## 3 Attacks on 5G NSA
### MITM Relay  
An MITM relay consists of a fake enodeB and a fake UE connected locally or remotely (e.g., via another communication technology). The fake enodeB part of the relay is used to lure victim UE to connect to the relay, while the fake UE part acts as a legitimate UE towards the core. The adversary can replay the messages between victim UE and the legitimate enodeB with and without tampering. 1. **Tampering**. In these attacks, the MITM attacker tampers with some messages before sending them to the recipient. Selected attacks belonging to this category are described below. 2. In an _Alter Attack_, the adversary exploits the lack of integrity protection for user plane data to perform a chosen ciphertext attack [28]. Specifically, the adversary performs a Domain Name System (DNS) redirection attack by manipulating the destination IP address of a DNS request, redirecting the request to a malicious server. In order to perform the attack, the adversary operates a malicious relay between the UE and the enodeB and applies a manipulation mask on the DNS packet to change its destination IP address to a malicious IP address. Then, it forwards the packets to the external network (e.g., the Internet). At the core network, the malicious DNS request is decrypted and routed to the malicious DNS server, which replies with a spoofed DNS response. 3. An _IMP4 GT Attack_ is an extension of the Alter attack. In this attack, the adversary exploits the missing integrity protection for user plane data and the reflection mechanism of the IP stack mobile operating system to impersonate a user toward the network and vice versa [29]. In the uplink impersonation variant, the adversary can access the Internet using the victim's IP address. In the downlink variant, the adversary establishes a TCP/IP connection to the phone to bypass the firewall mechanisms. 4. The _Null Ciphering Attack_[30] allows the UE to establish an unencrypted radio connection with gnodeB. It occurs due to the implementation flaw within the network of not rejecting UEs with invalid UE Additional Security Capabilities and the implementation flaw in the UE of not verifying replayed UE Additional Security Capabilities and Hash of the Attach message. These additional capabilities specify information about the supported algorithms for protecting the data transferred over gnodeB in 5G NSA. The capabilities are sent in the Attach Request message and are later replayed back to detect manipulation. In order to perform the attack, the adversary needs to set up an MITM relay to lure the UE to connect to it. Once the UE sends the Attach request message to the MITM relay, the attacker modifies the UE Additional Security Capabilities to include support only for null ciphering (NEA0). The network receives this message and replays the UE Additional Security Capabilities back to the UE in an integrity-protected NAS Security mode command without checking the capabilities. In the next phase, the MME would inform the target gnodeB about the additional security capabilities. Since the only ciphering algorithm left in the 5G security capability set is NEA0, enodeB would instruct the UE to establish an unencrypted radio connection to gnodeB via the RRC Connection Reconfiguration message. Note that it is also possible to perform a _battery draining attack on IoT devices_ (implementation flaw) or a DoS attack [8] (implementation flaw) using a similar attack procedure. * In the _Radio Service Hijacking_ attack [8], the adversary exploits an implementation flaw where the enodeB requests the UE Radio Capabilities before establishing the RRC security. The enodeB requests the Radio Capability from the UE by sending a UECapabilityEnquiry message, and the UE responds with the UECapabilityInformation message. In order to perform the attack, the adversary operates an MITM relay at a different TAC and lures the UE into initiating a new registration procedure via the relay.