:
@Configuration
@Order(ManagementServerProperties.BASIC_AUTH_ORDER + 1)
public class ApplicationConfigurerAdapter extends WebSecurityConfigurerAdapter {
@Override
protected void configure(HttpSecurity http) throws Exception {
http.antMatcher("/foo/**")
...;
}
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Note
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Spring Security in the web tier is currently tied to the Servlet API, so it is only really applicable when running an application in a servlet container, either embedded or otherwise. It is not, however, tied to Spring MVC or the rest of the Spring web stack, so it can be used in any servlet application — for instance, one using JAX-RS. |
As well as support for securing web applications, Spring Security
offers support for applying access rules to Java method
executions. For Spring Security, this is just a different type of
“protected resource”. For users, it means the access rules are declared
using the same format of ConfigAttribute strings (for example, roles or
expressions) but in a different place in your code. The first step is
to enable method security — for example, in the top level configuration
for our application:
@SpringBootApplication
@EnableGlobalMethodSecurity(securedEnabled = true)
public class SampleSecureApplication {
}Then we can decorate the method resources directly:
@Service
public class MyService {
@Secured("ROLE_USER")
public String secure() {
return "Hello Security";
}
}This example is a service with a secure method. If Spring creates a
@Bean of this type, it is proxied and callers have to
go through a security interceptor before the method is actually
executed. If access is denied, the caller gets an
AccessDeniedException instead of the actual method result.
There are other annotations that you can use on methods to enforce
security constraints, notably @PreAuthorize and @PostAuthorize,
which let you write expressions containing references to method
parameters and return values, respectively.
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Tip
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It is not uncommon to combine Web security and method security. The filter chain provides the user experience features, such as authentication and redirect to login pages and so on, and the method security provides protection at a more granular level. |
Spring Security is fundamentally thread-bound, because it needs to make
the current authenticated principal available to a wide variety of
downstream consumers. The basic building block is the
SecurityContext, which may contain an Authentication (and when a
user is logged in it is an Authentication that is explicitly
authenticated). You can always access and manipulate the
SecurityContext through static convenience methods in
SecurityContextHolder, which, in turn, manipulate a
ThreadLocal. The following example shows such an arrangement:
SecurityContext context = SecurityContextHolder.getContext();
Authentication authentication = context.getAuthentication();
assert(authentication.isAuthenticated);It is not common for user application code to do this, but it can be
useful if you, for instance, need to write a custom authentication
filter (although, even then, there are base classes in Spring Security
that you can use so that you could avoid needing to use the
SecurityContextHolder).
If you need access to the currently authenticated user in a web
endpoint, you can use a method parameter in a @RequestMapping, as follows:
@RequestMapping("/foo")
public String foo(@AuthenticationPrincipal User user) {
... // do stuff with user
}This annotation pulls the current Authentication out of the
SecurityContext and calls the getPrincipal() method on it to yield
the method parameter. The type of the Principal in an
Authentication is dependent on the AuthenticationManager used to
validate the authentication, so this can be a useful little trick to get a type-safe reference to your user data.
If Spring Security is in use, the Principal from the
HttpServletRequest is of type Authentication, so you can also
use that directly:
@RequestMapping("/foo")
public String foo(Principal principal) {
Authentication authentication = (Authentication) principal;
User = (User) authentication.getPrincipal();
... // do stuff with user
}This can sometimes be useful if you need to write code that works when
Spring Security is not in use (you would need to be more defensive
about loading the Authentication class).
Since the SecurityContext is thread-bound, if you want to do any
background processing that calls secure methods (for example, with @Async),
you need to ensure that the context is propagated. This boils down to
wrapping the SecurityContext with the task (Runnable,
Callable, and so on) that is executed in the background. Spring Security
provides some helpers to make this easier, such as wrappers for
Runnable and Callable. To propagate the SecurityContext to
@Async methods, you need to supply an AsyncConfigurer and ensure
the Executor is of the correct type:
@Configuration
public class ApplicationConfiguration extends AsyncConfigurerSupport {
@Override
public Executor getAsyncExecutor() {
return new DelegatingSecurityContextExecutorService(Executors.newFixedThreadPool(5));
}
}