free5GC is an open-source implementation of the 5G core network. In version 1.4.4 and earlier, the AUSF component stores per-subscriber authentication state in a global sync.Map named AUSFContext.UePool in internal/context/context.go, keyed only by SUPI. Every request handled by internal/sbi/processor/ue_authentication.go creates an AusfUeContext, and AddAusfUeContextToPool executes ausfContext.UePool.Store(ausfUeContext.Supi, ausfUeContext), unconditionally replacing the active context for that SUPI. An attacker with access to the AUSF SBI/N12 interface can send concurrent POST /nausf-auth/v1/ue-authentications requests for the same target SUPI, causing all attempts to share one logical authentication context URL while K_aut, XRES, and EapID are repeatedly overwritten. A valid EAP-AKA' response for an earlier challenge is then checked against the latest context, causing AT_MAC verification to fail and denying authentication to the selected subscriber while the request flood continues. No fixed version is available as of this review.

Affected: Go: github.com/free5gc/ausf

### Summary

The AUSF component of free5GC stores per-subscriber authentication state in a global `sync.Map` keyed only by SUPI. Every incoming authentication request creates a new `AusfUeContext` and stores it under that SUPI key without checking whether an authentication procedure is already in progress and without generating a per-session unique identifier.

An attacker with access to the AUSF SBI/N12 interface can send concurrent `POST /nausf-auth/v1/ue-authentications` requests for the same target SUPI. Each request is accepted and overwrites the previous authentication context. A valid EAP-AKA' response for an earlier challenge is then verified against the latest overwritten context, whose `K_aut`, `XRES`, and `EapID` no longer match the challenge. The result is a targeted authentication denial of service for that SUPI while the request flood is maintained.

This issue was confirmed on `github.com/free5gc/ausf` v1.4.4 and current main as of June 2026.

### Details

The vulnerable context pool is defined in `internal/context/context.go`.

`UePool` is a `sync.Map`, which makes individual map operations safe, but it does not make the authentication procedure state safe. The problem is the session design: the key is only the SUPI, and `Store()` unconditionally replaces any active context for that SUPI.

```go type AUSFContext struct { suciSupiMap sync.Map UePool sync.Map // ... }

type AusfUeContext struct { Supi string // ...

// for EAP-AKA'…