Description

Open redirection vulnerabilities arise when an application incorporates user-controllable data into the target of a redirection in an unsafe way. An attacker can construct a URL within the application that causes a redirection to an arbitrary external domain.
https://github.com/Rudloff/alltube is vulnerable to open redirects as shown below:

Proof of concept

Vuln variable: $_SERVER['REQUEST_URI']
Snippet:

if (isset($_SERVER['REQUEST_URI']) && strpos($_SERVER['REQUEST_URI'], '/index.php') !== false) {
    header('Location: ' . str_ireplace('/index.php', '/', $_SERVER['REQUEST_URI']));

Payload

In a browser perform a request to index.php resource:

http://localhost/index.php/example.com

Observe the user is redirected to example.com

Impact

This behavior can be leveraged to facilitate phishing attacks against users of the application. The ability to use an authentic application URL, targeting the correct domain and with a valid SSL certificate (if SSL is used), lends credibility to the phishing attack because many users, even if they verify these features, will not notice the subsequent redirection to a different domain.

References

https://portswigger.net/kb/issues/00500100_open-redirection-reflected
https://www.netsparker.com/blog/web-security/open-redirect-vulnerabilities-netsparker-pauls-security-weekly/