Method, system, and computer-readable medium for preventing unauthorized use of certificate management protocol (CMP) client identity private keys and public key certificates associated with network functions

Abstract

A method for preventing unauthorized use of a CMP client identity private key and a CMP public key certificate associated with a NF includes, by a CMP CA proxy, receiving a first CMP certificate request to update a security certificate associated with a first NF, the CMP certificate request including a public key certificate associated with the first NF and protected by the CMP client identity private key associated with the first NF. The method further includes: determining that the first NF is registered with an NRF; in response to determining that the first NF is registered with the NRF, the CMP CA proxy checking whether the first CMP certificate request includes an NRF-issued access token for the first NF, determining that the CMP certificate request does not include an NRF-issued access token for the first NF; and, in response to determining that the first CMP certificate request does not include an NRF-issued access token for the first NF, performing a network security action with respect to the first CMP certificate request.

Description

[Technical Field]

[0001] Priority claims This application claims the benefit of priority to U.S. Patent Application No. 18 / 113,041, filed February 22, 2023, the entire disclosure of which is incorporated herein by reference.

[0002] Technical Field The subject matter described herein relates to preventing fraudulent use of credentials that may be used to impersonate a 5G Network Function (NF), such as a 5G Network Function (NF) in a 5G network. More specifically, the subject matter described herein relates to methods, systems, and computer-readable media for preventing fraudulent use of CMP client identity private keys and public key certificates associated with an NF. [Background technology]

[0003] background In a 5G communication network, a network function that provides a service is called a producer NF or an NF service producer. A network function that consumes a service is called a consumer NF or an NF service consumer. A network function may be a producer NF, a consumer NF, or both, depending on whether the network function is consuming, generating, or consuming and generating the service. The terms "producer NF" and "NF service producer" are used interchangeably herein. Similarly, the terms "consumer NF" and "NF service consumer" are used interchangeably herein.

[0004] A producer NF may have many service endpoints, which are contact points for one or more NF instances hosted by the producer NF. A service endpoint is identified by a combination of an Internet Protocol (IP) address and port number on the network node hosting the producer NF, or a fully qualified domain name (FQDN) that resolves to an IP address and port number. An NF instance is an instance of a producer NF that provides a service. A producer NF may contain two or more NF instances. It should also be noted that multiple NF instances can share the same service endpoint.

[0005] NFs register with a Network Capability Repository Function (NRF). The NRF maintains profiles of available NF instances that identify the services supported by each NF instance. The profile of an NF instance is called an NF profile in 3GPP TS 29.510. An NF instance can obtain information about other NF instances registered with the NRF through an NF discovery service operation. According to the NF discovery service operation, a consumer NF sends an NF discovery request to the NRF. The NF discovery request includes query parameters that the NRF uses to find NF profiles of producer NFs that can provide the service identified by the query parameters. An NF profile is a data structure that defines the types of services provided by the NF instance as well as contact and capability information about the NF instance.

[0006] The Service Communication Proxy (SCP) can also invoke the NF discovery service operation to learn about available producer NF instances. The case where the SCP uses the NF discovery service operation to obtain information about producer NF instances on behalf of a consumer NF is called delegated discovery. The consumer NF contacts the SCP, and the SCP load balances traffic among the producer NF service instances that offer the required service, or routes traffic directly to the destination producer NF instance.

[0007] In addition to the SCP, another example of an intermediate proxy that forwards traffic between producer and consumer NFs is the Security Edge Protection Proxy (SEPP). The SEPP is a network function used to protect the control plane traffic exchanged between different 5G public land mobile networks (PLMNs). To that end, the SEPP performs message filtering, policing, and topology hiding for all application programming interface (API) messages sent between PLMNs.

[0008] One issue in 5G and other types of networks is that NF client identity private keys and public key certificates can be stolen and used without authorization to obtain a Transport Layer Security (TLS) certificate for the NF. The TLS certificate for the NF includes the NF instance ID of the NF in the TLS certificate's subject alternate name. The TLS certificate can then be used to impersonate the NF and perform malicious activities within the network.

[0009] The 3rd Generation Partnership Project (3GPP) specifies the use of the Certificate Management Protocol (CMP) for automated certificate management. CMP messages are protected using CMP client private keys and public key certificates, which may be manually configured in a Certificate Manager (CM) by a network operator. If the CMP client private keys and CMP public key certificates are stolen, a hacker could successfully obtain a TLS certificate for a registered NF and use that TLS certificate to impersonate the NF and perform malicious activities within the network.

[0010] Therefore, in light of these and other challenges, there is a need to prevent the unauthorized use of CMP client identity private keys and public key certificates in order to prevent hackers from obtaining and using NF's TLS certificates. Summary of the Invention [Means for solving the problem]

[0011] overview A method for preventing unauthorized use of a Certificate Management Protocol (CMP) client identity private key and a CMP public key certificate associated with a network function (NF) includes receiving, by a CMP certificate authority (CA) proxy, a first CMP certificate request for updating a security certificate associated with a first NF, the CMP certificate request including a public key certificate associated with the first NF and protected by the CMP client identity private key associated with the first NF. The method further includes the CMP CA proxy checking whether the first NF is registered with an NF Repository Function (NRF) and determining that the first NF is registered with the NRF. The method further includes, in response to the CMP CA proxy determining that the first NF is registered with the NRF, the CMP CA proxy checking whether the first CMP certificate request includes an NRF-issued access token for the first NF and determining that the CMP certificate request does not include an NRF-issued access token for the first NF. The method further includes, in response to the CMP CA proxy determining that the first CMP certificate request does not include an NRF-issued access token for the first NF, performing a network security action with respect to the first CMP certificate request.

