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背景： 客户进行渗透测试，验证上传文件的程序没有对上传文件作任何过滤，导致可以上传任意文件到服务器，甚至是病毒文件和Webshell木马文件。

解决办法：对于上传的附件，验证程序要做严格验证，使用服务器端校验，而不能仅用前端验证。

代码实例

// 允许上传文件后缀
private static final String[] ALLOWED_FILE_EXTENSIONS = {"jpg", "jpeg", "png", "gif", "doc", "docx", "xls", "xlsx", "pdf", "ppt", "pptx"};
// 允许上传文件头魔数十六进制字符串
private static final List<String> ALLOWED_MAGIC_NUMBERS = Arrays.asList("FFD8FF", "89504E47", "47494638", "25504446", "D0CF11E0", "504B0304"
);	// JPEG (jpg),PNG (png),GIF (gif),pdf,(doc、xls、ppt),(xls、pptx)// 允许上传文件的MIME类型  
private static final Set<String> ALLOWED_MIME_TYPES = new HashSet<>();
static {ALLOWED_MIME_TYPES.add("image/jpeg"); // jpg, jpeg  ALLOWED_MIME_TYPES.add("image/png");  // png  ALLOWED_MIME_TYPES.add("image/gif");  // gif  ALLOWED_MIME_TYPES.add("application/msword"); // doc  ALLOWED_MIME_TYPES.add("application/vnd.openxmlformats-officedocument.wordprocessingml.document"); // docx  ALLOWED_MIME_TYPES.add("application/vnd.ms-excel"); // xls  ALLOWED_MIME_TYPES.add("application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"); // xlsx  ALLOWED_MIME_TYPES.add("application/pdf"); // pdf  ALLOWED_MIME_TYPES.add("application/vnd.ms-powerpoint"); // pptALLOWED_MIME_TYPES.add("application/vnd.openxmlformats-officedocument.presentationml.presentation"); // pptx  
}  @SuppressWarnings("unchecked")
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {request.setCharacterEncoding("UTF-8");String fileType="";// 1.创建文件上传工厂类DiskFileItemFactory fac = new DiskFileItemFactory();// 2.创建文件上传核心类对象ServletFileUpload upload = new ServletFileUpload(fac);// 【一、设置单个文件最大1024M】upload.setFileSizeMax(1024 * 1024 * 1024);// 1024MM// 【二、设置总文件大小：2048M】upload.setSizeMax(2048 * 1024 * 1024); // 2048MList<String> StringArr=new  ArrayList<String>();// 判断，当前表单是否为文件上传表单if (upload.isMultipartContent(request)) {try {// 3.把请求数据转换为FileItem对象的集合List<FileItem> list = upload.parseRequest(request);Calendar calendar = Calendar.getInstance();int year = calendar.get(Calendar.YEAR);// 遍历，得到每一个上传项for (FileItem item : list) {// 判断：是普通表单项，还是文件上传表单项if (item.isFormField()) {// 普通表单xString fieldName = item.getFieldName();// 获取元素名称String value = item.getString("UTF-8"); // 获取元素值fileType=value;System.out.println(fieldName + " : " + value);} else {// 文件上传表单//文件保存目录路径String savePath = getServletContext().getRealPath("/") + "uploadFiles/wjgl/"+fileType+"/"+year+"/";File uploadFile = new File(savePath);if (!uploadFile.exists()) {uploadFile.mkdirs();}String oldname = item.getName(); // 上传的文件名称String fileExtension = item.getName().substring(item.getName().lastIndexOf(".") + 1);//获取到文件后缀if (!Arrays.asList(ALLOWED_FILE_EXTENSIONS).contains(fileExtension)) {//验证文件后缀response.setStatus(500);throw new FileUploadException("if1无效的文件扩展名: " + fileExtension);}String contentType = item.getContentType();if(!ALLOWED_MIME_TYPES.contains(contentType)){response.setStatus(500);throw new FileUploadException("if2无效的文件扩展名: " + fileExtension);}if (!isFileValid(item)) {// 文件有效，进行处理response.setStatus(500);throw new FileUploadException("被改了后缀，判断文件内容魔术无效的文件扩展名: " + fileExtension);}//时间戳String time=String.valueOf(new Date().getTime());String name = time+oldname;// 【三、上传到指定目录：获取上传目录路径】String realPath =  "uploadFiles/wjgl/"+fileType+"/"+year+"/";// 创建文件对象File file = new File(savePath, name);item.write(file);item.delete();response.setContentType("application/json");response.setCharacterEncoding("UTF-8");JSONObject obj = new JSONObject();obj.put("fileName", oldname);obj.put("filePath", realPath + name);StringArr.add(obj.toString());}}response.getWriter().println(StringArr);} catch (Exception e) {e.printStackTrace();}} else {System.out.println("不处理！");}}//校验是否为jpg,jpeg,png,gif,doc,docx,xls,xlsx,pdf,pptx格式
public static boolean isFileValid(FileItem fileItem) throws IOException {String fileName = fileItem.getName();try (InputStream inputStream = fileItem.getInputStream()) {return isValidFileMagicNumber(inputStream);}
}
//验证文件魔数
public static boolean isValidFileMagicNumber(InputStream inputStream) throws IOException {boolean bl=false;byte[] buffer = new byte[8];inputStream.read(buffer, 0, 8);String hexMagicNumber = bytesToHex(buffer);for(int i = 0; i<ALLOWED_MAGIC_NUMBERS.size(); i++){String ms=ALLOWED_MAGIC_NUMBERS.get(i);if(hexMagicNumber.toUpperCase().startsWith(ms)){bl=true;break;}}return bl;
}
//字节转换16进制
private static String bytesToHex(byte[] bytes) {StringBuilder hexString = new StringBuilder();for (byte b : bytes) {hexString.append(String.format("%02X", b));}return hexString.toString();
}