To ensure security of new 5G telecom networks, NCC Group has been providing guidance, conducting code reviews, red team engagements and pentesting 5G standalone and non-standalone networks since 2019. As with any network various attackers are motivated by different reasons. An attacker could be motivated to either gain information about subscribers on an operator’s network by targeting signalling, accessing the customers private data such as billing records, taking control over the management network or taking down the network. In most cases, the main avenue of attack is via the management layer into the core network – either utilising the operator’s support personnel or via the 3rd party vendor. In all cases attacking a 5G network will take a number of weeks or months, with the main group of attackers being Advanced Persistent Threat (APT) groups. Many governments around the world including the UK government are legislating and demanding operators and vendors reduce telecoms security gaps to ensure a resilient 5G network.

But many operators are unclear on the typical threats and how they could affect their business or if they do at all. Many companies are understandably investing significant time and effort into testing and reviewing threats to make sure they adhere to the compliance requirements.

Here, we aim to cover some of the main issues we have discovered during our pentesting and consultancy engagements with clients and explain not only what they are but how likely the threat is to disrupt the 5G network.

Background

Any typical 5G network deployment be it a Non Standalone (NSA) or Standalone (SA) core, can have various security threats or risks associated with it. These threats can be exploited by either known (i.e. default credentials) or unknown vulnerabilities (i.e. zero day). Primarily the main focus of any attack is via the existing core management network, be it via a malicious insider or an attacker who has leveraged access to a suitably high level administrator account or utilising default credentials. We have seen this first hand with red teaming attacks against various operators. Secondary attack vectors are via insecure remote sites hosting RAN infrastructure, which in turn allow access to the core network utilising the management network. Various mechanisms (i.e. firewalls, IDS etc) are put in place to manage these risks but vulnerable networks and systems have to be tested thoroughly to limit attacks. Having a good understanding of the 5G network topology and associated risks/threats is key and NCC Group has the necessary experience and knowledge to scope and deliver this testing.

Typical perceived threats and severity if compromised are illustrated below. The high risk vector is via the corporate and vendor estate, medium risk vectors via the external internet and rogue operators and low risk vector via the RAN edge nodes. This factors in ease of access plus the degree of severity should an attacker leverage access. For example, if an attacker was to gain access to the corporate network and suitable credentials to access the cloud network equipment running the 5G network, that would have a high level impact if a DoS attack was conducted. This is opposed to an attacker leveraging access to a RAN edge node to conduct a DoS attack, where the exposed risks would be limited to the cell site in question.

“Attack scenarios against a typical 4G/5G mobile network”

So a bit of background on 5G. A 5G NSA network consists of a 5G OpenRAN deployment or a gNodeB utilising a 4G LTE core. A 5G StandAlone (SA) network consists of a 5G RAN (Radio Access Network) plus a 5G core only. Within an NSA deployment, a secondary 5G carrier is provided in addition to the primary 4G carrier. A 5G NSA user equipment (UE) device connects first to the 4G carrier before also connecting to the secondary 5G carrier. The 4G anchor carrier is used for control plane signalling while the 5G carrier is used for high-speed data plane traffic. This approach has been used for the majority of commercial 5G network deployments to date. It provides improved data rates while leveraging existing 4G infrastructure. The main benefits of 5G NSA are an operator can build out a 5G network on top of their existing 4G infrastructure instead of investing in a new, costly 5G core, the NSA network uses 4G infrastructure which operators are already familiar with and deployment of a 5G network can be quicker by using the existing infrastructure. A 5G SA network helps reduce latency, improves network performance and has centrally controlling network management functions. The 5G SA can deliver new essential 5G services such as network slicing, allowing multiple tenants or networks to exist separate from each other on the same physical infrastructure. While services like smart meters require security, low power and high reliability are more forgiving with respect to latency, others like driver-less cars may need ultra-low latency (URLLC) and high data speeds. Network slicing in 5G supports these diverse services and facilitates the efficient reassignment of resources from one virtual network slice to another. However, the main disadvantage of implementing a 5G SA network is the cost to implement and training of staff to learn and configure correctly all parts of the new 5G SA core infrastructure.