[0012] According to another aspect of the subject matter described herein, receiving the first CMP certificate request includes receiving the first CMP certificate request from a hacker masquerading as the first NF.

[0013] According to another aspect of the subject matter described herein, performing a network security action with respect to the first CMP certificate request includes preventing the CMP CA from providing a security certificate to a hacker.

[0014] According to another aspect of the subject matter described herein, preventing the CMP CA from providing a security certificate to the hacker includes blocking a first CMP certificate request from reaching the CMP CA.

[0015] According to another aspect of the subject matter described herein, receiving the first CMP certificate request at a CMP CA proxy includes receiving the first CMP certificate request at a CMP CA proxy that is a component of the NRF or a CMP CA proxy that is separate from the NRF.

[0016] In accordance with another aspect of the subject matter described herein, receiving the first CMP certificate request includes intercepting the first CMP certificate request.

[0017] According to another aspect of the subject matter described herein, receiving the first CMP certificate request includes receiving the first CMP certificate request for a Transport Layer Security (TLS) or Client Credential Assertion (CCA) certificate of the first NF.

[0018] According to another aspect of the subject matter described herein, checking whether the first CMP certificate request includes an NRF-issued access token for the first NF includes checking whether the first CMP request includes an OAuth 2.0 access token issued by the NRF to the first NF.

[0019] According to another aspect of the subject matter described herein, a method for preventing unauthorized use of a CMP client identity private key and a CMP public key certificate associated with an NF includes: a CMP CA proxy receiving a second CMP certificate request to renew a security certificate associated with a second NF, the request including a public key certificate associated with the second NF and protected by the CMP client identity private key associated with the second NF. The method further includes the CMP CA proxy checking whether the second NF is registered with an NRF and determining that the second NF is not registered with the NRF. The method further includes, in response to determining that the second NF is not registered with the NRF, allowing processing of the second CMP certificate request by forwarding the second CMP certificate request to the CMP CA. The method for preventing unauthorized use of a CMP client identity private key and a CMP public key certificate associated with an NF includes generating, at the CMP CA, a CMP certificate response including the security certificate associated with the second NF and forwarding the CMP certificate response to an originator of the second CMP certificate request.

[0020] According to another aspect of the subject matter described herein, a method for preventing unauthorized use of a CMP client identity private key and a CMP public key certificate associated with an NF includes a second NF sending a second CMP certificate request to a CMP CA proxy to update a second security certificate associated with the second NF, the second CMP certificate request including the public key certificate is protected by the CMP client identity private key associated with the second NF and includes an NRF-issued access token associated with the second NF, the method including: the CMP CA proxy receiving the second CMP certificate request; the CMP CA proxy checking whether the second NF is registered with the NRF and determining that the second NF is registered with the NRF; in response to determining by the CMP CA proxy that the second NF is registered with the NRF, the CMP CA proxy checking whether the second CMP certificate request includes an NRF-issued access token associated with the second NF and determining that the second CMP certificate request includes an NRF-issued access token associated with the second NF; and further including, in response to determining, by the CA proxy, that the second CMP certificate request includes an NRF-issued access token associated with the second NF, allowing processing of the second CMP certificate request by forwarding the second CMP certificate request to the CMP CA proxy.

[0021] According to another aspect of the subject matter described herein, a system for preventing unauthorized use of a CMP client identity private key and a CMP public key certificate associated with an NF includes a CMP CA proxy including at least one processor and a memory. The system further includes a CMP certificate request validator implemented by the at least one processor, wherein the validator receives a first CMP certificate request for updating a security certificate associated with a first NF, the CMP certificate request including the public key certificate associated with the first NF and protected by the CMP client identity private key associated with the first NF. The validator checks whether the first NF is registered with an NF Repository Function (NRF) and determines that the first NF is registered with the NRF. In response to determining that the first NF is registered with the NRF, the validator checks whether the first CMP certificate request includes an NRF-issued access token for the first NF and determines that the CMP certificate request does not include an NRF-issued access token for the first NF. In response to determining that the first CMP certificate request does not include an NRF-issued access token for the first NF, the validator performs a network security action with respect to the first CMP certificate request.

[0022] According to another aspect of the subject matter described herein, the first CMP certificate request originates from a hacker masquerading as the first NF.

[0023] According to another aspect of the subject matter described herein, when performing a network security action with respect to the first CMP certificate request, the CMP certificate request validator is configured to prevent the CMP CA from providing a security certificate to a hacker by blocking the first CMP certificate request from reaching the CMP CA.

[0024] According to another aspect of the subject matter described herein, the CMP CA proxy may include components of the NRF or may be separate from the NRF.

