[DDCTF 2019]homebrew event loop

0x00 知识点

逻辑漏洞：

异步处理导致可以先调用增加钻石，再调用计算价钱的。也就是先货后款。

eval函数存在注入,可以通过#注释，我们可以传入路由action:eval#;arg1#arg2#arg3这样注释后面语句并可以调用任意函数，分号后面的#为传入参数，参数通过#被分割为参数列表.

flask session解密
网上有脚本

0x01解题

题目给了我们源码了

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')

先贴上师傅博客：

https://blog.cindemor.com/post/ctf-web-16.html
分析一下：

# flag获取函数def FLAG()

# 以下三个函数负责对参数进行解析。
# 1. 添加log，并将参数加入队列def trigger_event(event)

# 2. 工具函数，获取prefix与postfix之间的值
def get_mid_str(haystack, prefix, postfix=None):

# 3. 从队列中取出函数，并分析后，进行执行。（稍后进行详细分析）
def execute_event_loop()

# 网站入口点
def entry_point()

# 页面渲染，三个页面:
index/shop/resetdef view_handler()

# 下载源码
def index_handler(args)

# 增加钻石
def buy_handler(args)

# 计算价钱，进行减钱
def consume_point_function(args)

# 输出flagdef show_flag_function(args)
def get_flag_handler(args)

有这么两个跟 flag 有关的函数：

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:
        trigger_event('func:show_flag;' + FLAG())
    trigger_event('action:view;index')

可以看到show_flag_function()无法直接展示出 flag，先看看get_flag_handler()中用到的trigger_event()函数：

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:

这个函数往 session 里写了日志，而这个日志里就有 flag，并且 flask 的 session 是可以被解密的。只要后台成功设置了这个 session 我们就有机会获得 flag。

但若想正确调用show_flag_function()，必须满足session['num_items'] >= 5。

购买num_items需要花费points，而我们只有 3 个points，如何获得 5 个num_items？

先看看购买的机制：

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

buy_handler()这个函数会先把num_items的数目给你加上去，然后再执行consume_point_function()，若points不够consume_point_function()会把num_items的数目再扣回去。
其实就是先给了货后，无法扣款，然后货被拿跑了

那么我们只要赶在货被抢回来之前，先执行get_flag_handler()即可。
函数trigger_event()维护了一个命令执行的队列，只要让get_flag_handler()赶在consume_point_function()之前进入队列即可。看看最关键的执行函数：

仔细分析execute_event_loop，会发现里面有一个eval函数，而且是可控的！

利用eval()可以导致任意命令执行，使用注释符可以 bypass 掉后面的拼接部分。

若让eval()去执行trigger_event()，并且在后面跟两个命令作为参数，分别是buy和get_flag，那么buy和get_flag便先后进入队列。

根据顺序会先执行buy_handler()，此时consume_point进入队列，排在get_flag之后，我们的目标达成。

所以最终 Payload 如下：

action:trigger_event%23;action:buy;5%23action:get_flag;

要注意执行buy_handler函数后事件列表末尾会加入consume_point_function函数，在最后执行此函数时校验会失败，抛出RollBackException()异常，但是不会影响session的返回

参考链接：

https://blog.cindemor.com/post/ctf-web-16.html