• A Guide to CSRF Protection in Spring Security

1. CsrfFilter.

• CsrfFilter
  • CsrfFilter는 Synchronizer Token 패턴을 사용하여 CSRF 공격을 방지함.
  • 개발자는 상태를 변경하는 모든 요청에 대해 CsrfFilter가 호출되도록 해야함.
  • 일반적으로 CsrfTokenRepository를 구현한 HttpSessionCsrfTokenRepository를 사용하여 CSRF 토큰을 HttpSession에 저장하는 방식을 사용함.
• Cross Site Request Forgery(CSRF)
  • CSRF(Cross-Site Request Forgery, 사이트 간 요청 위조) 공격은 사용자가 의도하지 않은 요청을 강제로 실행하게 만드는 공격 기법.
  • 공격자는 사용자의 인증된 세션을 악용해서 사용자가 원치 않는 액션을 실행하게 함.

• CSRF 토큰 클라이언트측으로 발급해주기.
  • 기본 설정은 SSR(Server-Side Rendering, 서버 측에서 페이지를 렌더링하는 방식) 세션 방식으로 되어있음.
    (STATELESS REST API에서는 사용할 일이 거의 없기 때문)
  • Controller에서 View 응답시 HTML form 영역에 서버에 저장되어 있는 _csrf 토큰 값을 넣어주면 됨.

• STATELESS한 API 서버에서는 서버 세션을 사용하지 않기 때문에, CSRF 공격 위험이 낮음.
  • CSRF 공격은 일반적으로 서버 세션을 이용한 인증 방식에서 발생할 수 있기 때문.
• JWT와 같은 토큰 방식을 사용하고 해당 토큰을 쿠키에 저장할 경우 CSRF 공격의 위험이 있을 수 있기 때문에 활성화하는 게 좋음.

1-1. CsrfTokenRepository.

package org.springframework.security.web.csrf;

import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;
import jakarta.servlet.http.HttpSession;

/**
 * An API to allow changing the method in which the expected {@link CsrfToken} is
 * associated to the {@link HttpServletRequest}. For example, it may be stored in
 * {@link HttpSession}.
 *
 * @author Rob Winch
 * @author Steve Riesenberg
 * @since 3.2
 * @see HttpSessionCsrfTokenRepository
 */
public interface CsrfTokenRepository {

	/**
	 * Generates a {@link CsrfToken}
	 * @param request the {@link HttpServletRequest} to use
	 * @return the {@link CsrfToken} that was generated. Cannot be null.
	 */
	CsrfToken generateToken(HttpServletRequest request);

	/**
	 * Saves the {@link CsrfToken} using the {@link HttpServletRequest} and
	 * {@link HttpServletResponse}. If the {@link CsrfToken} is null, it is the same as
	 * deleting it.
	 * @param token the {@link CsrfToken} to save or null to delete
	 * @param request the {@link HttpServletRequest} to use
	 * @param response the {@link HttpServletResponse} to use
	 */
	void saveToken(CsrfToken token, HttpServletRequest request, HttpServletResponse response);

	/**
	 * Loads the expected {@link CsrfToken} from the {@link HttpServletRequest}
	 * @param request the {@link HttpServletRequest} to use
	 * @return the {@link CsrfToken} or null if none exists
	 */
	CsrfToken loadToken(HttpServletRequest request);

	/**
	 * Defers loading the {@link CsrfToken} using the {@link HttpServletRequest} and
	 * {@link HttpServletResponse} until it is needed by the application.
	 * <p>
	 * The returned {@link DeferredCsrfToken} is cached to allow subsequent calls to
	 * {@link DeferredCsrfToken#get()} to return the same {@link CsrfToken} without the
	 * cost of loading or generating the token again.
	 * @param request the {@link HttpServletRequest} to use
	 * @param response the {@link HttpServletResponse} to use
	 * @return a {@link DeferredCsrfToken} that will load the {@link CsrfToken}
	 * @since 5.8
	 */
	default DeferredCsrfToken loadDeferredToken(HttpServletRequest request, HttpServletResponse response) {
		return new RepositoryDeferredCsrfToken(this, request, response);
	}

