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