A OpenRAN network allows deployment of a Radio Access Network (RAN) with vendor neutral hardware or software. The interfaces linking components use open interface specifications between the components (eg RU/DU/CU) plus with different architecture options. A Radio Unit (RU) is used to handle the radio link and antenna connectivity, a Distributed unit (DU) is used to handle the baseband protocols and interconnections to the Centralised Unit (CU). The architecture options include RAN with just Radio Units (RU) and Base Band units (BBU), or split between RU,DU,CU. Normally the Radio Unit is a physical amplifier device connected over a fibre or coaxial link to a DU component that is normally virtualised. A CU component is normally located back in a secure datacentre or exchange and provides the eNodeB/gNodeB connectivity into the core. In most engagements we have seen the use of Kubernetes running DU/CU pods as docker containers on primarily Dell hardware, with a software defined network layer linking into the 5G core.

In 5G a user identity (i.e. IMSI) is never sent over the air in the clear. On the RAN/edge datacentre the control and user planes are encrypted over air and on the wire (i.e. IPSEC), with 5G core utilising encrypted and authenticated signalling traffic. The 5G network components have externally and internally exposed HTTP2 Service Based Interface (SBI) APIs and provide access directly to the 5G core components for management, logging and monitoring. Usually the SBI interface is secured using TLS client and server certificates. The network can now support different tenants by implementing network slices, with the Software Defined Networking (SDN) layer isolating network domains for different users.

So what are the main security threats?

Shown below is a high level overview of a 5G network with a summary of threats. A radio unit front end containing the gNodeB (i.e. basestation) handles interconnects to the user equipment (UE). A RU/DU/CU together form the gNodeB. The midhaul (i.e. Distributed Unit) handles the baseband layer to the RU over the fronthaul to the midhaul Centralised Unit (CU). The DU does not have any access to customer communications as it may be deployed in unsupervised sites. The CU and Non-3GPP Inter Working Function (N3IWF), which terminates the Access Stratum (AS) security, will be deployed in sites with more restricted access. The DU and CU components can be collocated or separate, usually running as virtualised components within a cluster on standard servers. To support low latency applications, Multi-Access Edge computing (MEC) servers are now being developed to reduce network congestion and application latency to users by pushing the computing resources, including storage, to the edge of the network collocating them with the front RF equipment. The MEC offers application developers and content providers cloud computing capabilities and an IT service environment at the edge of the external data network to provide processing capacity for high demand streaming applications like virtual reality games as well as low latency processing for driverless cars. All links are connected over Nx links. The main threats against the DU/CU/MEC components are physical attacks against the infrastructure either to cause damage (ie arson) or to compromise the operating system to glean information about users on the RAN signalling plane. In some cases, attacking the core via these components by compromising management platforms is also possible. Targeting the MEC by a poorly configured CI/CD pipeline and the ingest of malicious code could also be a threat.

The N1/N2 link carrying the NAS protocol provides mobility management and session management between the User equipment (UE) and Access and Mobility Management Function (AMF). It is carried over the RRC protocol to/from the UE. A User Plane Function (UPF) is used as a router of user data connections. The Core Network consists of an AMF, a gateway to the core network, which talks to the AUSF/UDM to authenticate the handset with the network, plus the network also authenticates using a public key with the handset. In the core network all components including a lot of legacy 4G components are now virtualised, running as Kubernetes pods, with worker nodes running on either custom cloud environment or an opensource instance like Openstack.  Targeting the 5G NFVI or mobile core cloud via the corporate access is a likely attack vector, either disrupting the service by a DoS attack or acquiring billing data. Similar signalling attacks as in 4G are now prevalent in 5G, due to the same 4G components and associated protocols (ie. SS7, DIAMETER, GTP) being collocated with 5G components, utilising the legacy 4G network to provide service for the 5G network. Within 5G, HTTP/2 SBI interfaces are now in use between the core components (ie AMF/UPF etc), however due to no or poor encryption it is still possible to either view this traffic or query APIs directly. The diagram below illustrates the various threats against a typical 5G deployment. A full more compromise hiearchy of threats are detailed within the Mitre FiGHT attack framework.

