OpenRun: Redirect URL validation bypass using  //host  paths leads to Open Redirect

Summary

The restrictions on redirect URLs in openrun can be bypassed by attackers, leading to open redirect attacks.

Details

In the current project, the referrer header value is used for subsequent redirects, so there is currently a validation for this redirect value. The current validation logic requires that the host and schema of the redirect URL be the same as the current website's URL, and finally, the path part is used for redirection. This check seems robust, but it can still be bypassed by attackers.

Here's the problem: Assuming the current website is http://127.0.0.1:25222/, if the attacker passes in a redirect URL of http://127.0.0.1:25222//fushuling.com, its host and schema are obviously the same as the current website, thus bypassing the verification. However, the issue lies in the final redirect URL, which is the path part of the URL, i.e., //fushuling.com.

Browsers automatically complete the HTTP header for URLs starting with //, ultimately successfully bypassing the restriction and redirecting to the external address http://fushuling.com.

This vulnerable behavior was successfully reproduced locally. Normally, specifying an external address directly will be blocked, so it will not redirect.

However, if the redirect URL is http://127.0.0.1:25222//fushuling.com, the existing validation logic is bypassed, and the Location header is successfully set to //fushuling.com.

 1POST /redirecttest/abc/frag HTTP/1.1
 2Host: 127.0.0.1:25222
 3Referer: http://127.0.0.1:25222//fushuling.com
 4Cache-Control: max-age=0
 5sec-ch-ua: "Not(A:Brand";v="24", "Chromium";v="122"
 6sec-ch-ua-mobile: ?0
 7sec-ch-ua-platform: "Windows"
 8Upgrade-Insecure-Requests: 1
 9User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/122.0.6261.57 Safari/537.36
10Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.7
11Sec-Fetch-Site: none
12Sec-Fetch-Mode: navigate
13Sec-Fetch-User: ?1
14Sec-Fetch-Dest: document
15Accept-Encoding: gzip, deflate, br
16Accept-Language: zh-CN,zh;q=0.9
17Connection: close
18Content-Type: application/x-www-form-urlencoded
19Content-Length: 0

The user was then successfully redirected to the external address http://fushuling.com.

PoC

 1http://127.0.0.1:25222//fushuling.com

Impact

Open Redirect