free5GC is an open-source implementation of the 5G core network. Prior to 4.2.2, the AMF in Free5GC 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. This vulnerability is fixed in 4.2.2.

Affected: Go: github.com/free5gc/amf

### 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:** `…