}

• CSRF 토큰의 생성 및 관리는 CsrfTokenRepository 인터페이스를 구현한 클래스에게 위임 시킴.
  • HttpSessionCsrfTokenRepository.
    • 서버 세션에 토큰을 저장하고 관리함. (default)
  • CookieCsrfTokenRepository.
    • 쿠키에 토큰을 저장하고 관리함.
  • 직접 구현도 가능.
• CSRF 토큰 저장소 설정

http
        .csrf((csrf) -> csrf.csrfTokenRepository(new HttpSessionCsrfTokenRepository()));

1-1-1. HttpSessionCsrfTokenRepository.

package org.springframework.security.web.csrf;

import java.util.UUID;

import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;
import jakarta.servlet.http.HttpSession;

import org.springframework.util.Assert;

/**
 * A {@link CsrfTokenRepository} that stores the {@link CsrfToken} in the
 * {@link HttpSession}.
 *
 * @author Rob Winch
 * @since 3.2
 */
public final class HttpSessionCsrfTokenRepository implements CsrfTokenRepository {

	private static final String DEFAULT_CSRF_PARAMETER_NAME = "_csrf";

	private static final String DEFAULT_CSRF_HEADER_NAME = "X-CSRF-TOKEN";

	private static final String DEFAULT_CSRF_TOKEN_ATTR_NAME = HttpSessionCsrfTokenRepository.class.getName()
		.concat(".CSRF_TOKEN");

	private String parameterName = DEFAULT_CSRF_PARAMETER_NAME;

	private String headerName = DEFAULT_CSRF_HEADER_NAME;

	private String sessionAttributeName = DEFAULT_CSRF_TOKEN_ATTR_NAME;

	@Override
	public void saveToken(CsrfToken token, HttpServletRequest request, HttpServletResponse response) {
		if (token == null) {
			HttpSession session = request.getSession(false);
			if (session != null) {
				session.removeAttribute(this.sessionAttributeName);
			}
		}
		else {
			HttpSession session = request.getSession();
			session.setAttribute(this.sessionAttributeName, token);
		}
	}

	@Override
	public CsrfToken loadToken(HttpServletRequest request) {
		HttpSession session = request.getSession(false);
		if (session == null) {
			return null;
		}
		return (CsrfToken) session.getAttribute(this.sessionAttributeName);
	}

	@Override
	public CsrfToken generateToken(HttpServletRequest request) {
		return new DefaultCsrfToken(this.headerName, this.parameterName, createNewToken());
	}

	/**
	 * Sets the {@link HttpServletRequest} parameter name that the {@link CsrfToken} is
	 * expected to appear on
	 * @param parameterName the new parameter name to use
	 */
	public void setParameterName(String parameterName) {
		Assert.hasLength(parameterName, "parameterName cannot be null or empty");
		this.parameterName = parameterName;
	}

	/**
	 * Sets the header name that the {@link CsrfToken} is expected to appear on and the
	 * header that the response will contain the {@link CsrfToken}.
	 * @param headerName the new header name to use
	 */
	public void setHeaderName(String headerName) {
		Assert.hasLength(headerName, "headerName cannot be null or empty");
		this.headerName = headerName;
	}

	/**
	 * Sets the {@link HttpSession} attribute name that the {@link CsrfToken} is stored in
	 * @param sessionAttributeName the new attribute name to use
	 */
	public void setSessionAttributeName(String sessionAttributeName) {
		Assert.hasLength(sessionAttributeName, "sessionAttributename cannot be null or empty");
		this.sessionAttributeName = sessionAttributeName;
	}

	private String createNewToken() {
		return UUID.randomUUID().toString();
	}

}

1-1-2. CookieCsrfTokenRepository

package org.springframework.security.web.csrf;

import java.util.UUID;
import java.util.function.Consumer;

import jakarta.servlet.http.Cookie;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;

import org.springframework.http.HttpHeaders;
import org.springframework.http.ResponseCookie;
import org.springframework.util.Assert;
import org.springframework.util.StringUtils;
import org.springframework.web.util.WebUtils;

