CSRF（Cross-site request forgery）跨站请求伪造，也被称为One Click Attack或者Session Riding，通常缩写为CSRF或XSRF，是一种对网站的恶意利用。尽管听起来像跨站脚本（XSS），但它与XSS非常不同，XSS利用站点内的信任用户，而CSRF则通过伪装成受信任用户的请求来利用受信任的网站。与XSS攻击相比，CSRF攻击往往不大流行（因此对其进行防范的资源也相当稀少）和难以防范，所以被认为比XSS更具危险性。
CSRF是一种依赖web浏览器的、被混淆过的代理人攻击（deputy attack）。
如何防御
使用POST请求时，确实避免了如img、script、iframe等标签自动发起GET请求的问题，但这并不能杜绝CSRF攻击的发生。一些恶意网站会通过表单的形式构造攻击请求
public final class CsrfFilter extends OncePerRequestFilter {
public static final RequestMatcher DEFAULT_CSRF_MATCHER = new
CsrfFilter.DefaultRequiresCsrfMatcher();
private final Log logger = LogFactory.getLog(this.getClass());
private final CsrfTokenRepository tokenRepository;
private RequestMatcher requireCsrfProtectionMatcher;
private AccessDeniedHandler accessDeniedHandler;
public CsrfFilter(CsrfTokenRepository csrfTokenRepository) {
this.requireCsrfProtectionMatcher = DEFAULT_CSRF_MATCHER;
this.accessDeniedHandler = new AccessDeniedHandlerImpl();
Assert.notNull(csrfTokenRepository, "csrfTokenRepository cannot be null");
this.tokenRepository = csrfTokenRepository;
}
//通过这里可以看出SpringSecurity的csrf机制把请求方式分成两类来处理
protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response,
FilterChain filterChain) throws ServletException, IOException {
request.setAttribute(HttpServletResponse.class.getName(), response);
CsrfToken csrfToken = this.tokenRepository.loadToken(request);
boolean missingToken = csrfToken == null;
if (missingToken) {
csrfToken = this.tokenRepository.generateToken(request);
this.tokenRepository.saveToken(csrfToken, request, response);
}
request.setAttribute(CsrfToken.class.getName(), csrfToken);
request.setAttribute(csrfToken.getParameterName(), csrfToken);
//第一类："GET", "HEAD", "TRACE", "OPTIONS"四类请求可以直接通过
if (!this.requireCsrfProtectionMatcher.matches(request)) {
filterChain.doFilter(request, response);
} else {
//第二类：除去上面四类，包括POST都要被验证携带token才能通过
String actualToken = request.getHeader(csrfToken.getHeaderName());
if (actualToken == null) {
actualToken = request.getParameter(csrfToken.getParameterName());
}
if (!csrfToken.getToken().equals(actualToken)) {
if (this.logger.isDebugEnabled()) {
this.logger.debug("Invalid CSRF token found for " +
UrlUtils.buildFullRequestUrl(request));
}
if (missingToken) {
this.accessDeniedHandler.handle(request, response, new
MissingCsrfTokenException(actualToken));
} else {
this.accessDeniedHandler.handle(request, response, new
InvalidCsrfTokenException(csrfToken, actualToken));
}
} else {
filterChain.doFilter(request, response);
}
}
}
public void setRequireCsrfProtectionMatcher(RequestMatcher requireCsrfProtectionMatcher) {
Assert.notNull(requireCsrfProtectionMatcher, "requireCsrfProtectionMatcher cannot be
null");
this.requireCsrfProtectionMatcher = requireCsrfProtectionMatcher;
}
public void setAccessDeniedHandler(AccessDeniedHandler accessDeniedHandler) {
Assert.notNull(accessDeniedHandler, "accessDeniedHandler cannot be null");
this.accessDeniedHandler = accessDeniedHandler;
}
private static final class DefaultRequiresCsrfMatcher implements RequestMatcher {
private final HashSet<String> allowedMethods;
private DefaultRequiresCsrfMatcher() {
this.allowedMethods = new HashSet(Arrays.asList("GET", "HEAD", "TRACE", "OPTIONS"));
}
public boolean matches(HttpServletRequest request) {
return !this.allowedMethods.contains(request.getMethod());
}
}
}