“Threats against a typical 5G network”

Reducing the vulnerabilities will decrease the risks and threats an operator will face. However, there is a fine line between testing time and finding vulnerabilities, and we can never guarantee we have found all the issues with a component. When scoping pentesting assessments, we always start with the edge and work our way into the centre of the network, trying to peel away the layers of functionality to expose potential security gaps. The same testing methodology applies to any network, but detailed below are some of the key points that we cover when brought into consult on 5G network builds.

Segment, restrict and deny all

Simple idea – if an attacker cannot see the service or endpoint then they cannot leverage access to it. A segmented network can improve network performance by containing specific traffic only to the parts of the network that need to see it. It can help to reduce attack surface by limiting lateral movement and preventing an attack from spreading. For example, segmentation ensures malware in one section does not affect systems in another. Segmentation reduces the number of in-scope systems, thereby limiting costs associated with regulatory compliance. However, we still see poor segmentation during engagements, where it was possible to directly connect to management components from the corporate operator network. Implementing VLANs to segment a 5G network is down to the security team and network architects. When considering a network architecture, segmenting the management network from signalling and user data traffic is key. Limiting access to the 5G core, NFVI services and exposed management to a small set of IP ranges using robust firewall rules with an implicit “deny all” statement is required. The Operations Support System (OSS) and Business Support Systems (BSS) are instrumental in managing the network but if not properly segmented from the corporate network can allow an unauthenticated attacker to leverage access to the entire 5G core network. Implementing robust role based access controls and multi-factor access controls to these applications is key, with suitably hardened Privileged Access Workstations (PAW) in place, with access closely monitored. Do not implement a secure 5G core but then allow all 3rd party vendors access to the entire network. Limit access using the principle of least privilege – should vendor A have access by default to vendor B’s management system? The answer is a clear no.

Limit access to the underlying network switches and routers – be sure to review the configuration of the devices and review the firmware versions. During recent 5G pentesting we have discovered poor default passwords for privileged accounts still in use, allowing access to network components, plus even end of support switch and router firmware. If an attacker was able to leverage access to the underlying network components any virtualised cloud network could be simply removed from the rest of the enterprise network. Within the new 5G network, software-defined networking (SDN) is used to provide greater network automation and programmability through the use of a centralised controller. However, the SDN controller provides a single point of failure and must have robust security policies in place to protect it. Check the configuration of the SDN controller software. Perhaps it is a java application with known vulnerabilities. Or is there an unauthenticated northbound REST API exposed to everyone in the enterprise network? Has the SDN controller OS not been hardened – perhaps no account lockout policy and default/weak SSH credentials used?

In short follow a zero trust principle when designing 5G network infrastructure.

Secure the exposed management edge

An attacker will likely enable access to the corporate network first before horizontally pivoting into the enterprise network via a jumpbox. So secure any services supplying access to the 5G core either at the NFVI application layer such as hardware running the cloud instance, the exposed OSS/BSS web applications or any interconnects (i.e. N1/N2 NAS) back to the core. Limit access to the exposed web applications with strong Role Based Access Controls (RBAC) and monitor access. Use a centralised access management platform (i.e. CyberARK) to control and police access to the OSS/BSS platforms. If you have to expose the cloud hardware processing layer to users (i.e. Dell iDRAC/HP iLO), don’t use default credentials or limit the recovery of remote password hashes. Exposing these underlying hardware control layers to multiple users due to poor segmentation could lead to an attacker conducting a DoS attack by simply turning off servers within the cluster and locking administrators out of the platforms used to manage services.

The myriad of exposed web APIs used for monitoring or control are also a vector for attack. During a recent engagement we discovered an XML External Entity Injection (XXE) vulnerability within an exposed management API and it was possible for an authenticated low privileged attacker to use a vulnerability in the configuration of the XML processor to read any file on the host system.