/**
 * A {@link CsrfTokenRepository} that persists the CSRF token in a cookie named
 * "XSRF-TOKEN" and reads from the header "X-XSRF-TOKEN" following the conventions of
 * AngularJS. When using with AngularJS be sure to use {@link #withHttpOnlyFalse()}.
 *
 * @author Rob Winch
 * @author Steve Riesenberg
 * @author Alex Montoya
 * @since 4.1
 */
public final class CookieCsrfTokenRepository implements CsrfTokenRepository {

	static final String DEFAULT_CSRF_COOKIE_NAME = "XSRF-TOKEN";

	static final String DEFAULT_CSRF_PARAMETER_NAME = "_csrf";

	static final String DEFAULT_CSRF_HEADER_NAME = "X-XSRF-TOKEN";

	private static final String CSRF_TOKEN_REMOVED_ATTRIBUTE_NAME = CookieCsrfTokenRepository.class.getName()
		.concat(".REMOVED");

	private String parameterName = DEFAULT_CSRF_PARAMETER_NAME;

	private String headerName = DEFAULT_CSRF_HEADER_NAME;

	private String cookieName = DEFAULT_CSRF_COOKIE_NAME;

	private boolean cookieHttpOnly = true;

	private String cookiePath;

	private String cookieDomain;

	private Boolean secure;

	private int cookieMaxAge = -1;

	private Consumer<ResponseCookie.ResponseCookieBuilder> cookieCustomizer = (builder) -> {
	};

	/**
	 * Add a {@link Consumer} for a {@code ResponseCookieBuilder} that will be invoked for
	 * each cookie being built, just before the call to {@code build()}.
	 * @param cookieCustomizer consumer for a cookie builder
	 * @since 6.1
	 */
	public void setCookieCustomizer(Consumer<ResponseCookie.ResponseCookieBuilder> cookieCustomizer) {
		Assert.notNull(cookieCustomizer, "cookieCustomizer must not be null");
		this.cookieCustomizer = cookieCustomizer;
	}

	@Override
	public CsrfToken generateToken(HttpServletRequest request) {
		return new DefaultCsrfToken(this.headerName, this.parameterName, createNewToken());
	}

	@Override
	public void saveToken(CsrfToken token, HttpServletRequest request, HttpServletResponse response) {
		String tokenValue = (token != null) ? token.getToken() : "";

		ResponseCookie.ResponseCookieBuilder cookieBuilder = ResponseCookie.from(this.cookieName, tokenValue)
			.secure((this.secure != null) ? this.secure : request.isSecure())
			.path(StringUtils.hasLength(this.cookiePath) ? this.cookiePath : this.getRequestContext(request))
			.maxAge((token != null) ? this.cookieMaxAge : 0)
			.httpOnly(this.cookieHttpOnly)
			.domain(this.cookieDomain);

		this.cookieCustomizer.accept(cookieBuilder);

		ResponseCookie responseCookie = cookieBuilder.build();
		if (!StringUtils.hasLength(responseCookie.getSameSite())) {
			Cookie cookie = mapToCookie(responseCookie);
			response.addCookie(cookie);
		}
		else if (request.getServletContext().getMajorVersion() > 5) {
			Cookie cookie = mapToCookie(responseCookie);
			response.addCookie(cookie);
		}
		else {
			response.addHeader(HttpHeaders.SET_COOKIE, responseCookie.toString());
		}

		// Set request attribute to signal that response has blank cookie value,
		// which allows loadToken to return null when token has been removed
		if (!StringUtils.hasLength(tokenValue)) {
			request.setAttribute(CSRF_TOKEN_REMOVED_ATTRIBUTE_NAME, Boolean.TRUE);
		}
		else {
			request.removeAttribute(CSRF_TOKEN_REMOVED_ATTRIBUTE_NAME);
		}
	}

	@Override
	public CsrfToken loadToken(HttpServletRequest request) {
		// Return null when token has been removed during the current request
		// which allows loadDeferredToken to re-generate the token
		if (Boolean.TRUE.equals(request.getAttribute(CSRF_TOKEN_REMOVED_ATTRIBUTE_NAME))) {
			return null;
		}
		Cookie cookie = WebUtils.getCookie(request, this.cookieName);
		if (cookie == null) {
			return null;
		}
		String token = cookie.getValue();
		if (!StringUtils.hasLength(token)) {
			return null;
		}
		return new DefaultCsrfToken(this.headerName, this.parameterName, token);
	}

