An attacker with network-level access to the Service Based Architecture (SBA) or the SBI/N12 interface can flood the AUSF with concurrent POST /nausf-auth/v1/ue-authentications requests for a targeted SUPI. By replacing the active authentication context mid-procedure, the attacker ensures that the legitimate EAP-AKA' response from the user is validated against mismatched session material (specifically K_aut and XRES). This causes integrity check failures and prevents the subscriber from completing authentication, effectively denying the service to the target.

Attack Chain

1. The legitimate subscriber initiates a 5G authentication procedure.
2. The AUSF processes the initial request and stores the authentication context (ctx_LEGIT) in the global UePool map under the target's SUPI key.
3. The attacker identifies a target SUPI/SUCI and initiates a flood of concurrent POST /nausf-auth/v1/ue-authentications requests for that same identity.
4. Each malicious request causes the AUSF to overwrite the previous entry in the UePool map with a new context (ctx_ATTACK).
5. The target subscriber receives an authentication challenge and computes a valid EAP-AKA' response based on ctx_LEGIT.
6. The subscriber transmits the response to the AUSF /eap-session endpoint.
7. The AUSF retrieves the current (overwritten) context (ctx_ATTACK) from the map.
8. The integrity check (AT_MAC verification) fails because the response was signed using parameters from ctx_LEGIT while the AUSF validates against ctx_ATTACK, resulting in a service denial.

Impact

Successful exploitation results in a targeted denial of authentication service for any selected subscriber in a 5G network running vulnerable versions of free5GC. While the AUSF service remains operational and no sensitive authentication material is leaked, the impacted subscriber is unable to connect to the network as long as the attacker maintains the request flood. The scope of impact is limited to those with direct or proxied access to the AUSF internal SBI/N12 interface.

Recommendation

1. Upgrade free5GC AUSF to a version that addresses the context management vulnerability (patch status for v1.4.4+).
2. Implement a unique session identifier in the AusfUeContext and ensure authentication state lookups use this identifier rather than the SUPI.
3. Deploy logic to check for ongoing authentication procedures for a specific SUPI and reject concurrent attempts with HTTP 409 Conflict.
4. Enforce strict mTLS and OAuth2 requirements on all internal SBI traffic to prevent unauthorized access to the AUSF interface.
5. Enable rate limiting on authentication request endpoints to mitigate the impact of flooding attacks.