漏洞概述

VMware Spring Framework是美国威睿（VMware）公司的一套开源的Java、JavaEE应用程序框架。该框架可帮助开发人员构建高质量的应用。

近期，网宿安全演武实验室监测到Spring Framework在特定条件下，存在目录遍历漏洞（网宿评分：高危、CVSS 3.1 评分：7.5）：

当同时满足使用 RouterFunctions 和 FileSystemResource 来处理和提供静态文件时，攻击者可构造恶意请求遍历读取系统上的文件。

目前该漏洞POC状态已在互联网公开，建议客户尽快做好自查及防护。

受影响版本

漏洞分析

根据漏洞描述（https://spring.io/security/cve-2024-38816）可知，关键变更在于如何处理静态资源路径。

https://github.com/spring-projects/spring-framework/commit/d86bf8b2056429edf5494456cffcb2b243331c49#diff-25869a3e3b3d4960cb59b02235d71d192fdc4e02ef81530dd6a660802d4f8707R4

webmvc --> org.springframework.web.servlet.function.PathResourceLookupFunction

它们都旨在为 Web 应用程序提供静态内容的访问。

以webmvc --> org.springframework.web.servlet.function.PathResourceLookupFunction为例，展开分析。

先是动态处理了资源请求，确保只返回有效并且可访问的资源。

@Override
public Optional<Resource> apply(ServerRequest request) {
        PathContainer pathContainer = request.requestPath().pathWithinApplication();
        if (!this.pattern.matches(pathContainer)) {
                return Optional.empty();
        }
 
        pathContainer = this.pattern.extractPathWithinPattern(pathContainer);
        String path = processPath(pathContainer.value());
        if (path.contains("%")) {
                path = StringUtils.uriDecode(path, StandardCharsets.UTF_8);
        }
        if (!StringUtils.hasLength(path) || isInvalidPath(path)) {
                return Optional.empty();
        }
 
        try {
                Resource resource = this.location.createRelative(path);
                if (resource.isReadable() && isResourceUnderLocation(resource)) {
                        return Optional.of(resource);
                }
                else {
                        return Optional.empty();
                }
        }
        catch (IOException ex) {
                throw new UncheckedIOException(ex);
        }
}

接着对路径字符串进行规范化处理，确保返回的路径格式是有效的。

private 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);
                        return path;
                }
        }
        return (slash ? "/" : "");
}

最后从安全角度，确保路径不指向敏感目录，并且避免出现路径穿越的情况。

private boolean isInvalidPath(String path) {
        if (path.contains("WEB-INF") || path.contains("META-INF")) {
                return true;
        }
        if (path.contains(":/")) {
                String relativePath = (path.charAt(0) == '/' ? path.substring(1) : path);
                if (ResourceUtils.isUrl(relativePath) || relativePath.startsWith("url:")) {
                        return true;
                }
        }
        return path.contains("..") && StringUtils.cleanPath(path).contains("../");
}

简单阐明以后，不难发现上述代码做了敏感目录检查、url检查、路径穿越检查等操作，暂时没发现可疑点，所以我们需要进一步跟进

public static String cleanPath(String path) {
        if (!hasLength(path)) {
                return path;
        }
 
        String normalizedPath;
        // Optimize when there is no backslash
        if (path.indexOf('\\') != -1) {
                normalizedPath = replace(path, DOUBLE_BACKSLASHES, FOLDER_SEPARATOR);
                normalizedPath = replace(normalizedPath, WINDOWS_FOLDER_SEPARATOR, FOLDER_SEPARATOR);
        }
        else {
                normalizedPath = path;
        }
        String pathToUse = normalizedPath;
 
        // Shortcut if there is no work to do
        if (pathToUse.indexOf('.') == -1) {
                return pathToUse;
        }
 