	/**
	 * Sets the name of the HTTP request parameter that should be used to provide a token.
	 * @param parameterName the name of the HTTP request parameter that should be used to
	 * provide a token
	 */
	public void setParameterName(String parameterName) {
		Assert.notNull(parameterName, "parameterName cannot be null");
		this.parameterName = parameterName;
	}

	/**
	 * Sets the name of the HTTP header that should be used to provide the token.
	 * @param headerName the name of the HTTP header that should be used to provide the
	 * token
	 */
	public void setHeaderName(String headerName) {
		Assert.notNull(headerName, "headerName cannot be null");
		this.headerName = headerName;
	}

	/**
	 * Sets the name of the cookie that the expected CSRF token is saved to and read from.
	 * @param cookieName the name of the cookie that the expected CSRF token is saved to
	 * and read from
	 */
	public void setCookieName(String cookieName) {
		Assert.notNull(cookieName, "cookieName cannot be null");
		this.cookieName = cookieName;
	}

	/**
	 * @deprecated Use {@link #setCookieCustomizer(Consumer)} instead.
	 */
	@Deprecated(since = "6.1")
	public void setCookieHttpOnly(boolean cookieHttpOnly) {
		this.cookieHttpOnly = cookieHttpOnly;
	}

	private String getRequestContext(HttpServletRequest request) {
		String contextPath = request.getContextPath();
		return (contextPath.length() > 0) ? contextPath : "/";
	}

	/**
	 * Factory method to conveniently create an instance that creates cookies where
	 * {@link Cookie#isHttpOnly()} is set to false.
	 * @return an instance of CookieCsrfTokenRepository that creates cookies where
	 * {@link Cookie#isHttpOnly()} is set to false.
	 */
	public static CookieCsrfTokenRepository withHttpOnlyFalse() {
		CookieCsrfTokenRepository result = new CookieCsrfTokenRepository();
		result.cookieHttpOnly = false;
		return result;
	}

	private String createNewToken() {
		return UUID.randomUUID().toString();
	}

	private Cookie mapToCookie(ResponseCookie responseCookie) {
		Cookie cookie = new Cookie(responseCookie.getName(), responseCookie.getValue());
		cookie.setSecure(responseCookie.isSecure());
		cookie.setPath(responseCookie.getPath());
		cookie.setMaxAge((int) responseCookie.getMaxAge().getSeconds());
		cookie.setHttpOnly(responseCookie.isHttpOnly());
		if (StringUtils.hasLength(responseCookie.getDomain())) {
			cookie.setDomain(responseCookie.getDomain());
		}
		if (StringUtils.hasText(responseCookie.getSameSite())) {
			cookie.setAttribute("SameSite", responseCookie.getSameSite());
		}
		return cookie;
	}

	/**
	 * Set the path that the Cookie will be created with. This will override the default
	 * functionality which uses the request context as the path.
	 * @param path the path to use
	 */
	public void setCookiePath(String path) {
		this.cookiePath = path;
	}

	/**
	 * Get the path that the CSRF cookie will be set to.
	 * @return the path to be used.
	 */
	public String getCookiePath() {
		return this.cookiePath;
	}

	/**
	 * @since 5.2
	 * @deprecated Use {@link #setCookieCustomizer(Consumer)} instead.
	 */
	@Deprecated(since = "6.1")
	public void setCookieDomain(String cookieDomain) {
		this.cookieDomain = cookieDomain;
	}

	/**
	 * @since 5.4
	 * @deprecated Use {@link #setCookieCustomizer(Consumer)} instead.
	 */
	@Deprecated(since = "6.1")
	public void setSecure(Boolean secure) {
		this.secure = secure;
	}

