Apply extra checks to static resource handling

- remove leading '/' and control chars
- improve url and relative path checks
- account for URL encoding
- add isResourceUnderLocation final verification

Issue: SPR-12354

spring-webmvc/src/main/java/org/springframework/web/servlet/resource/PathResourceResolver.java

@@ -17,10 +17,14 @@
 package org.springframework.web.servlet.resource;
 
 import java.io.IOException;
+import java.net.URLDecoder;
 import java.util.List;
+
 import javax.servlet.http.HttpServletRequest;
 
+import org.springframework.core.io.ClassPathResource;
 import org.springframework.core.io.Resource;
+import org.springframework.core.io.UrlResource;
 
 /**
  * A simple {@code ResourceResolver} that tries to find a resource under the given
@@ -36,6 +40,35 @@ import org.springframework.core.io.Resource;
  */
 public class PathResourceResolver extends AbstractResourceResolver {
 
+	private Resource[] allowedLocations;
+
+
+	/**
+	 * By default when a Resource is found, the path of the resolved resource is
+	 * compared to ensure it's under the input location where it was found.
+	 * However sometimes that may not be the case, e.g. when
+	 * {@link org.springframework.web.servlet.resource.CssLinkResourceTransformer}
+	 * resolves public URLs of links it contains, the CSS file is the location
+	 * and the resources being resolved are css files, images, fonts and others
+	 * located in adjacent or parent directories.
+	 * <p>This property allows configuring a complete list of locations under
+	 * which resources must be so that if a resource is not under the location
+	 * relative to which it was found, this list may be checked as well.
+	 * <p>By default {@link ResourceHttpRequestHandler} initializes this property
+	 * to match its list of locations.
+	 * @param locations the list of allowed locations
+	 * @since 4.1.2
+	 * @see ResourceHttpRequestHandler#initAllowedLocations()
+	 */
+	public void setAllowedLocations(Resource... locations) {
+		this.allowedLocations = locations;
+	}
+
+	public Resource[] getAllowedLocations() {
+		return this.allowedLocations;
+	}
+
+
 	@Override
 	protected Resource resolveResourceInternal(HttpServletRequest request, String requestPath,
 			List<? extends Resource> locations, ResourceResolverChain chain) {
@@ -84,7 +117,79 @@ public class PathResourceResolver extends AbstractResourceResolver {
 	 */
 	protected Resource getResource(String resourcePath, Resource location) throws IOException {
 		Resource resource = location.createRelative(resourcePath);
-		return (resource.exists() && resource.isReadable() ? resource : null);
+		if (resource.exists() && resource.isReadable()) {
+			if (checkResource(resource, location)) {
+				return resource;
+			}
+			else {
+				if (logger.isTraceEnabled()) {
+					logger.trace("resourcePath=\"" + resourcePath + "\" was successfully resolved " +
+							"but resource=\"" +	resource.getURL() + "\" is neither under the " +
+							"current location=\"" + location.getURL() + "\" nor under any of the " +
+							"allowed locations=" + getAllowedLocations());
+				}
+			}
+		}
+		return null;
+	}
+
+	/**
+	 * Perform additional checks on a resolved resource beyond checking whether
+	 * the resources exists and is readable. The default implementation also
+	 * verifies the resource is either under the location relative to which it
+	 * was found or is under one of the {@link #setAllowedLocations allowed
+	 * locations}.
+	 * @param resource the resource to check
+	 * @param location the location relative to which the resource was found
+	 * @return "true" if resource is in a valid location, "false" otherwise.
+	 * @since 4.1.2
+	 */
+	protected boolean checkResource(Resource resource, Resource location) throws IOException {
+		if (isResourceUnderLocation(resource, location)) {
+			return true;
+		}
+		if (getAllowedLocations() != null) {
+			for (Resource current : getAllowedLocations()) {
+				if (isResourceUnderLocation(resource, current)) {
+					return true;
+				}
+			}
+		}
+		return false;
+	}
+
+	private boolean isResourceUnderLocation(Resource resource, Resource location) throws IOException {
+		if (!resource.getClass().equals(location.getClass())) {
+			return false;
+		}
+		String resourcePath;
+		String locationPath;
+		if (resource instanceof ClassPathResource) {
+			resourcePath = ((ClassPathResource) resource).getPath();
+			locationPath = ((ClassPathResource) location).getPath();
+		}
+		else if (resource instanceof UrlResource) {
+			resourcePath = resource.getURL().toExternalForm();
+			locationPath = location.getURL().toExternalForm();
+		}
+		else {
+			resourcePath = resource.getURL().getPath();
+			locationPath = location.getURL().getPath();
+		}
+		locationPath = (locationPath.endsWith("/") || locationPath.isEmpty() ? locationPath : locationPath + "/");
+		if (!resourcePath.startsWith(locationPath)) {
+			return false;
+		}
+		if (resourcePath.contains("%")) {
+			// Use URLDecoder (vs UriUtils) to preserve potentially decoded UTF-8 chars
+			if (URLDecoder.decode(resourcePath, "UTF-8").contains("../")) {
+				if (logger.isTraceEnabled()) {
+					logger.trace("Resolved resource path contains \"../\" after decoding: " + resourcePath);
+				}
+				return false;
+			}
+		}
+		return true;
 	}
 