        // Strip prefix from path to analyze, to not treat it as part of the
        // first path element. This is necessary to correctly parse paths like
        // "file:core/../core/io/Resource.class", where the ".." should just
        // strip the first "core" directory while keeping the "file:" prefix.
        int prefixIndex = pathToUse.indexOf(':');
        String prefix = "";
        if (prefixIndex != -1) {
                prefix = pathToUse.substring(0, prefixIndex + 1);
                if (prefix.contains(FOLDER_SEPARATOR)) {
                        prefix = "";
                }
                else {
                        pathToUse = pathToUse.substring(prefixIndex + 1);
                }
        }
        if (pathToUse.startsWith(FOLDER_SEPARATOR)) {
                prefix = prefix + FOLDER_SEPARATOR;
                pathToUse = pathToUse.substring(1);
        }
 
        String[] pathArray = delimitedListToStringArray(pathToUse, FOLDER_SEPARATOR);
        // we never require more elements than pathArray and in the common case the same number
        Deque<String> pathElements = new ArrayDeque<>(pathArray.length);
        int tops = 0;
 
        for (int i = pathArray.length - 1; i >= 0; i--) {
                String element = pathArray[i];
                if (CURRENT_PATH.equals(element)) {
                        // Points to current directory - drop it.
                }
                else if (TOP_PATH.equals(element)) {
                        // Registering top path found.
                        tops++;
                }
                else {
                        if (tops > 0) {
                                // Merging path element with element corresponding to top path.
                                tops--;
                        }
                        else {
                                // Normal path element found.
                                pathElements.addFirst(element);
                        }
                }
        }
 
        // All path elements stayed the same - shortcut
        if (pathArray.length == pathElements.size()) {
                return normalizedPath;
        }
        // Remaining top paths need to be retained.
        for (int i = 0; i < tops; i++) {
                pathElements.addFirst(TOP_PATH);
        }
        // If nothing else left, at least explicitly point to current path.
        if (pathElements.size() == 1 && pathElements.getLast().isEmpty() && !prefix.endsWith(FOLDER_SEPARATOR)) {
                pathElements.addFirst(CURRENT_PATH);
        }
 
        final String joined = collectionToDelimitedString(pathElements, FOLDER_SEPARATOR);
        // avoid string concatenation with empty prefix
        return prefix.isEmpty() ? joined : prefix + joined;
}

这个方法主要对用户输入路径做了规范化处理，具体包括长度检查、不同操作系统下的路径分隔符处理等。看起来也做了严格的处理，但这一步存在问题。

String[] pathArray = delimitedListToStringArray(pathToUse, FOLDER_SEPARATOR);

具体来说，它是允许空元素存在的，假设路径字符串形如：

再来看这一串：String pathToUse = "/static/../../Windows/win.ini";

显然，pathArray中存在空元素会影响上级目录的处理，导致返回不同的结果，即存在安全隐患。

漏洞复现

实现目录穿越需要用到"../"，结合上述分析，可通过这种方式实现。

package org.example.demo;

import org.springframework.util.ResourceUtils;
import org.springframework.util.StringUtils;

public class test {
public static void main(String[] args) {
String path = "/static///../../Windows/win.ini";
System.out.println(isInvalidPath(path));
}

private static boolean isInvalidPath(String path) {
if (path.contains("WEB-INF") || path.contains("META-INF")) {
return true;
}
if (path.contains(":/")) {
String relativePath = (path.charAt(0) == '/' ? path.substring(1) : path);
if (ResourceUtils.isUrl(relativePath) || relativePath.startsWith("url:")) {
return true;
}
}
return path.contains("..") && StringUtils.cleanPath(path).contains("../");
}
}

但还需要结合上下文，继续构造payload。首先路径以斜杠开头时，StringUtils.cleanPath()方法会去掉路径的第⼀个斜杠。

if (pathToUse.startsWith(FOLDER_SEPARATOR)) {
    prefix = prefix + FOLDER_SEPARATOR;
    pathToUse = pathToUse.substring(1);
}

pathContainer = this.pattern.extractPathWithinPattern(pathContainer);
String path = processPath(pathContainer.value());

所以需要将多余的"/"变为"\"，再借助StringUtils.cleanPath()方法重新转换回来。

normalizedPath = replace(normalizedPath, WINDOWS_FOLDER_SEPARATOR, FOLDER_SEPARATOR);

修复方案

目前官方已有可更新版本，建议受影响用户升级至最新版本：

https://github.com/spring-projects/spring-framework/tags