知识点：

  • 逻辑漏洞

  • flask session解密

先放flask源码

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from flask import Flask, session, request, Response
import urllib

app = Flask(__name__)
app.secret_key = '*********************'  # censored
url_prefix = '/d5afe1f66147e857'


def FLAG():
    return '*********************'  # censored


def trigger_event(event):
    session['log'].append(event)
    if len(session['log']) > 5:
        session['log'] = session['log'][-5:]
    if type(event) == type([]):
        request.event_queue += event
    else:
        request.event_queue.append(event)


def get_mid_str(haystack, prefix, postfix=None):
    haystack = haystack[haystack.find(prefix)+len(prefix):]
    if postfix is not None:
        haystack = haystack[:haystack.find(postfix)]
    return haystack


class RollBackException:
    pass


def execute_event_loop():
    valid_event_chars = set(
        'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_0123456789:;#')
    resp = None
    while len(request.event_queue) > 0:
        # `event` is something like "action:ACTION;ARGS0#ARGS1#ARGS2......"
        event = request.event_queue[0]
        request.event_queue = request.event_queue[1:]
        if not event.startswith(('action:', 'func:')):
            continue
        for c in event:
            if c not in valid_event_chars:
                break
        else:
            is_action = event[0] == 'a'
            action = get_mid_str(event, ':', ';')
            args = get_mid_str(event, action+';').split('#')
            try:
                event_handler = eval(
                    action + ('_handler' if is_action else '_function'))
                ret_val = event_handler(args)
            except RollBackException:
                if resp is None:
                    resp = ''
                resp += 'ERROR! All transactions have been cancelled. <br />'
                resp += '<a href="./?action:view;index">Go back to index.html</a><br />'
                session['num_items'] = request.prev_session['num_items']
                session['points'] = request.prev_session['points']
                break
            except Exception, e:
                if resp is None:
                    resp = ''
                # resp += str(e) # only for debugging
                continue
            if ret_val is not None:
                if resp is None:
                    resp = ret_val
                else:
                    resp += ret_val
    if resp is None or resp == '':
        resp = ('404 NOT FOUND', 404)
    session.modified = True
    return resp


@app.route(url_prefix+'/')
def entry_point():
    querystring = urllib.unquote(request.query_string)
    request.event_queue = []
    if querystring == '' or (not querystring.startswith('action:')) or len(querystring) > 100:
        querystring = 'action:index;False#False'
    if 'num_items' not in session:
        session['num_items'] = 0
        session['points'] = 3
        session['log'] = []
    request.prev_session = dict(session)
    trigger_event(querystring)
    return execute_event_loop()

# handlers/functions below --------------------------------------


def view_handler(args):
    page = args[0]
    html = ''
    html += '[INFO] you have {} diamonds, {} points now.<br />'.format(
        session['num_items'], session['points'])
    if page == 'index':
        html += '<a href="./?action:index;True%23False">View source code</a><br />'
        html += '<a href="./?action:view;shop">Go to e-shop</a><br />'
        html += '<a href="./?action:view;reset">Reset</a><br />'
    elif page == 'shop':
        html += '<a href="./?action:buy;1">Buy a diamond (1 point)</a><br />'
    elif page == 'reset':
        del session['num_items']
        html += 'Session reset.<br />'
    html += '<a href="./?action:view;index">Go back to index.html</a><br />'
    return html


def index_handler(args):
    bool_show_source = str(args[0])
    bool_download_source = str(args[1])
    if bool_show_source == 'True':

        source = open('eventLoop.py', 'r')
        html = ''
        if bool_download_source != 'True':
            html += '<a href="./?action:index;True%23True">Download this .py file</a><br />'
            html += '<a href="./?action:view;index">Go back to index.html</a><br />'

        for line in source:
            if bool_download_source != 'True':
                html += line.replace('&', '&amp;').replace('\t', '&nbsp;'*4).replace(
                    ' ', '&nbsp;').replace('<', '&lt;').replace('>', '&gt;').replace('\n', '<br />')
            else:
                html += line
        source.close()

        if bool_download_source == 'True':
            headers = {}
            headers['Content-Type'] = 'text/plain'
            headers['Content-Disposition'] = 'attachment; filename=serve.py'
            return Response(html, headers=headers)
        else:
            return html
    else:
        trigger_event('action:view;index')


def buy_handler(args):
    num_items = int(args[0])
    if num_items <= 0:
        return 'invalid number({}) of diamonds to buy<br />'.format(args[0])
    session['num_items'] += num_items
    trigger_event(['func:consume_point;{}'.format(
        num_items), 'action:view;index'])


def consume_point_function(args):
    point_to_consume = int(args[0])
    if session['points'] < point_to_consume:
        raise RollBackException()
    session['points'] -= point_to_consume


def show_flag_function(args):
    flag = args[0]
    # return flag # GOTCHA! We noticed that here is a backdoor planted by a hacker which will print the flag, so we disabled it.
    return 'You naughty boy! ;) <br />'


def get_flag_handler(args):
    if session['num_items'] >= 5:
        # show_flag_function has been disabled, no worries
        trigger_event('func:show_flag;' + FLAG())
    trigger_event('action:view;index')


if __name__ == '__main__':
    app.run(debug=False, host='0.0.0.0')

从 get_flag_handler可以看出来得到flag的条件，也就是让session['num_items'] >= 5:满足。

接着跟进这个num_items ，在buy_handler里，对num_items先进行了加操作，再调用consume_point_handler判断是否足够，不够的话扣除掉num_items，这里就存在了一个明显的逻辑漏洞，我们如果能在，执行buy_handler结束，consume_point_handler调用之前直接执行get_flag_handler，就可以拿到flag。

继续看源码，缕清执行顺序：

entry_point()函数为入口，querystring是我们的传参

为了达成目的，我们需要主意两个重要的方法： trigger_event(event) 和execute_event_loop()

trigger_event传参进入event_queue然后在execute_evenr_loop中执行。

get_mid_str函数的作用就是在第一个参数的字符串中截取第二个参数和第三个参数中间的字符串，如果第三个参数为空，则截取出字符串中第二个参数之后的所有字符。
以#为分隔符分隔event中的action，然后和_handler进行拼接，eval的作用是执行语句并返回结果，因为可以随意传递参数，所以我们可以利用这个拼接buy_handler函数和get_flag_handler函数，然后执行这些函数拿到flag。

构造?action:trigger_event%23;action:buy;7%23action:get_flag;

%23为#的urlencode，有注释作用，该payload传递之后，entry_point函数中的trigger_event(querystring)其实就是trigger_event(trigger_event)，然后trigger_event被传入event_queue，接下来在execute_event_loop中eventaction:trigger_event#;action:buy;7#action:get_flag;，到下面action为trigger_event#，args[‘action:buy;7’, ‘action:get_flag;’]，在eval中由于action中的#，所以trigger_event不会被拼接，所以event_handler==trigger_event，然后下面的event_handler(args)也就是trigger_event(args)，然后action:buy;7和action:get_flag;被append到event_queue中，然后就会在while循环中被eval拼接并先后执行buy_handler和get_flag_handler，而且买的是7个diamonds。

payload打进去之后，在cookie里面找到加密后的session，然后用脚本解密。

然后base64decode得到flag。