[0025] According to another aspect of the subject matter described herein, the CMP certificate request validator is configured to receive a first CMP certificate request by intercepting the first CMP certificate request.

[0026] According to another aspect of the subject matter described herein, the security credentials include a Transport Layer Security (TLS) or Client Credential Assertion (CCA) certificate of the first NF.

[0027] According to another aspect of the subject matter described herein, the NRF-issued access token for the first NF includes an OAuth 2.0 access token issued by the NRF to the first NF.

[0028] According to another aspect of the subject matter described herein, a CMP certificate request validator is configured to receive a second CMP certificate request for updating a security certificate associated with a second NF, the request including a public key certificate associated with the second NF and protected by a CMP client identity private key associated with the second NF. The CMP certificate request validator is further configured to check whether the second NF is registered with an NRF and determine that the second NF is not registered with the NRF. In response to determining that the second NF is not registered with the NRF, the CMP certificate request validator is further configured to allow processing of the second CMP certificate request by forwarding the second CMP certificate request to a CMP CA.

[0029] According to another aspect of the subject matter described herein, a system for protecting a CMP client identity private key and a CMP public key certificate includes a second MF that sends a second CMP certificate request to a CMP CA proxy to renew a second CMP certificate associated with a second NF, the second CMP certificate request including a public key certificate associated with the second NF, protected by a CMP client identity private key associated with the second NF, and including an NRF-issued access token associated with the second NF. The CMP certificate request validator is configured to receive a second CMP certificate request, check whether the second NF is registered with the NRF and determine that the second NF is registered with the NRF, in response to determining that the second NF is registered with the NRF, check whether the second CMP certificate request includes an NRF-issued access token associated with the second NF and determine that the second CMP certificate request includes an NRF-issued access token associated with the second NF, and in response to determining that the second CMP certificate request includes an NRF-issued access token associated with the second NF, allow processing of the second CMP certificate request by forwarding the second CMP certificate request to a CMP CA.

[0030] According to another aspect of the subject matter described herein, a non-transitory computer-readable medium having stored thereon executable instructions that, when executed by a processor of a computer, control the computer to perform steps including receiving, by a Certificate Management Protocol (CMP) Certificate Authority (CA) proxy, a first CMP certificate request for updating a security certificate associated with a first network function (NF), the CMP certificate request being protected by a public key certificate associated with the first NF and a CMP client identity private key associated with the first NF. The steps further include checking, by the CMP CA proxy, whether the first NF is registered with a Network Repository Function (NRF) and determining that the first NF is registered with the NRF. The steps further include, in response to the CMP CA proxy determining that the first NF is registered with the NRF, checking, by the CMP CA proxy, whether the first CMP certificate request includes an NRF-issued access token for the first NF and determining that the CMP certificate request does not include an NRF-issued access token for the first NF. The steps further include performing, by the CMP CA proxy, a network security action with respect to the first CMP certificate request in response to determining that the first CMP certificate request does not include an NRF-issued access token for the first NF.

[0031] The subject matter described herein can be implemented in software combined with hardware and / or firmware. For example, the subject matter described herein can be implemented in software executed by a processor. In one exemplary embodiment, the subject matter described herein can be implemented using a non-transitory computer-readable medium having stored thereon computer-executable instructions that, when executed by a computer's processor, control the computer to perform steps. Exemplary computer-readable media suitable for implementing the subject matter described herein include non-transitory computer-readable media such as disk memory devices, chip memory devices, programmable logic devices, and application-specific integrated circuits. In addition, computer-readable media implementing the subject matter described herein can be located on a single device or computing platform or distributed across multiple devices or computing platforms.

[0032] Exemplary embodiments of the subject matter described herein will now be described with reference to the accompanying drawings. [Brief explanation of the drawings]

[0033] [Figure 1] FIG. 1 is a network diagram illustrating an example 5G system network architecture. [Figure 2] FIG. 1 is a network diagram illustrating an example security architecture for certificate management for 5G NFs. [Figure 3] 3 is a message flow diagram illustrating exemplary messages exchanged when obtaining a TLS certificate using the security architecture shown in FIG. 2. [Figure 4] FIG. 10 is a message flow diagram illustrating exemplary messages exchanged in an NFRegister service operation. [Figure 5]FIG. 10 is a message flow diagram illustrating example messages exchanged when a hacker obtains a NF's TLS certificate using a stolen CMP client identity private key and certificate. [Figure 6A] FIG. 1 is a network diagram illustrating an example security architecture in which a CA proxy intercepts a CMP certificate request and requests an NRF-issued OAuth 2.0 access token for a registered NF before providing the TLS certificate of that NF to the querying entity. [Figure 6B] FIG. 10 is a network diagram illustrating an alternative security architecture in which a CA proxy intercepts a CMP certificate request and requests an NRF-issued OAuth 2.0 access token for a registered NF before providing the TLS certificate of that NF to the querying entity. [Figure 6C] FIG. 10 is a network diagram illustrating yet another alternative security architecture in which a CA proxy intercepts a CMP certificate request and requests an NRF-issued OAuth 2.0 access token for a registered NF before providing the TLS certificate of that NF to the querying entity. [Figure 7] FIG. 10 is a message flow diagram illustrating exemplary messages exchanged with a CMP CA proxy to validate a CMP certificate request. [Figure 8] FIG. 2 is a block diagram illustrating an example architecture for a CMP CA proxy. [Figure 9] 10 is a flowchart illustrating an example process for preventing unauthorized use of a CMP client identity private key and a CMP public key certificate associated with an NF. DETAILED DESCRIPTION OF THE INVENTION