It was possible to send crafted payloads to the endpoint OSS application located at https://10.1.2.3/oss/conf/ and trigger this vulnerability, which would allow an attacker to:

• Read the filesystem (including listing directories), which ended in getting a valid user to log into the server running the API alongside the credentials to successfully log into the SSH service of the mentioned machine.
• Trigger Server Side Request forgery.

The resulting authenticated XXE request and response is illustrated below:

Request

POST /oss/conf/blah HTTP/1.1
Host: 10.1.2.3:443
Cookie: JSESSIONID=xxxxxx
[…SNIP…]
<!DOCTYPE root []<nc:rpc xmlns:……..
none test;

Response

HTTP/1.1 200
error-message xml:lang=”en”>/edit-someconfig/default: “noneroot:x:0:0:root:/
root:/bin/bash bin:x:1:1:bin:/bin:/sbin/nologin daemon:x:2:2:daemon:/sbin:/sbin/nologin
user:x:1000:0::/home/user:/bin/bash

Using this XXE vulnerability, it was possible to read a properties file and recover LDAP credential information and then SSH directly into the host running the API server. In this particular case, once on the host running the containerised web application, the user could read all encrypted password hashes that were stored on the host, utilising the same decryption process and poorly stored key values that were used to encrypt the hashes. The same password was used for the root account and allowed for trivial privileged escalation to root. With the root access to the running API server, which in turn was a docker container running as a Kubernetes pod, it was possible to leverage a vulnerability with the Kubernetes configuration to compromise the container and escalate privileges to the underlying cluster worker node host. To prevent this type of escalation a defense in depth approach is paramount on any Linux host plus on any containers. More on this below.

Implement exploit mitigation measures on binaries

If you expose a service externally be sure to check it is compiled with exploit mitigation measures. Exploitation can be significantly simplified due to the manner in which any service/binary has been built. If a binary has an executable stack, and lacks any modern exploit mitigations such as ASLR, NX, stack cookies, hardened C functions, etc, then an attacker can utilise any issues they might find such as a stack buffer overflow, to get remote code execution (RCE) on the host. This was discovered whilst testing a 5G instance and an exposed sensitive encrypted and proprietary service. This service was exposed externally to the enterprise network, and after a brief analysis showed that it was likely a high risk process due to –

• It was exposed on all network interfaces, making it reachable across the network
• It ran as the root user
• It was built with an executable stack, and no exploit mitigations
• It used unsafe functions such as memcpy, strcat, system, popen etc.

The service took a simple encrypted stream of data that was easily decrypt-able into a configuration message. Analysis of the message/data stream showed an issue with how the buffer data was stored and it was possible to trigger a memory corruption via a stack buffer overflow. After decompiling the binary using Ghidra, it was clear one important value was not used as an input to the function processing a certain string of data making up the configuration message – the size of the buffer used to store the parts of the string. Many of the instances where the function was used were safe due to the size and location of the target buffers. However, one of the elements of the message string was split into 12 parts, the first of which was stored in a short buffer (20 bytes in length) that was located at the end of the stack frame. Due to its length it was possible to overwrite data that was adjacent to the buffer, and due to the buffer’s location, this was the saved instruction pointer. When the function completed, the saved instruction pointer was used to determine where to continue execution. As the attacker could control this value they could take control over the process’s execution flow.

Knowing how to crash the stack it was possible using Metasploit to determine the offset of the instruction pointer and to determine how much data could be written to the stack. As the stack was executable it was straightforward to find a ROP gadget that would perform the command ‘JMP ESP’. An initial 100 byte payload was generated using Metasploit (pattern_create.rb). This was used to find the offset to over write the instruction pointer, using the Metasploit pattern_offset.rb script. The shellcode was generated by Metasploit and simply created a remote listener on port 5600. The shellcode was written to the stack after the bytes that control the instruction pointer.

To find and generate suitable exploit code took around 5-10 days work and would require an attacker with good reverse engineering skills. This service was running as root on the 5G virtualised network component, and due to the virtualised component accesses within the 5G network, could have been leveraged by attacker to compromise all other components. During this review the AFL fuzzer was used to determine any other locations within the input stream that could potentially cause a crash. A number of crashes were found revealing multiple issues with the binary.

