2016年 9月 23日

Spring Security 学习笔记（英文原文）

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Major building blocks of Spring Security

• SecurityContextHolder to provide SecurityContext
• SecurityContext to hold the Authentication
• Authentication to represent the principal
• GrantedAuthority to reflect the application-wide permissions granted to a principal
• UserDetails to provide the user details information to build an Authentication object
• UserDetailsService to create a UserDetailswhen passed in a String-based uername.

Authentication in Spring Security

Consider the following scenario:

1. A user is prompted to log in with a username and password.
2. The system (successfully) verifies that the password is correct for the username.
3. The context information for that user is obtained (their list of roles and so on).
4. A security context is established for the user
5. The user proceeds, potentially to perform some operation which is potentially protected by an access control mechanism which checks the required permissions for the operation against the current security context information.

The first three items constitute the authentication process so we’ll take a look at how these take place within Spring Security.

1. The username and password are obtained and combined into an instance of UsernamePasswordAuthenticationToken (an instance of the Authentication interface, which we saw earlier).
2. The token is passed to an instance of AuthenticationManager for validation.
3. The AuthenticationManager returns a fully populated Authentication instance on successful authentication.
4. The security context is established by calling SecurityContextHolder.getContext().setAuthentication(…), passing in the returned authentication object.

From that point on, the user is considered to be authenticated.

Authentication Abstraction in Spring Security for Web Application

Consider a typical web application’s authentication process:

1. You visit the home page, and click on a link.
2. A request goes to the server, and the server decides that you’ve asked for a protected resource.
3. As you’re not presently authenticated, the server sends back a response indicating that you must authenticate. The response will either be an HTTP response code, or a redirect to a particular web page.
4. Depending on the authentication mechanism, your browser will either redirect to the specific web page so that you can fill out the form, or the browser will somehow retrieve your identity (via a BASIC authentication dialogue box, a cookie, a X.509 certificate etc.).
5. The browser will send back a response to the server. This will either be an HTTP POST containing the contents of the form that you filled out, or an HTTP header containing your authentication details.
6. Next the server will decide whether or not the presented credentials are valid. If they’re valid, the next step will happen. If they’re invalid, usually your browser will be asked to try again (so you return to step two above).
7. The original request that you made to cause the authentication process will be retried. Hopefully you’ve authenticated with sufficient granted authorities to access the protected resource. If you have sufficient access, the request will be successful. Otherwise, you’ll receive back an HTTP error code 403, which means “forbidden”.

The Main participants are

• ExceptionTranslationFilter - a Spring Security filter that has responsibility for detecting any Spring Security exceptions that are thrown (generally be thrown by AbstractSecurityInterceptor). This filter offers the service to translate the exceptions into actual action(url redirection or returning error code 403) or launching an AuthenticationEntryPoint.
• AuthenticationEntryPoint - responsible for step 3 in the above list.
• Authentication Mechansim - responsible for collect authentication details from user (username and password etc.) and buile Authentication request object then presented to AuthenticationManager to valid the authentication request and set back to SecurityContextHolder (step 6 - 7 above)

The Security Filter Chain

Filter delegating chain

DelegatingFilterProxy(in web.xml) -> FilterChainProxy(Bean in context) -> each security filter

Security FIlter ordering

The order that security filters are defined in the chain is very import, should be as follows:

• ChannelProcessingFilter, because it might need to redirect to a different protocol
• SecurityContextPersistenceFilter, so a SecurityContext can be set up in the SecurityContextHolder at the beginning of a web request, and any changes to the SecurityContext can be copied to the HttpSession when the web request ends (ready for use with the next web request)
• ConcurrentSessionFilter, because it uses the SecurityContextHolder functionality and needs to update the SessionRegistry to refect ongoing requests from the principal.
• Authentication processing mechanisms (UsernamePasswordAuthenticationFilter, CasAuthenticationFilter, BasicAuthenticationFilter etc), so that the SecurityContextHolder can be modified to contain a valid Authentication request token.
• The SecurityContextHolderAwareRequestFilter, install Spring Security aware HttpServletRequestWrapper into your servlet container.
• The JaasApiIntegrationFilter, if a JaasAuthenticationToken is in the SecurityContextHolder this will process the FilterChain as the Subject in the JaasAuthenticationToken
• RememberMeAuhenticationFilter, if no earlier authentication processing mechanism updated the SecurityContextHolder, and the request presents a cookie that enables remember-me services to take place, a suitable remembered Authentication object will be put here
• AnonymousAuthenticationFilter, if no earlier authentication processing mechanism updated the SecurityContextHolder, an anonymous Authentication object will be put here.
• ExceptionTranslationFilter to catch any Spring Security exceptions so that either an HTTP error response can be returned or an appropriate AuthenticationEntryPoint can be launched
• FilterSecurityInterceptor to protect web URIs and raise exceptions when access is denied

The complete order of filters can be found at org.springframework.security.config.annotation.web.builders.FilterComparator

Core Security Filters

There are some key filters which will always be used in Spring Security web application.

