FirstBlood-#341Referer header reflection leads 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.


The endpoint /login.php has Return to previous page element. This href attribute in a tag reflects Referer header's value, it causes Reflective XSS.

Vulnerability Description(PoC)

First of all, to set a malicious value in Referer header access to such a URL like'/onmouseover="alert(document.cookie)"/a='

Note: if you have a problem in URL encoded letters, you can use proxy tool to capture the request to reproduce.

Next, go to /login.php and move your mouse cursor on Return to previous page.

Then, you can see popup alert.



  • FirstBlood v2 still doesn't have a HttpOnly flag in Cookie so malicious attackers can steal victim's cookie if Reflective XSS is executed correctly.
  • Phishing

Regards, kinako

P3 Medium

Endpoint: /login.php

Parameter: Referer header

Payload: Referer:'/onmouseover="alert(1)"/a='

FirstBlood ID: 19
Vulnerability Type: Reflective XSS

The parameter ?ref= on login.php was fixed and instead the use of $_SERVER['HTTP_REFERER']; was used. Patrice tested in Chrome and Firefox and saw it was secure, but some users still use Internet Explorer 10 (governments for example!) and the Referer header is vulnerable to reflective XSS.

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Creator & Administrator

Nice find, thanks for providing a working PoC and mentioning it only works on IE. Some users reported that the referrer was vulnerable but the payload provided wouldn't work, and no mention of browser. So great work with clear repo steps. Perfect!