FirstBlood-#349Register.php is still vulenerable to Reflective XSS
This issue was discovered on FirstBlood v2

On 2021-10-25, kinako Level 5 reported:

Dear FirstBlood security team, I found a vulnerability on your service. I hope this report will help you.


Because of not enough sanitization, the attackers still can perform reflective XSS on /register.php.

Vulnerability Description(PoC)

First of all, I enumerated a hidden param on /register.php and I found that this endpoint reflects the hidden parameter ref value in href attribute of Return to previous page.

Next, firstblood server-side seems to sanitize user's input by replacing some malicious words into nope.

However, we can bypass it by using %09.

Then, finally our malicious input is reflected and leads to Reflective XSS.


  • Firstblood session cookie does not have HttpOnly attribute so the attackers can steal victim's session cookie easily
  • Phishing

Regards, kinako

P3 Medium

Endpoint: /register.php

Parameter: ref

Payload: ref=ja%09va%09sc%09ri%09pt%09:%09alert(1)

FirstBlood ID: 32
Vulnerability Type: Reflective XSS

The parameter ?ref on register.php was poorly fixed and can be bypassed in various ways. Firstly the developer failed to use strtolower() when comparing strings, and the use of characters such as %09 will also bypass the filter.