一、NaNNaNNaNNaN-Batman

题目链接：https://adworld.xctf.org.cn/task/task_list?type=web&number=3&grade=1&page=1

二、使用步骤

1.下载附件

<script>_='function $(){e=getEleById("c").value;length==16^be0f23
233ac
e98aa$
c7be9){tfls_aie}na_h0lnrg{e_0iit\'_ns=[t,n,r,i];for(o=0;o<13;++o){	[0]);.splice(0,1)}}}	\'<input id="c">< onclick=$()>Ok</>\');delete _
var ","docu.)match(/"];/)!=null=["	write(s[o%4]buttonif(e.ment';for(Y in $='	
')with(_.split($[Y]))_=join(pop());eval(_)</script>

修改后缀为html，浏览器打开

将eval函数改为alert得到

function $(){
var e=document.getElementById("c").value;
if(e.length==16)//构造长度为16
   if(e.match(/^be0f23/)!=null)//开头匹配到be0f23
      if(e.match(/233ac/)!=null)//e中有233ac
         if(e.match(/e98aa$/)!=null)//结尾匹配到e98aa
             if(e.match(/c7be9/)!=null){//e中有c7be9
             var t=["fl","s_a","i","e}"];
             var n=["a","_h0l","n"];
             var r=["g{","e","_0"];
             var i=["it'","_","n"];
             var s=[t,n,r,i];
             for(var o=0;o<13;++o){
             document.write(s[o%4][0]);s[o%4].splice(0,1)}
             }
}
document.write('<input id="c"><button οnclick=$()>Ok</button>');
delete _

给的数正好可构造：e=be0f233ac7be98aa

把alert改回eval，在提交框中输入，即可得到flag：flag{it's_a_h0le_in_0ne}

总结

js源码审计