[0034] Detailed Description FIG. 1 is a block diagram illustrating an example 5G system network architecture. The architecture of FIG. 1 includes an NRF 100 and an SCP 101, which may be located within the same Home Public Land Mobile Network (HPLMN). As described above, the NRF 100 maintains a profile of available NF instances and their supported services, allowing consumer NFs or SCPs to subscribe to new / updated NF instances and be notified of their registration. The SCP 101 can also support service discovery and NF instance selection. The SCP 101 can perform load balancing of connections between consumer NFs and producer NFs.

[0035] The NRF 100 is a repository of profiles of NF instances. To communicate with a producer NF instance, a consumer NF or SCP must obtain the NF profile of the producer NF instance from the NRF 100. The NF profile is a JavaScript Object Notation (JSON) data structure defined in 3GPP TS 29.510. The NF profile contains attributes that indicate the type of service provided, the capacity of the NF instance, and information for contacting the NF instance.

[0036] 1, any of the network functions may be consumer NFs, producer NFs, or both, depending on whether they are requesting, providing, or both, a service. In the illustrated example, the NFs include a Policy Control Function (PCF) 102 that performs policy-related operations within the network, a Unified Data Manager (UDM) 104 that manages user data, and an Application Function (AF) 106 that provides application services.

[0037] 1 further includes an Access and Mobility Management Function (AMF) 110 and a Session Management Function (SMF) 108 that manages sessions between the PCF 102. The AMF 110 performs mobility management operations similar to those performed by a Mobility Management Entity (MME) in 4G networks. An Authentication Server Function (AUSF) 112 performs authentication services for user equipment (UE), such as user equipment (UE) 114, that seeks access to the network.

[0038] The Network Slice Selection Function (NSSF) 116 provides network slicing services to devices that seek to access specific network capabilities and characteristics associated with a network slice. The NSSF 116 provides the NSSelection service, which allows NFs to request information about network slices and NSSAI reachability services, which allows NFs to update and subscribe to receive communication of updates in Network Slice Selection Assistance Information (NSSAI) reachability information.

[0039] The Network Exposure Function (NEF) 118 provides an application programming interface (API) for application functions that want to obtain information about Internet of Things (IoT) devices and other UEs attached to the network. The NEF 118 performs a function similar to the Service Capability Exposure Function (SCEF) in 4G networks.

[0040] The radio access network (RAN) 120 connects the user equipment (UE) 114 to the network via a wireless link. The radio access network 120 may be accessed using a gNB (not shown in FIG. 1) or other wireless access points. The user plane function (UPF) 122 may support various proxy functionalities for user plane services. One example of such proxy functionality is multipath transmission control protocol (MPTCP) proxy functionality. The UPF 122 may also support performance measurement functionality, which may be used by the UE 114 to obtain network performance measurements. Also shown in FIG. 1 is a data network (DN) 124 through which the UE accesses data network services, such as Internet services.

[0041] The SEPP 126 filters incoming traffic from another PLMN and performs topology hiding for traffic egressing from the home PLMN. The SEPP 126 may communicate with a SEPP in a foreign PLMN that manages security for the foreign PLMN. Thus, traffic between NFs in different PLMNs may traverse two SEPP functions, one for the home PLMN and the other for the foreign PLMN. The Unified Data Repository (UDR) 128 stores UE subscription data. The Binding Support Function (BSF) 130 manages binding between PDU sessions and PCFs.

[0042] As mentioned above, one problem that can arise in 5G and other types of networks is that NF client identity private keys and public key certificates can be stolen and used without authorization to obtain NF TLS certificates, which can then be used to impersonate the NF and gain unauthorized access to other NFs and information within the network.

[0043] According to aspects of the subject matter described herein, rather than issuing a TLS certificate to an entity presenting a request having only a signature generated using a CMP client identity private key and public key certificate, an NRF-issued access token, such as an OAuth 2.0 access token, may be required as additional authentication information before an NF TLS certificate is issued. In the context of a 5G communication network, an NF service consumer acts as an OAuth 2.0 resource client, an NF service producer acts as an OAuth 2.0 resource server, and the NRF acts as an authorization server. Thus, an NF service consumer seeking to access a service provided by an NF service producer signals with the NRF to obtain an access token to access the resource provided by the NF service producer. After the NF service consumer obtains the access token from the NRF, the NF service consumer sends a service request to the NF service producer, where the service request includes the access token. The NF service producer validates the access token and provides access to the service requested by the NF service consumer. Rather than simply using an OAuth 2.0 access token to authorize a consumer NF to access services provided by a producer NF, the subject matter described herein includes using the NF's OAuth 2.0 access token as a required credential for a requesting entity to obtain the NF's TLS certificate. By providing the OAuth 2.0 access token in addition to a signature and public key certificate generated using the CMP client identity private key in a CMP certificate request, the NF utilizes multi-factor authentication (MFA), thereby enhancing the security of the CMP protocol. In the example described herein, the multi-factor authentication is two-factor authentication, i.e., verification of the OAuth 2.0 access token is the first factor and verification of the signature generated using the CMP client identity private key is the second factor.