	/**
	 * @since 5.5
	 * @deprecated Use {@link #setCookieCustomizer(Consumer)} instead.
	 */
	@Deprecated(since = "6.1")
	public void setCookieMaxAge(int cookieMaxAge) {
		Assert.isTrue(cookieMaxAge != 0, "cookieMaxAge cannot be zero");
		this.cookieMaxAge = cookieMaxAge;
	}

}

1-2. CsrfFilter.

package org.springframework.security.web.csrf;

import java.io.IOException;
import java.security.MessageDigest;
import java.util.Arrays;
import java.util.HashSet;

import jakarta.servlet.FilterChain;
import jakarta.servlet.ServletException;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;
import jakarta.servlet.http.HttpSession;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.core.log.LogMessage;
import org.springframework.security.access.AccessDeniedException;
import org.springframework.security.crypto.codec.Utf8;
import org.springframework.security.web.access.AccessDeniedHandler;
import org.springframework.security.web.access.AccessDeniedHandlerImpl;
import org.springframework.security.web.util.UrlUtils;
import org.springframework.security.web.util.matcher.RequestMatcher;
import org.springframework.util.Assert;
import org.springframework.web.filter.OncePerRequestFilter;

/**
 * <p>
 * Applies
 * <a href="https://www.owasp.org/index.php/Cross-Site_Request_Forgery_(CSRF)" >CSRF</a>
 * protection using a synchronizer token pattern. Developers are required to ensure that
 * {@link CsrfFilter} is invoked for any request that allows state to change. Typically
 * this just means that they should ensure their web application follows proper REST
 * semantics (i.e. do not change state with the HTTP methods GET, HEAD, TRACE, OPTIONS).
 * </p>
 *
 * <p>
 * Typically the {@link CsrfTokenRepository} implementation chooses to store the
 * {@link CsrfToken} in {@link HttpSession} with {@link HttpSessionCsrfTokenRepository}.
 * This is preferred to storing the token in a cookie which can be modified by a client
 * application.
 * </p>
 *
 * @author Rob Winch
 * @author Steve Riesenberg
 * @since 3.2
 */
public final class CsrfFilter extends OncePerRequestFilter {

	/**
	 * The default {@link RequestMatcher} that indicates if CSRF protection is required or
	 * not. The default is to ignore GET, HEAD, TRACE, OPTIONS and process all other
	 * requests.
	 */
	public static final RequestMatcher DEFAULT_CSRF_MATCHER = new DefaultRequiresCsrfMatcher();

	/**
	 * The attribute name to use when marking a given request as one that should not be
	 * filtered.
	 * <p>
	 * To use, set the attribute on your {@link HttpServletRequest}: <pre>
	 * 	CsrfFilter.skipRequest(request);
	 * </pre>
	 */
	private static final String SHOULD_NOT_FILTER = "SHOULD_NOT_FILTER" + CsrfFilter.class.getName();

	private final Log logger = LogFactory.getLog(getClass());

	private final CsrfTokenRepository tokenRepository;

	private RequestMatcher requireCsrfProtectionMatcher = DEFAULT_CSRF_MATCHER;

	private AccessDeniedHandler accessDeniedHandler = new AccessDeniedHandlerImpl();

	private CsrfTokenRequestHandler requestHandler = new XorCsrfTokenRequestAttributeHandler();

	/**
	 * Creates a new instance.
	 * @param tokenRepository the {@link CsrfTokenRepository} to use
	 */
	public CsrfFilter(CsrfTokenRepository tokenRepository) {
		Assert.notNull(tokenRepository, "tokenRepository cannot be null");
		this.tokenRepository = tokenRepository;
	}

	@Override
	protected boolean shouldNotFilter(HttpServletRequest request) throws ServletException {
		return Boolean.TRUE.equals(request.getAttribute(SHOULD_NOT_FILTER));
	}