“Running AFL fuzzer against the target binary”

To illustrate this issue further please read our blog posts Exploit the Fuzz – Exploiting Vulnerabilities in 5G Core Networks. In this particular opensource case, exposing “external” protocols and associated services like the UPF component on a remotely hosted server, not directly within in the 5G core could be leveraged by attacker to compromise a server (ie SMF) within the 5G core. It is important to bear this in mind when deploying equipment out to the end of the network. Physical access to the component, even when within a roadside cabinet or semi-secure location such as an exchange is possible, allowing an attacker to leverage access to the 5G core via a not so closely monitored signalling or data plane service. This is more prevalent now with the deployment of OpenRAN components, where multiple services (RU,DU,CU) are now potentially exposed.

Secure the virtualised cloud layer

All 5G core run on a virtualised cloud system being a custom built environment or from a separate provider such as VMWare. The main question is can an attacker break out of one container or pod to compromise other containers or potentially other clusters? It might even be possible for an attack to exploit the underlying hypervisor infrastructure if suitably positioned. There are multiple capabilities assigned to a running pod/container – privileged containers, hostpid, sysadmin, docker.sock, hostpath, hostnetwork – that could be overly permissive so allowing an attacker to leverage a feature to mount the underlying host cluster file system or to take full control over the Kubernetes host. We have also seen issues with kernel patching with a kernel privileged escalation vulnerability leveraged to breakout of a container.

During recent testing, applying security controls on the deployment of pods in the cluster were not managed by an admission controller. This meant that privileged containers, containers with the node file system mounted, containers running as root users, and containers with host facilities, could be deployed. This would enable any cluster user or principal with pod deployment privileges to compromise the cluster, the workloads, the nodes, and potentially gain access to the wider 5G environment.

The risk to an operator is that any developer with deployment privileges, even to a single namespace, can compromise the underlying node and then access all containers running on that node – which may be from other namespaces they do not have direct privileges for, breaking the separation of role model in use.

Leveraging a vulnerability such as the previous XXE issue or brute forcing SSH login credentials to a Docker container running with overly permissive capabilities has been leveraged on various engagements and is illustrated below.

“Container breakout via initial XXE vulnerability”

As mentioned it was possible to recover ssh credentials with a XXE vulnerability. Utilising the SSH access an escalating to root permissions on the container, it was possible to abuse a known issue with cgroups to perform privilege escalation and compromise the nodes and cluster from an unprivileged container. The Linux kernel does not check that the process setting the cgroups release_agent file has correct administrative privileges – the CAP_SYS_ADMIN capability in the root namespace , and so an unprivileged container that can create a new namespace with a fake CAP_SYS_ADMIN capability through unshare, could force the kernel to execute arbitrary commands when a process completed.

It was possible to enter a namespace with CAP_SYS_ADMIN privileges, and use the notify_on_release feature of cgroups, that did not differentiate between root namespace CAP_SYS_ADMIN and user namespace CAP_SYS_ADMIN, to execute a shell script with root privileges on the underlying host. A syscall breakout was used to execute a reverse shell payload with cluster admin privileges on the underlying cluster host. This is shown below:

“Container breakout utilising cgroups”

Once a shell was created on the underlying kubernetes cluster host, it was then possible to SSH directly to the RAN cluster due to credentials seen in backup files and exploit any basestation equipment. It was also possible to leverage weak security controls on the deployment of pods in the cluster since there was no admission controller. As this exploited cluster user had pod deployment privileges, it was possible to deploy a manifest specifying a master node for the pod to be deployed to, the access gained was root privileges on a master node. This highly privileged access enabled compromise of the whole cluster through gaining cluster administer privileges from a kubeconfig file located on the node filesystem.