[0044] When an entity requests a renewal of an NF's TLS certificate, an NRF-issued access token can be a required credential. Using the NRF-issued access token as an additional required credential reduces the likelihood that a hacker can obtain a registered NF's TLS certificate even if the CMP client identity private key and public key certificate are compromised. In one example, a CA proxy is provided in front of the CMP CA to verify that an NRF-issued OAuth 2.0 access token for the NF for which the TLS certificate is being requested is embedded in the HTTP-based CMP certificate renewal request. To successfully renew the TLS certificate, the requesting entity must provide the NRF-issued OAuth 2.0 access token at the time of TLS certificate renewal. The CA proxy queries the NRF to check whether the NF is registered. If the NF is registered, the CA proxy validates the OAuth 2.0 access token using the NRF public key corresponding to the private key used to generate the token. To successfully obtain the NF's TLS certificate, the certificate request must include the NF's NRF-issued OAuth 2.0 access token.

[0045] Section 13.3.1 of 3GPP TS33.501 specifies the use of TLS and HTTPS with mutual authentication and further specifies the use of identities embedded in end-entity certificates for authentication and policy checking. If a hacker can gain access to the CMP client identity private key and public key certificate, the hacker can obtain a TLS certificate issued in the name of the NF and use that TLS certificate to impersonate the NF to the NRF or other NFs.

[0046] TLS certificates and private keys may be stolen by hackers and used in SBI requests (e.g., requests for NFRegister service operations). TLS / Public Key Infrastructure (PKI) provides the concept of revocation using Certificate Revocation Lists (CRLs) or Online Certificate Status Protocol (OCSP) for certificates whose owners are aware of the theft. However, when TLS certificates and private keys are stolen without the owner's knowledge, no mitigation is available at the transport layer. Even with CRLs and OCSP, the owner must notify an authority of the certificate theft, and the authority must update its database and mark the certificate as stolen. There is a time delay between the theft and the certificate's revocation, creating an opportunity for hackers to launch an attack. The subject matter described herein mitigates the problem when a certificate is stolen without the owner's knowledge. The proposed solution also works when a certificate is stolen but there is a delay in revoking the stolen certificate.

[0047] In one possible security architecture, a certificate manager (CM) can be used for automated certificate management for NFs. FIG. 2 illustrates an example of such an architecture, in which the CM 200 is configured with a CMP client identity private key and a CMP public key certificate corresponding to one or more NFs 202. The CMP client identity private key and CMP public key certificate are used for authentication with a CMP certificate authority (CA) 204. Communication between the 5G NFs 202 and the CM 200 can be secured using a service mesh or other similar solution. In the example architecture illustrated in FIG. 2, the NFs 202 and CM 200 are implemented within a Kubernetes cluster 206.

[0048] 3 shows exemplary messages exchanged when obtaining a TLS certificate using the security architecture shown in FIG. 2. Referring to FIG. 3, in step 1, the NF 202 sends a request to the CM 200 to obtain a TLS key and certificate. In step 2, the CM 200 generates a key pair for the TLS certificate request, and in step 3, generates a CMP certificate request to send to the CA 204. The key pair generated by the CM 200 includes a CMP client identity private key and public key for the NF 202. The CM 200 uses a signature generated using the CMP client identity private key for PKI message protection. In step 4, the CM 200 sends the CMP certificate request to the CA 204. The CMP certificate request includes a public key certificate (including the NF 202's public key and key metadata). The CMP certificate request also includes a signature generated using the CMP client identity private key. In step 5, the CA 204 validates the request using the public key in the request to verify the signature, generates a certificate response including the TLS key and the NF 202's certificate, and sends this response to the CM 200. In step 6, the CM 200 sends the TLS key and certificate to the NF 202. A hacker with access to the CMP client identity private key can impersonate the CM 200 and obtain the NF's TLS certificate from the CA 204.

[0049] As mentioned above, it is desirable to prevent hackers from masquerading as registered NFs. A registered NF is an NF that has registered with the NRF using the NFRegister service operation. The NFRegister service operation is described in section 5.2.2.2.1 of 3GPP TS 29.510. FIG. 4 shows exemplary messages exchanged in an NFRegister service operation. Referring to FIG. 4, in line 1, the NF service consumer 202 sends an NFRegister request to the NRF 100. The NFRegister request includes the NF profile of the NF service consumer 202. The NFRegister request is also secured by the TLS certificate of the NF service consumer 202. The NRF 100 receives the NFRegister request, verifies the TLS certificate, processes the request, and, if processing is successful, responds with a 201 Created message, as shown in line 2a. The 201 Created message includes the NF profile of the NF being registered. If the processing of the NF registration request is not successful, the NRF 100 responds with an NF4xx or 5xx message specifying the details of the problem, as shown in row 2b.

