[11] SessionManagementFilter-优快云博客

SessionManagementFilter

介绍

该Filter的主要作用是，当请求经过过滤器是，判断缓存中是否有同一session id的请求，如果不存在则从上下文中获取身份认证信息，并使用SessionAuthenticationStrategy对身份认证信息执行必要的操作，比如重新生成session id来防止session-fixation攻击。过滤器中有2个比较重要的全局变量，分别是SessionAuthenticationStrategy、SecurityContextRepository。SessionAuthenticationStrategy实例的实现是CompositeSessionAuthenticationStrategy，是一个复合型的会话认证策略，默认情况下仅包含一个ChangeSessionIdAuthenticationStrategy，用于改变session的id，可以防止session fixation攻击（建议百度）。SecurityContextRepository用户将SecurityContext上下文保存到session中，但是默认情况SecurityContextRepository的实现是NullSecurityContextRepository，可以通过修改配置为SessionCreationPolicy.IF_REQUIRED，进而给Filter注入HttpSessionSecurityContextRepository的实现，具体如何进行配置以及配置注入的代码分析可以参照SecurityContextPersistenceFilter这篇文章。

代码分析

步骤1

当请求经过SessionManagementFilter时，需要判断缓存中是否已经存在了同一session id的请求了，只有不存在时，才从上下文中取身份认证信息，并通过SessionAuthenticationStrategy对authentication进行认证操作，认证成功后写入到缓存中，代码如下：

public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
        throws IOException, ServletException {
    HttpServletRequest request = (HttpServletRequest) req;
    HttpServletResponse response = (HttpServletResponse) res;
	//过滤器已经应用过了，直接可以进入下个过滤器
    if (request.getAttribute(FILTER_APPLIED) != null) {
        chain.doFilter(request, response);
        return;
    }
	//防止重复进入验证代码，先打个标识
    request.setAttribute(FILTER_APPLIED, Boolean.TRUE);
	//判断repo中是否未存储过同一session id请求
    if (!securityContextRepository.containsContext(request)) {
        //从上下文中或身份认证信息
        Authentication authentication = SecurityContextHolder.getContext()
                .getAuthentication();
		//身份认证信息非空，并且不是匿名认证
        if (authentication != null && !trustResolver.isAnonymous(authentication)) {
            try {
                //这里CompositeSessionAuthenticationStrategy
                //1. ChangeSessionIdAuthenticationStrategy进行验证
                sessionAuthenticationStrategy.onAuthentication(authentication,
                        request, response);
            }
            catch (SessionAuthenticationException e) {
                SecurityContextHolder.clearContext();
                failureHandler.onAuthenticationFailure(request, response, e);

                return;
            }
            //将上下文中的身份认证信息存储的session中
            securityContextRepository.saveContext(SecurityContextHolder.getContext(),
                    request, response);
        }
        else {
            // No security context or authentication present. Check for a session
            // timeout
            if (request.getRequestedSessionId() != null
                    && !request.isRequestedSessionIdValid()) {
                if (invalidSessionStrategy != null) {
                    invalidSessionStrategy
                            .onInvalidSessionDetected(request, response);
                    return;
                }
            }
        }
    }

    chain.doFilter(request, response);
}

步骤2

sessionAuthenticationStrategy.onAuthentication()这行代码笔者有个疑问，sessionAuthenticationStrategy持有的策略默认只有一个ChangeSessionIdAuthenticationStrategy，这个策略使用修改session 的session id的，但是在笔者实际调用中始终hadSessionAlready=false，但笔者认为不应该修改alwaysCreateSession的默认值，因此代码始终执行不到修改session id的代码段，代码如下：

public void onAuthentication(Authentication authentication,
        HttpServletRequest request, HttpServletResponse response) {
    boolean hadSessionAlready = request.getSession(false) != null;
	//如果session不存在，就无法发起session攻击，直接返回即可
    if (!hadSessionAlready && !alwaysCreateSession) {
        // Session fixation isn't a problem if there's no session

        return;
    }

    //session已经存在，一下操作是替换session id
    // Create new session if necessary
    HttpSession session = request.getSession();

    if (hadSessionAlready && request.isRequestedSessionIdValid()) {

        String originalSessionId;
        String newSessionId;
        Object mutex = WebUtils.getSessionMutex(session);
        synchronized (mutex) {
            // We need to migrate to a new session
            originalSessionId = session.getId();

            session = applySessionFixation(request);
            newSessionId = session.getId();
        }

        if (originalSessionId.equals(newSessionId)) {
            logger.warn("Your servlet container did not change the session ID when a new session was created. You will"
                    + " not be adequately protected against session-fixation attacks");
        }

        onSessionChange(originalSessionId, session, authentication);
    }
}

步骤3

HttpSessionSecurityContextRepository#saveContext()方法最终会调到createNewSessionIfAllowed()，有个全局变量十分重要，allowSessionCreation的值决定着session能否成功被创建，我们可以配置SessionCreationPolicy为IF_REQUIRED或者ALWAYS。Spring Security SessionCreationPolicy的配置在ResourceServerConfigurerAdapter（资源服务配置）和AuthorizationServerSecurityConfigurer（认证服务配置），我们在微服务中使用Spring Security时，往往登录、登出、检查TOKEN调认证服务接口，而在其他业务系统中往往引入资源服务认证配置。但是对于授权认证服务，其代码有点不太一样，即使配置了也没有什么用，AuthorizationServerSecurityConfigurer的这段代码又晚于我们的自定义配置执行，因此这里的设置放入shareObject的SecurityContextRepository，最终都会被替换为NullSecurityContextRepository，代码如下：

private HttpSession createNewSessionIfAllowed(SecurityContext context) {
    if (isTransientAuthentication(context.getAuthentication())) {
        return null;
    }

    if (httpSessionExistedAtStartOfRequest) {
        return null;
    }

    if (!allowSessionCreation) {
        return null;
    }
    // Generate a HttpSession only if we need to

    if (contextObject.equals(context)) {
        return null;
    }

    try {
        return request.getSession(true);
    }
    catch (IllegalStateException e) {
    }

    return null;
}

@Override
public void init(HttpSecurity http) throws Exception {

    registerDefaultAuthenticationEntryPoint(http);
    if (passwordEncoder != null) {
        ClientDetailsUserDetailsService clientDetailsUserDetailsService = new ClientDetailsUserDetailsService(clientDetailsService());
        clientDetailsUserDetailsService.setPasswordEncoder(passwordEncoder());
        http.getSharedObject(AuthenticationManagerBuilder.class)
                .userDetailsService(clientDetailsUserDetailsService)
                .passwordEncoder(passwordEncoder());
    }
    else {
        http.userDetailsService(new ClientDetailsUserDetailsService(clientDetailsService()));
    }
    //这里默认使用的是NullSecurityContextRepository
    http.securityContext().securityContextRepository(new NullSecurityContextRepository()).and().csrf().disable()
            .httpBasic().realmName(realm);
    if (sslOnly) {
        http.requiresChannel().anyRequest().requiresSecure();
    }
}