 }

spring-webmvc/src/main/java/org/springframework/web/servlet/resource/ResourceHttpRequestHandler.java

@@ -18,8 +18,10 @@ package org.springframework.web.servlet.resource;
 
 import java.io.IOException;
 import java.io.InputStream;
+import java.net.URLDecoder;
 import java.util.ArrayList;
 import java.util.List;
+
 import javax.activation.FileTypeMap;
 import javax.activation.MimetypesFileTypeMap;
 import javax.servlet.ServletException;
@@ -28,7 +30,6 @@ import javax.servlet.http.HttpServletResponse;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
-
 import org.springframework.beans.factory.InitializingBean;
 import org.springframework.core.io.ClassPathResource;
 import org.springframework.core.io.Resource;
@@ -36,6 +37,8 @@ import org.springframework.http.MediaType;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.CollectionUtils;
+import org.springframework.util.ObjectUtils;
+import org.springframework.util.ResourceUtils;
 import org.springframework.util.StreamUtils;
 import org.springframework.util.StringUtils;
 import org.springframework.web.HttpRequestHandler;
@@ -158,6 +161,29 @@ public class ResourceHttpRequestHandler extends WebContentGenerator implements H
 			logger.warn("Locations list is empty. No resources will be served unless a " +
 					"custom ResourceResolver is configured as an alternative to PathResourceResolver.");
 		}
+		initAllowedLocations();
+	}
+
+	/**
+	 * Look for a {@link org.springframework.web.servlet.resource.PathResourceResolver}
+	 * among the {@link #getResourceResolvers() resource resolvers} and configure
+	 * its {@code "allowedLocations"} to match the value of the
+	 * {@link #setLocations(java.util.List) locations} property unless the "allowed
+	 * locations" of the {@code PathResourceResolver} is non-empty.
+	 */
+	protected void initAllowedLocations() {
+		if (CollectionUtils.isEmpty(this.locations)) {
+			return;
+		}
+		for (int i = getResourceResolvers().size()-1; i >= 0; i--) {
+			if (getResourceResolvers().get(i) instanceof PathResourceResolver) {
+				PathResourceResolver pathResolver = (PathResourceResolver) getResourceResolvers().get(i);
+				if (ObjectUtils.isEmpty(pathResolver.getAllowedLocations())) {
+					pathResolver.setAllowedLocations(getLocations().toArray(new Resource[getLocations().size()]));
+				}
+				break;
+			}
+		}
 	}
 
 	/**
@@ -214,18 +240,33 @@ public class ResourceHttpRequestHandler extends WebContentGenerator implements H
 		writeContent(response, resource);
 	}
 