[0050] 5 is a message flow diagram showing example messages exchanged when a hacker obtains a TLS certificate for an NF using a stolen CMP client identity private key and public key certificate. Referring to FIG. 5, in line 1, the NF 202 sends an NF certificate request to the CM 200. In line 2, the CM 200 sends a CMP certificate request to the CMP CA 204. In line 3, the CMP CA 204 sends a CMP certificate response to the CM 200. The CMP certificate response includes the TLS certificate for the NF 202. In line 4, the CM 200 provides the TLS certificate to the NF 202 in an NF certificate response message. In line 5, the NF 202 registers with the NRF 100 using the NFRegister service operation shown in FIG. 4. The NFRegister service operation is secured using the TLS certificate for the NF 202.

[0051] In line 6, the hacker obtains the NF 202's CMP client identity private key and public key certificate. In line 7, the hacker sends a CMP certificate request to the CMP CA 204. In line 8, the CMP CA 204 responds with a CMP certificate response that includes the NF 202's TLS certificate. In line 9, the hacker initiates service operations impersonating the NF 202 using the stolen TLS certificate.

[0052] To reduce the likelihood that a hacker will succeed in obtaining an NF's TLS certificate, the subject matter described herein includes a CA proxy that intercepts CMP requests and requires an NRF-issued OAuth 2.0 access token for TLS certificate renewal. A hacker needs an NRF-issued OAuth 2.0 access token in addition to a CMP client identity private key and certificate to request a TLS certificate renewal. The NF must provide an NRF-issued OAuth 2.0 access token for TLS certificate renewal or creation. If the NF is not yet registered, an NRF-issued OAuth 2.0 access token is not required for a TLS certificate request. In one embodiment of the subject matter described herein, direct access to the CMP CA is made available only to the CA proxy.

[0053] FIG. 6A illustrates one example of a security architecture in which a CA proxy intercepts a CMP certificate request and requests an NRF-issued OAuth 2.0 access token for a registered NF before providing the NF's TLS certificate to the querying entity. Referring to FIG. 6A , a CM 200 issues CMP certificate requests to a CA 204 on behalf of a 5G NF 202. The certificate requests each include the NF's public key certificate, each protected by the CMP client identity private key associated with the NF. Additionally, if the certificate request is for an NF previously registered with the NRF 100, the certificate request also includes an OAuth 2.0 access token. The CA proxy 600 receives or intercepts the certificate request, checks whether the NF is registered with the NRF 100, and, in response to determining that the NF is registered with the NRF, checks whether an OAuth 2.0 access token for the registered NF is present in the certificate request. If the certificate request includes an OAuth 2.0 access token for the registered NF (along with the NF's public key certificate), the CA proxy 600 allows the request through to the CMP CA 204. The CMP CA 204 renews the NF's TLS certificate and provides the TLS certificate to the requesting CM in a TLS certificate response.

[0054] If the NF is registered and the CMP certificate request does not include the NF's OAuth 2.0 access token, the CA proxy 600 may perform network security actions, such as preventing further processing of the CMP certificate request by the CA 204 by discarding the request.

[0055] Figure 6B shows an alternative security architecture in which a CA proxy intercepts CMP certificate requests for registered NFs and requests an NRF-issued OAuth 2.0 access token for the NF before providing the NF's TLS certificate to the querying entity. In the architecture of Figure 6B, each NF 202 has its own CM 200 and communicates directly with the CA proxy 600. The CA proxy 600 operates to validate the CMP certificate request based on the presence of an NRF-issued OAuth 2.0 access token, similar to that shown in Figure 6A.

[0056] FIG. 6C illustrates yet another alternative security architecture in which a CA proxy intercepts a CMP certificate request and requests an NRF-issued OAuth 2.0 access token for a registered NF before providing the NF's TLS certificate to the querying entity. In the architecture of FIG. 6C, similar to the architecture of FIG. 6B, each NF 202 has its own CM 200 and communicates directly with a CA proxy 600. Additionally, the CA proxy 600 is a component of the NRF 100 rather than being separate from the NRF 100. The CA proxy 600 operates similarly to that shown in FIG. 6A to validate a CMP certificate request based on the presence of an NRF-issued OAuth 2.0 access token, except that instead of querying a separate NRF, the CA proxy 600 shown in FIG. 6C can access the NRF 100's internal NF profile database to determine whether the NF is registered and can also obtain the registered NF's OAuth 2.0 access token from the NRF's internal database.