	@Override
	protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
			throws ServletException, IOException {
		DeferredCsrfToken deferredCsrfToken = this.tokenRepository.loadDeferredToken(request, response);
		request.setAttribute(DeferredCsrfToken.class.getName(), deferredCsrfToken);
		this.requestHandler.handle(request, response, deferredCsrfToken::get);
		if (!this.requireCsrfProtectionMatcher.matches(request)) {
			if (this.logger.isTraceEnabled()) {
				this.logger.trace("Did not protect against CSRF since request did not match "
						+ this.requireCsrfProtectionMatcher);
			}
			filterChain.doFilter(request, response);
			return;
		}
		CsrfToken csrfToken = deferredCsrfToken.get();
		String actualToken = this.requestHandler.resolveCsrfTokenValue(request, csrfToken);
		if (!equalsConstantTime(csrfToken.getToken(), actualToken)) {
			boolean missingToken = deferredCsrfToken.isGenerated();
			this.logger
				.debug(LogMessage.of(() -> "Invalid CSRF token found for " + UrlUtils.buildFullRequestUrl(request)));
			AccessDeniedException exception = (!missingToken) ? new InvalidCsrfTokenException(csrfToken, actualToken)
					: new MissingCsrfTokenException(actualToken);
			this.accessDeniedHandler.handle(request, response, exception);
			return;
		}
		filterChain.doFilter(request, response);
	}

	public static void skipRequest(HttpServletRequest request) {
		request.setAttribute(SHOULD_NOT_FILTER, Boolean.TRUE);
	}

	/**
	 * Specifies a {@link RequestMatcher} that is used to determine if CSRF protection
	 * should be applied. If the {@link RequestMatcher} returns true for a given request,
	 * then CSRF protection is applied.
	 *
	 * <p>
	 * The default is to apply CSRF protection for any HTTP method other than GET, HEAD,
	 * TRACE, OPTIONS.
	 * </p>
	 * @param requireCsrfProtectionMatcher the {@link RequestMatcher} used to determine if
	 * CSRF protection should be applied.
	 */
	public void setRequireCsrfProtectionMatcher(RequestMatcher requireCsrfProtectionMatcher) {
		Assert.notNull(requireCsrfProtectionMatcher, "requireCsrfProtectionMatcher cannot be null");
		this.requireCsrfProtectionMatcher = requireCsrfProtectionMatcher;
	}

	/**
	 * Specifies a {@link AccessDeniedHandler} that should be used when CSRF protection
	 * fails.
	 *
	 * <p>
	 * The default is to use AccessDeniedHandlerImpl with no arguments.
	 * </p>
	 * @param accessDeniedHandler the {@link AccessDeniedHandler} to use
	 */
	public void setAccessDeniedHandler(AccessDeniedHandler accessDeniedHandler) {
		Assert.notNull(accessDeniedHandler, "accessDeniedHandler cannot be null");
		this.accessDeniedHandler = accessDeniedHandler;
	}

	/**
	 * Specifies a {@link CsrfTokenRequestHandler} that is used to make the
	 * {@link CsrfToken} available as a request attribute.
	 *
	 * <p>
	 * The default is {@link XorCsrfTokenRequestAttributeHandler}.
	 * </p>
	 * @param requestHandler the {@link CsrfTokenRequestHandler} to use
	 * @since 5.8
	 */
	public void setRequestHandler(CsrfTokenRequestHandler requestHandler) {
		Assert.notNull(requestHandler, "requestHandler cannot be null");
		this.requestHandler = requestHandler;
	}

	/**
	 * Constant time comparison to prevent against timing attacks.
	 * @param expected
	 * @param actual
	 * @return
	 */
	private static boolean equalsConstantTime(String expected, String actual) {
		if (expected == actual) {
			return true;
		}
		if (expected == null || actual == null) {
			return false;
		}
		// Encode after ensure that the string is not null
		byte[] expectedBytes = Utf8.encode(expected);
		byte[] actualBytes = Utf8.encode(actual);
		return MessageDigest.isEqual(expectedBytes, actualBytes);
	}

	private static final class DefaultRequiresCsrfMatcher implements RequestMatcher {

		private final HashSet<String> allowedMethods = new HashSet<>(Arrays.asList("GET", "HEAD", "TRACE", "OPTIONS"));

		@Override
		public boolean matches(HttpServletRequest request) {
			return !this.allowedMethods.contains(request.getMethod());
		}

		@Override
		public String toString() {
			return "CsrfNotRequired " + this.allowedMethods;
		}

	}

}