wger: trainer_login open redirect - ?next= parameter not validated against host

Summary

The trainer_login view in wger redirects to request.GET['next'] directly via HttpResponseRedirect() without calling url_has_allowed_host_and_scheme(). After the trainer successfully enters impersonation mode, their browser is redirected to any attacker-controlled URL supplied in the ?next= parameter, enabling Referer exfiltration and phishing.

Details

File: wger/core/views/user.py, approximately line 203

# VULNERABLE - wger/core/views/user.py
if not own:
    request.session['trainer.identity'] = orig_user_pk
    if request.GET.get('next'):
        return HttpResponseRedirect(request.GET['next'])   # no host/scheme validation

After the impersonation logic succeeds, the view performs no validation of the next parameter before issuing the redirect. An attacker who can deliver a crafted link (e.g. /en/user/2/trainer-login?next=https://evil.example/steal) to a trainer can redirect the trainer's browser to any external host immediately after the impersonation session is established. The Location header contains the raw attacker-controlled URL.

Affected endpoint:

• GET /en/user/<user_pk>/trainer-login -> wger.core.views.user.trainer_login (the ?next= redirect branch)

Suggested patch:

--- a/wger/core/views/user.py
+++ b/wger/core/views/user.py
+from django.utils.http import url_has_allowed_host_and_scheme
+
 if not own:
     request.session['trainer.identity'] = orig_user_pk
-    if request.GET.get('next'):
-        return HttpResponseRedirect(request.GET['next'])
+    next_url = request.GET.get('next')
+    if next_url and url_has_allowed_host_and_scheme(
+        next_url, allowed_hosts={request.get_host()}, require_https=request.is_secure()
+    ):
+        return HttpResponseRedirect(next_url)
     return HttpResponseRedirect(reverse('core:index'))

Adding @require_POST to trainer_login (see also VULN-030) moves the next parameter to the POST body where CSRF protection applies and eliminates the combined CSRF + open-redirect attack surface entirely.