[0057] FIG. 7 is a message flow diagram illustrating exemplary messages exchanged with the CMP CA proxy to validate a CMP certificate request. Referring to FIG. 7, in line 1, the NF 202 sends the NF certificate request to the CM 200. In line 2, the CM 200 sends the CMP certificate request to the CA 204. The CMP CA proxy 600 intercepts the request and, in line 3, queries the NRF 100 to determine whether the NF identified in the CMP certificate request is registered. The query to determine whether the NF is registered may be an NFDiscover request in one example. In line 4, the NRF 100 responds indicating that the NF 202 is not registered. Because the NF 202 is not registered, in line 5, the CA proxy 600 forwards or allows the CMP certificate request to pass through to the CMP CA 204. The CMP CA 204 renews the NF 202's TLS certificate and, in line 6, sends a CMP certificate response including the NF 202's TLS certificate to the CMP CA proxy 600. On line 7, the CMP CA proxy 600 sends a CMP certificate response to the CM 200. On line 8, the CMP CA 204 sends a NF certificate response to the NF 202.

[0058] In line 9, the NF 202 registers with the NRF 100 using the NFRegister service operation. In line 10, the hacker, having obtained the NF 202's CMP client identity private key and public key certificate, sends a CMP certificate request to the CA 204. The CMP certificate request includes the NF 202's public key certificate but does not include the NF 202's OAuth 2.0 access token. The CMP CA proxy 600 intercepts the CMP certificate request, determines that the request is for an already registered NF's TLS certificate, determines that the request does not include an OAuth 2.0 access token, and in step 11 performs network security action, such as blocking the request.

[0059] 8 is a block diagram illustrating an example architecture for a CMP CA proxy 600. Referring to FIG. 8, the CMP CA proxy 600 includes at least one processor 800 and a memory 802. The CMP CA proxy 600 may also include a CMP certificate request validator 804 for performing the steps described herein to validate a CFP certificate request before forwarding the request to the CMP CA 204. The CMP certificate request validator 804 may be implemented using computer-executable instructions stored in the memory 802 and executed by the processor 800. The CMP CA proxy 600 may be a standalone entity or may be a component of an NRF.

[0060] The CMP CA proxy 600 may be registered as a custom NF with the NRF 100. Registering the CMP CA proxy 600 with the NRF 100 may be required so that the CMP CA proxy is authorized to query the NRF 100 to determine whether an NF is registered. The NRF-issued OAuth 2.0 access token for the CA proxy 600 may be scoped to the CA proxy service.

[0061] FIG. 9 is a flowchart illustrating an example process for preventing unauthorized use of a CMP client identity private key and a CMP public key certificate associated with an NF. Referring to FIG. 9, in step 900, the process includes receiving, by a CMP certificate authority (CA) proxy, a first CMP certificate request for updating a security certificate associated with a first NF. The CMP certificate request includes a public key certificate associated with the first NF. For example, the CMP CA proxy 600 may receive a CMP certificate request requesting a TLS or other security certificate for the NF. If the request is legitimate, the request may have originated from a CM associated with the first NF. If the request is not legitimate, the request may have originated from a hacker who has stolen the CMP client identity private key and public key certificate associated with the first NF.

[0062] In step 902, the process further includes the CMP CA proxy checking whether the first NF is registered with an NF repository function (NRF) and determining that the first NF is registered with the NRF. For example, the CMP CA proxy 600 may query the NRF to determine whether the first NF is registered with the NRF, for example, using an NFDiscover request or other type of query message. The CMP CA proxy 600 may determine that the first NF is registered with the NRF by receiving an NFDiscover or other response message indicating that the first NF is registered with the NRF.

[0063] In step 904, the process further includes, in response to the CMP CA proxy determining that the first NF is registered with the NRF, the CMP CA proxy checking whether the first CMP certificate request includes an NRF-issued access token for the first NF and determining that the CMP certificate request does not include an NRF-issued access token for the first NF. For example, the CMP CA proxy 600 may determine whether the CMP certificate request includes an OAuth 2.0 access token in addition to a signature generated using the first NF's public key certificate and a CMP client identity private key associated with the first NF. The CMP CA proxy 600 may extract the OAuth 2.0 access token from the request and verify the OAuth 2.0 access token using the NRF's public key. In step 904, the CMP CA proxy 600 determines that the OAuth 2.0 access token is not present in the CMP certificate request.

[0064] In step 906, the process further includes performing, by the CMP CA proxy, a network security action with respect to the first CMP certificate request in response to determining that the first CMP certificate request does not include an NRF-issued access token for the first NF. For example, the CMP CA proxy 600 may block the CMP CA request from reaching the CA, thereby preventing the originator of the CMP certificate request from obtaining a TLS certificate for the first NF.

[0065] While the examples described above relate to reducing the likelihood that a hacker could successfully renew an NF's TLS certificate by impersonating the NF, the subject matter described herein is not limited to preventing unauthorized renewal of TLS certificates. The same protections and protocols can be used to reduce the likelihood that a hacker could successfully renew or obtain a Client Credential Assertion (CCA) certificate associated with an NF. The message flow for protecting against unauthorized renewal of a CCA certificate is similar to that described above, except that the CMP certificate request identifies the request as a request for a CCA certificate rather than a TLS certificate. An OAuth 2.0 access token is a required parameter in the CMP certificate request to obtain the NF's CCA certificate.

