Kiwi TCMS account confirmation open redirect through protocol-relative URLs

## Summary

In Kiwi TCMS version 16.1, the fix for CVE-2026-54724 (open redirect via unvalidated `next` parameter) in the account confirmation endpoint is incomplete. The validation uses `startswith("/")` to check if the redirect URL is local, but this is bypassed by protocol-relative URLs like `//evil.com` which start with `/` but redirect the browser to an external attacker-controlled domain. 16.1’s incomplete mitigation still allows protocol-relative destinations and earlier releases lack the mitigation.

## Vulnerable Code

In `tcms/kiwi_auth/views.py`, the `Confirm` view (line ~113):

def get_redirect_url(self, *args, **kwargs):
self.url = self.request.GET.get("next", reverse("core-views-index"))
if not self.url.startswith("/"):
self.url = reverse("core-views-index")

The check `startswith("/")` passes for `//evil.com` because it begins with `/`, but browsers interpret `//evil.com` as a protocol-relative URL that redirects to `https://evil.com` (or `http://evil.com` depending on the current page's protocol).

## Proof of Concept

GET /confirm/<activation_key>/?next=//evil.com/phishing HTTP/1.1
Host: kiwi.example.com

Response:

HTTP/1.1 302 Found
Location: //evil.com/phishing

The browser follows the redirect to `https://evil.com/phishing`.

Verification:

>>> url = "//evil.com/phishing"
>>> url.startswith("/")
True # passes the check!

## Impact

An attacker can craft a confirmation link that, after the user confirms their account, redirects them to a phishing page. This is useful for:

- Credential phishing (fake login page on attacker domain)
- Malware delivery after trusted action
- Social engineering (user trusts the link because it's from the legitimate Kiwi TCMS instance)

The attack requires sending a crafted activation link to a user who is registering, or intercepting/predicting the activation key.

## Suggested Fix

Use Django's `url_has_allowed_host_and_scheme()` utility instead of a simple `startswith` check:

from django.utils.http import url_has_allowed_host_and_scheme

def get_redirect_url(self, *args, **kwargs):
self.url = self.request.GET.get("next", reverse("core-views-index"))
if not url_has_allowed_host_and_scheme(
self.url,
allowed_hosts={self.request.get_host()},
require_https=self.request.is_secure(),
):
self.url = reverse("core-views-index")

This properly rejects protocol-relative URLs, absolute URLs to other hosts, and other bypass patterns.

Fixed in Version 16.2