As a proof of concept attack, the following deployment specification can be used to target the master node by chroot’ing to the underlying host :

“Deploying a bad pod to gain access to master node”

With the kubeconfig file from the master node it is then possible to read all namespaces on the cluster. It would also be possible from the master node to access the underlying hypervisor or virtualisation platform. We have also had in some cases due to discovered credentials, the ability to log directly into the VSphere client and disable hosts.

Strict enforcement of privilege limitations is essential to ensuring that users, containers, and services cannot bridge the containerisation layers of container, namespace, cluster, node, and hosting service. It should be noted that if only a small number of principals have access to a cluster, and they all require cluster administration privileges then, a cluster admin could likely modify any admission controller policies. However, best practice is to implement business policies and enforce the blocking of containers with weak security controls. Equally, if more roles are included with the administration model at a later date, then the likelihood of value in implementing admission controllers increases. In short the main recommendation is to ensure appropriate privilege security controls are enforced to prevent deployments having access or the ability to compromise other layers of the orchestration model. Consider implementing limitations to which worker nodes containers can deploy, and insecure manifest configurations can be deployed.

Scan, verify, monitor and patch all images regularly

It is important when deploying virtualised container images to check regularly for any changes to the underlying OS, audit any events such as login events plus patch all critical vulnerabilities as soon as possible. Basic vulnerability management is key – identifying and prevent risks to all the hosts, images and functions. Scanning images before they are deployed should be done by default on a regular interval.

For instance, if a Kubernetes cluster is utilising a Harbor registry, simply enabling vulnerability scanning “Automatically scan images on push” with a suitable tool such as Trivy with a regularly updated set of vulnerabilities will suffice. Even preventing vulnerable images from running is possible for images with a certain severity. Implement signed images or content trust also gives you the ability to verify both the integrity and the publisher of all the data received from a registry over any channel.

“Setting harbor to automatically scan images”

Enforce with tighter contracts with vendors the need to supply patches to images quicker and verify as much as possible all patches have had no change to the underlying functionality. Enforcing the use of harden Linux OS images is best practice, utilising CIS benchmarks scans to verify OS images have been hardened. This is also important on the underlying cluster hosts. Our recommendation is to move security back to the developer or vendor with a secure Continuous Integration and Continuous Development (CI/CD) pipeline with Open Policy Agent integrations to secure workloads across the Software Development Life Cycle (SDLC). NCC Group conducts regular reviews of CI/CD pipelines and can help you understand the issues. Please check out 10 real world stories of how we’ve compromised ci/cd pipelines for further details.

If possible get a software build of materials (SBOM) from vendors. SBOM is an industry best practice part of secure software development that enhances the understanding of the upstream software supply chain, so that vulnerability notifications and updates can be properly and safely handled across the installed customer base. The SBOM documents proprietary and third-party software, including commercial and free and open source software (FOSS), used in software products. The SBOM should be maintained and used by the software supplier and stored and viewed by the network operator. Operators should be periodically checking against known vulnerability databases to identify potential risk. However, the level of risk for a vulnerability should be determined by the software vendor and operator with consideration of the software product, use case, and network environment.

Once an image is running, verifying the running services is key with some kind of runtime defences. This will entail implementing strong auditing utilising auditd and syslog to monitor kernel, process and access logs. We have seen no use of this service plus no use of any antivirus. Securing containers with Seccomp and either AppArmor or SELinux would be enough to prevent container escape. Taking all the logging data into a suitable active defence engine could allow for more predictive and threat-based active protection for running containers. Predictive protection could include capabilities like determining when a container runs a process not included in the origin image or creates an unexpected network socket. Threat-based protection includes capabilities like detecting when malware is added to a container or when a container connects to a botnet. Utilising a machine learning model to create a model for each running container in the cluster is highly recommended. Applied intelligence used for monitoring log data is key for any threat prevention, aiding in the SOC identifying quickly key 5G attack vectors.

Implement 5G security functions

Previous generations of cellular networks failed on providing confidentiality/integrity protection on some pre-authentication signalling messages, allowing attackers to exploit multiple vulnerabilities such as IMSI sniffing or downgrade attacks to 5G. The 5G standard facilitates a base level of security with various security features. However, we have seen during engagements these are not enabled.

