Improper Data Validation / Unvalidated Input

How it was found

Steps to reproduce:

1 - Be logged in a account 2 - Go to: https://liberapay.com/{user}/edit/statement 3 - Click on Visualize 4 - Submit (and edit POST parameters to fuzz infinitely) 5 - Wait the server proccess the request.

I send only 2.813.054 characters.

Improper input size validation...

I'm sorry. The server gets a bit lagged. My account in website is cardangi, if u guys want to check the characters that I got stored in DB...

POST /cardangi/edit/statement HTTP/1.1
Host: liberapay.com
User-Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:52.0) Gecko/20100101 Firefox/52.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: pt-BR,pt;q=0.8,en-US;q=0.5,en;q=0.3
Accept-Encoding: gzip, deflate
Referer: https://liberapay.com/cardangi/edit/statement
Cookie: __cfduid=dec7fa01079ce07bb54844ee12fafb51c1528593599; csrf_token=Y1wbnOu1ykM7eNq0DuCNs5fDoGInaDa5; session="647226:1:GER3tosCuKW0BtbQP2zgtOxn_VHaGn6-"
Connection: close
Upgrade-Insecure-Requests: 1
Content-Type: application/x-www-form-urlencoded
Content-Length: 2813138

csrf_token=Y1wbnOu1ykM7eNq0DuCNs5fDoGInaDa5&lang=pt&summary=&statement={VULNERABLE}&preview=true

Impact

An attacker can send UNLIMITATED data to the server. He can do a unlimited fuzzing too. Affects totally availability, server response...

Maybe null terminators? Who knows.

Basically, no captcha to register, real attack scenario:

1 - Create with a bot 100+ accounts 2 - Send packets of GB's and GB's of strings to the field 3 - Wait for the CHAOS! :]