Free5GC AMF has Missing Concurrent NAS SMC Validation During NGAP Handover

Summary

The AMF in Free5GC v4.2.1 does not enforce the concurrent security procedure rules defined in 3GPP TS 33.501 §6.9.5.1. The AMF does not check for ongoing N2 handover procedures before initiating a NAS Security Mode Command, and vice versa. This can lead to mismatches between NAS and AS security contexts in the network and the UE.

Details

Vulnerability Type: CWE-358 (Improperly Implemented Security Check for Standard)

Affected File: internal/ngap/handler.go — handleHandoverRequiredMain() and internal/gmm/sm.go — SecurityMode()

Root Cause:

3GPP TS 33.501 §6.9.5.1 states:

"Concurrent runs of security procedures may, in certain situations, lead to mismatches between security contexts in the network and the UE. In order to avoid such mismatches, the following rules shall be adhered to:

1. AMF shall not initiate any of the N2 procedures including a new key towards a UE if a NAS Security Mode Command procedure is ongoing with the UE.
2. The AMF shall not initiate a NAS Security Mode Command towards a UE if one of the N2 procedures including a new key is ongoing with the UE."

Free5GC AMF uses an OnGoing state tracking mechanism (SetOnGoing(), GetOnGoing()) with OnGoingProcedureN2Handover type. However, the cross-procedure checks required by §6.9.5.1 are not implemented:

Rule 2 violation: SecurityMode() in internal/gmm/sm.go sends SMC on EntryEvent without checking if N2 handover is ongoing.

Rule 1 violation: handleHandoverRequiredMain() in internal/ngap/handler.go calls SetOnGoing(OnGoingProcedureN2Handover) without checking if SMC is ongoing.

Why NH/NCC and SMC are related:

SMC activates a new KAMF, which changes the basis for NH key derivation. The N2 HandoverRequest includes NH/NCC derived from the old KAMF. If both procedures run concurrently, the target gNB and UE derive different KgNB keys, breaking AS security.

PoC

Source code evidence:

Free5GC AMF internal/gmm/sm.go — SecurityMode():

func SecurityMode(state *fsm.State, event fsm.EventType, args fsm.ArgsType) {
    switch event {
    case fsm.EntryEvent:
        // No check for OnGoing N2 procedure
        // Directly proceeds to SMC