The 5G network uses data encryption and integrity protection mechanisms to safeguard data transmitted by the enterprise, prevent information leakage and enhance data security for the enterprise. Not implementing these will compromise the confidentiality, integrity and availability (CIA).

5G introduces novel protection mechanisms specifically designed for signalling and user data. 5G security controls outlined in 3GPP Release 15 include:

• Subscriber permanent identifier (SUPI) – a unique identifier for the subscriber
• Dual authentication and key agreement (AKA)
• Anchor key is used to identify and authenticate UE. The key is used to create secure access throughout the 5G infrastructure
• X509 certificates and PKI are used to protect various non-UE devices
• Encryption keys are used to demonstrate the integrity of signalling data
• Authentication when moving from 3GPP to non-3GPP networks
• Security anchor function (SEAF) allows reauthentication of the UE when it moves between different network access points
• The home network carries out the original authentication based on the home profile (home control)
• Encryption keys will be based on IP network protocols and IPSec
• Security edge protection proxy (SEPP) protects the home network edge
• 5G separates control and data plane traffic

Besides increasing the length of the key algorithms (to 256-bit expected for future 3GPP releases), 5G forces mandatory integrity support of the user plane, and extends confidentiality and integrity protection to the initial NAS messages. The table below summarises in various columns the standard requirements in terms of confidentiality and integrity protection as defined in the 3GPP specs. 5G also secures the UE network capabilities, a field within the initial NAS message, which is used to allow UEs to report to the AMF about the supported integrity and encryption algorithms in the initial NAS message.

In general there has been an increase in the number of security features in 5G to address issues found with the legacy 2G, 3G and 4G network deployments and various published exploits. These have been included within the different 3GPP specifications and adopted by the various vendors. It should be noted that a lot of the security features are optional and the implementation of these is down to the operator rather than the vendor.

The only security features that are defined as mandatory within the 5G standards are integrity checking of the RRC/NAS signalling plane and on the IPX interface the mandatory use of a Security Edge Protection Proxy (SEPP). The SUPI encryption is optional but in the UK this is required due to GDPR.

“Table illustrating various 4G / 5G security functions”

As shown, the user plane integrity protection is still optional so still in theory vulnerable to attack such as malicious redirect of traffic using a DNS response. Some providers now by default turn on the new integrity protection feature for the user plane and prevent an attacker forcing the network to use a less secure algorithm. In 4G, a series of GRX firewalls are in place to limit attacks via the IPX network but due to the use of HTTPS in 5G control messages a new SEPP device is mandated to allow matching of control and user plane sessions.

By collecting 5G signalling traffic it is possible to check implementations and analyse the vulnerabilities. NCC Group conducts these assessments and advises clients on implementing various optional security features either related to 5G or with other legacy systems such as enabling A5/4 algorithm on GSM networks. This issue is illustrated clearly within the paper European 5G Security in the Wild: Reality versus Expectations. This paper highlights the issues with no concealment of permanent identifiers and the fact it was possible to capture the permanent IMSI and IMEI values, which are sent without protection within the NAS Identity Response message. Issues with the temporary identifier and GUTI refresh have also been observed. After receiving the NAS Attach Accept and RRC Connection Request messages, the freshness of m-TMSI value was not changed, only changing during a Registration procedure. This would allow TMSI tracking and possible geolocation of 5G user handsets.

As 5G networks become more mature and deployments progress to full 5G SA deployments, it is likely issues affecting the network will be addressed. However, it is important to implement and test these new security features as soon as possible to prevent a compromise.

Summary

The 5G network is a complex environment, requiring methodical comprehensive reviews to secure the entire stack. Often a company may lack the time, specialist security knowledge, and people needed to secure their network. Fundamentally, a 5G network must be configured properly, robustly tested and security features enabled.

As seen from above, most compromises have the following root causes or can be traced back to:

• Lack of segmentation and segregation
• Default configurations
• Over permissive permissions and roles
• Poor patching
• Lack of security controls

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