FilterSecurityInterceptor

• Responsible for handling the security of HTTP resources.
• has a reference to an AuthenticationManager and a AccessDecisionManager
• supplied with configuration attributes (or SecurityMetadataSource) that apply to different HTTP URL request.
• extends AbstractSecurityInterceptor

ExceptionTranslationFilter

• sits above FilterSecurityInterceptor
• doesn’t do any actual security enforcement itself
• handles exceptions thrown by the security interceptors and provide suitable HTTP response
• AuthenticationEntryPoint will be called if the user requests a secure HTTP resource but they are not authenticated. (AuthenticationException will be thrown by a security interceptor futher down the call stace, triggerint the commence method on the entry point.)
• AccessDeniedHandler will be called if the user authenticated but don’t have enough permissions to access the resources. (AccessDeniedException will be thrown by a security interceptor futher down the call stack)
• Also responsible for saving the current request before AuthenticationEntryPoint is invoked and restoring after the user has authenticated. (SavedRequests and RequestCache)

SecurityContextPersistenceFilter

• Responsible for storage of the SecurityContext contents between HTTP requests
• Responsible for clearing the SecurityContextHolder when a request is completed.
• v3.0 onward, SecurityContextRepository is used for different storage strategy.

Authorization in Spring Security

The main interface responsible for making access-control decisions is AccessDecisionManager, it decides the principal represented by an Authentication object can or can not access the “secure object” using a list of security metadata attributes which apply for the object.

Secure Object

“secure object” refers to any object that can have security (such as an authorization decision) applied to it, for example: method invocations and web requests.

workflow

Each supported secure object type has its own implementation of AbstractSecurityInterceptor, but the workflow for handling secure object requests is consistent:

1. Look up the “configuration attributes” associated with the present request
2. Submitting the secure object, current Authentication and configuration attributes to the AccessDecisionManager for an authorization decision
3. Optionally change the Authentication under which the invocation takes place
4. Allow the secure object invocation to proceed (assuming access was granted)
5. Call the AfterInvocationManager if configured, once the invocation has returned. If the invocation raised an exception, the AfterInvocationManager will not be invoked.

configuration attributes

Can be thought of as a String that has special meaning to the classes used by AbstractSecurityInterceptor, represented by the interface ConfigAttribute within the framework. May be simple role names or have more complex meaning, depending on the implementation of AccessDecisionManager.

AbstractSecurityInterceptor uses SecurityMetadataSource to look up the attributes for a secure object. Usually configuration attributes will be entered as annotations on secured methods or as access attributes on secured URLs. For exmple, <intercept-url pattern='/secure/**' access='ROLE_A,ROLE_B' /> is saying that the configuration attributes ROLE_A and ROLE_B apply to web requests matching the given pattern.

Authorities

All Authentication implementations store a list of GrantedAuthority objects, represent the authorities that have been granted to the principal, and are inserted by AuthenticationManager and are later read by AccessDecisionManager when making authorization decisions.

AccessDecisionManager

• decide all the authorization decisions for the “secured object”. Throw AccessDeniedException if access is denied.
• supports(ConfigAttribute) is used by AbstractSecurityInterceptor at startup time to determine if the AccessDecisionManager can process the passed ConfigAttribute.
• supports(Class) is used by a security interceptor implementation to ensure the AccessDecisionManager supports the type of secure object.
• AccessDecisionManager delegates authorization decisions to AccessDecisionVoters

Voting-Based Implementations

A series of AccessDecisionVoter implementations are polled on authorization decision, then AccessDecisionManager decides whether or not to throw an AccessDeniedException based on its assessment of the votes.

Concrete implementations of AccessDecisionVoter returns int result, possible values are: (All are reflected in the AccessDecisionVoter static fields)

• ACCESS_ABSTAIN (a voting implementation has no opinion on an authorization decision),
• ACCESS_DENIED,
• ACCESS_GRANTED

There are 3 concrete AccessDecisionManagers that tally the votes:

• AffirmativeBased - will grant access if one or more ACCESS_GRANTED votes were received.
• ConsensusBased - will grant or deny access based on the consensus of non-abstain votes
• UnanimousBased - expects unanimous ACCESS_GRANTED votes in order to grant access, ignoring abstains. It will deny access if there is any ACCESS_DENIED vote ( Properties are provided to controls the behaviour if all votes are abstain or in the event of an equality of votes. )

Voters

• RoleVoter - most commonly used, vote against ROLE_ prefixed ConfigAttribute
• AuthenticatedVoter - used to differentiate between anonymous, fully-authenticated and remember-me authenticated users.
• Custom voters - you can implement a custom AccessDecisionVoter