### Summary

Five routine detail action endpoints check a cache before calling `self.get_object()`. Cache keys are scoped only by `pk` — no user ID is included. When a victim has previously accessed their routine via the API, an attacker can retrieve the cached response for the same PK without any ownership check.

### Details

`wger/manager/api/views.py` — five actions follow this pattern (lines 134–201):

```python
@action(detail=True)
def date_sequence_display_mode(self, request, pk=None):
cache_key = make_routine_api_date_sequence_display_cache_key(pk)
cached = cache.get(cache_key)
if cached:
return Response(cached) # returned WITHOUT calling self.get_object()
# only reaches ownership check on cache miss
routine = self.get_object()
...
```

Cache key construction in `wger/utils/cache.py:89–106`:

```python
def make_routine_api_date_sequence_display_cache_key(routine_id):
return f"routine-api-date-sequence-display-{routine_id}"
# No user ID in key
```

Cache TTL: 1 month (`4 * 604800` seconds, `settings_global.py:461`).

Affected endpoints:
```
GET /api/v2/routine/{pk}/date-sequence-display/
GET /api/v2/routine/{pk}/date-sequence-gym/
GET /api/v2/routine/{pk}/structure/
GET /api/v2/routine/{pk}/logs/
GET /api/v2/routine/{pk}/stats/
```

### PoC

```
1. Victim (user A) visits GET /api/v2/routine/5/structure/ → response cached under key "routine-api-structure-5"
2. Attacker (user B) visits GET /api/v2/routine/5/structure/ → cache hit → returns user A's routine structure without any ownership check
```

Requires the victim to have previously accessed the endpoint (cache must be populated). Once populated, the cache entry is valid for 1 month.

### Impact

An attacker with a registered account can retrieve another user's routine details — workout day sequences, exercise structure, training logs, and statistics — from cache without ownership verification.

**Fix**: Include the user ID in the cache key:
```python
def make_routine_api_date_sequence_display_cache_key(routine_id, user_id):
return f"routine-api-date-sequence-display-{user_id}-{routine_id}"
```

Or move `self.get_object()` before the cache lookup so ownership is always verified first.

CWE-639 Authorization Bypass Through User-Controlled Key