-	protected Resource getResource(HttpServletRequest request) throws IOException{
+	protected Resource getResource(HttpServletRequest request) throws IOException {
 		String path = (String) request.getAttribute(HandlerMapping.PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE);
 		if (path == null) {
 			throw new IllegalStateException("Required request attribute '" +
 					HandlerMapping.PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE + "' is not set");
 		}
+		path = processPath(path);
 		if (!StringUtils.hasText(path) || isInvalidPath(path)) {
 			if (logger.isTraceEnabled()) {
 				logger.trace("Ignoring invalid resource path [" + path + "]");
 			}
 			return null;
 		}
+		if (path.contains("%")) {
+			try {
+				// Use URLDecoder (vs UriUtils) to preserve potentially decoded UTF-8 chars
+				if (isInvalidPath(URLDecoder.decode(path, "UTF-8"))) {
+					if (logger.isTraceEnabled()) {
+						logger.trace("Ignoring invalid resource path with escape sequences [" + path + "].");
+					}
+					return null;
+				}
+			}
+			catch (IllegalArgumentException ex) {
+				// ignore
+			}
+		}
 		ResourceResolverChain resolveChain = new DefaultResourceResolverChain(getResourceResolvers());
 		Resource resource = resolveChain.resolveResource(request, path, getLocations());
 		if (resource == null || getResourceTransformers().isEmpty()) {
@@ -237,14 +278,76 @@ public class ResourceHttpRequestHandler extends WebContentGenerator implements H
 	}
 
 	/**
-	 * Validates the given path: returns {@code true} if the given path is not a valid resource path.
-	 * <p>The default implementation rejects paths containing "WEB-INF" or "META-INF" as well as paths
-	 * with relative paths ("../") that result in access of a parent directory.
+	 * Process the given resource path to be used.
+	 * <p>The default implementation replaces any combination of leading '/' and
+	 * control characters (00-1F and 7F) with a single "/" or "". For example
+	 * {@code "  // /// ////  foo/bar"} becomes {@code "/foo/bar"}.
+	 * @since 3.2.12
+	 */
+	protected String processPath(String path) {
+		boolean slash = false;
+		for (int i = 0; i < path.length(); i++) {
+			if (path.charAt(i) == '/') {
+				slash = true;
+			}
+			else if (path.charAt(i) > ' ' && path.charAt(i) != 127) {
+				if (i == 0 || (i == 1 && slash)) {
+					return path;
+				}
+				path = slash ? "/" + path.substring(i) : path.substring(i);
+				if (logger.isTraceEnabled()) {
+					logger.trace("Path after trimming leading '/' and control characters: " + path);
+				}
+				return path;
+			}
+		}
+		return (slash ? "/" : "");
+	}
+
+	/**
+	 * Identifies invalid resource paths. By default rejects:
+	 * <ul>
+	 * <li>Paths that contain "WEB-INF" or "META-INF"
+	 * <li>Paths that contain "../" after a call to
+	 * {@link org.springframework.util.StringUtils#cleanPath}.
+	 * <li>Paths that represent a {@link org.springframework.util.ResourceUtils#isUrl
+	 * valid URL} or would represent one after the leading slash is removed.
+	 * </ul>
+	 * <p><strong>Note:</strong> this method assumes that leading, duplicate '/'
+	 * or control characters (e.g. white space) have been trimmed so that the
+	 * path starts predictably with a single '/' or does not have one.
 	 * @param path the path to validate
-	 * @return {@code true} if the path has been recognized as invalid, {@code false} otherwise
+	 * @return {@code true} if the path is invalid, {@code false} otherwise
 	 */
 	protected boolean isInvalidPath(String path) {
-		return (path.contains("WEB-INF") || path.contains("META-INF") || StringUtils.cleanPath(path).startsWith(".."));
+		if (logger.isTraceEnabled()) {
+			logger.trace("Applying \"invalid path\" checks to path: " + path);
+		}
+		if (path.contains("WEB-INF") || path.contains("META-INF")) {
+			if (logger.isTraceEnabled()) {
+				logger.trace("Path contains \"WEB-INF\" or \"META-INF\".");
+			}
+			return true;
+		}
+		if (path.contains(":/")) {
+			String relativePath = (path.charAt(0) == '/' ? path.substring(1) : path);
+			if (ResourceUtils.isUrl(relativePath) || relativePath.startsWith("url:")) {
+				if (logger.isTraceEnabled()) {
+					logger.trace("Path represents URL or has \"url:\" prefix.");
+				}
+				return true;
+			}
+		}
+		if (path.contains("../")) {
+			path = StringUtils.cleanPath(path);
+			if (path.contains("../")) {
+				if (logger.isTraceEnabled()) {
+					logger.trace("Path contains \"../\" after call to StringUtils#cleanPath.");
+				}
+				return true;
+			}
+		}
+		return false;
 	}
 
 	/**