[0066] An example advantage of the subject matter described herein includes the ability to prevent hackers from obtaining an NF's TLS certificate using a stolen CMP client identity private key and public key certificate. This further prevents hackers from impersonating an NF to 5G entities that rely on transport layer-based security mechanisms to authenticate the NF. The CMP CA proxy can be used to validate CMP certificate requests for any NF, including those shown in FIG. 1. The CMP CA proxy's function of validating certificate requests using an NRF-issued OAuth 2.0 access token can be extended to enhance the security / protection of certificate management protocols that secure certificate types other than TLS certificates, which may be used in future 5G or later generation deployments. The CMP CA proxy can be used in combination with any 5G or other generation network function, including, but not limited to, the NRF, SCP, SEPP, PCF, BSF, NEF, NSSF, UDR, Equipment Identity Register (EIR), or Network Data Analysis Function (NWDAF).

[0067] The disclosure of each of the following references is incorporated herein by reference in its entirety. References 1.3rd Generation Partnership Project; Technical Specification Group Services and System Aspects for the 5G System (5GS); Stage 2; (Release 17) 3GPP TS 23.501 V17.7.0 (2022-12) 2. 3 rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Procedures for the 5G System (5GS); Stage 2 (Release 17) 3GPP TS 23.502 V17.7.0 (2022-12) 3. 3 rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; 5G System; Network Function Repository Services; Stage 3 (Release 18) 3GPP TS 29.510 V18.1.0 (2022-12) 4. 3 rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; 5G System; Technical Realization of Service Based Architecture; Stage 3 (Release 18) 3GPP TS 29.500 V18.0.0 (2022-12) 5. 3 rdGeneration Partnership Project; Technical Specification Group Core Network and Terminals; 5G System; Principles and Guidelines for Services Definition; Stage 3 (Release 18) 3GPP TS 29.501 V18.0.0 (2022-12) 6.3 rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Security architecture and procedures for 5G system (Release 18) 3GPP TS 33.501 V18.0.0 (2022-12) It will be understood that various details of the subject matter described herein can be changed without departing from the scope of the subject matter described herein. Moreover, the above description is for purposes of illustration and not limitation, as the subject matter described herein is defined by the claims set forth below.

Claims

1. 1. A method for preventing unauthorized use of a Certificate Management Protocol (CMP) client identity private key and a CMP public key certificate associated with a Network Function (NF), comprising: a CMP certificate authority (CA) proxy receiving a first CMP certificate request to update a security certificate associated with a first NF; the CMP certificate request includes a public key certificate associated with the first NF and is protected by a CMP client identity private key associated with the first NF; The method comprises: the CMP CA proxy checking whether the first NF is registered with an NF Repository Function (NRF) and determining that the first NF is registered with the NRF; In response to the CMP CA proxy determining that the first NF is registered with the NRF, the CMP CA proxy checks whether the first CMP certificate request includes an NRF-issued access token for the first NF, and determines that the CMP certificate request does not include the NRF-issued access token for the first NF; and performing a network security action with respect to the first CMP certificate request in response to the CMP CA proxy determining that the first CMP certificate request does not include the NRF-issued access token for the first NF.

2. 2. The method of claim 1, wherein receiving the first CMP certificate request comprises receiving the first CMP certificate request from a hacker masquerading as a first NF.

3. 3. The method of claim 2, wherein performing a network security action with respect to the first CMP certificate request includes preventing a CMP CA from providing the security certificate to the hacker.

4. 4. The method of claim 3, wherein preventing the CMP CA from providing the security certificate to the hacker comprises blocking the first CMP certificate request from reaching the CMP CA.

5. 10. The method of claim 9, wherein receiving the first CMP certificate request at a CMP CA proxy comprises receiving the first CMP certificate request at a CMP CA proxy that is a component of the NRF or a CMP CA proxy that is separate from the NRF.

6. 10. The method of any preceding claim, wherein receiving the first CMP certificate request includes intercepting the first CMP certificate request.

7. 10. The method of any preceding claim, wherein receiving the first CMP certificate request includes receiving a first CMP certificate request for a Transport Layer Security (TLS) or Client Credential Assertion (CCA) certificate of the first NF.

8. 10. The method of claim 9, wherein checking whether the first CMP certificate request includes the NRF-issued access token for the first NF includes checking whether the first CMP request includes an OAuth 2.0 access token issued by the NRF to the first NF.

9. The method comprises: receiving, by the CMP CA proxy, a second CMP certificate request for updating a security certificate associated with a second NF; the request includes a public key certificate associated with the second NF and is protected by a CMP client identity private key associated with the second NF; The method comprises: the CMP CA proxy checking whether the second NF is registered with the NRF and determining that the second NF is not registered with the NRF; responsive to determining that the second NF is not registered with the NRF, allowing processing of the second CMP certificate request.

10. 10. The method of claim 9, wherein allowing the second CMP certificate request to be processed includes providing the second CMP certificate request to the CMP CA, generating a CMP certificate response at the CMP CA that includes the security certificate associated with the second NF, and forwarding the CMP certificate response to an originator of the second CMP certificate request.

11. The method comprises: a second NF sending a second CMP certificate request to the CMP CA proxy to update a second security certificate associated with the second NF; the second CMP certificate request includes a public key certificate and an NRF-issued access token associated with the second NF; The method comprises: receiving the second CMP certificate request at the CMP CA proxy; the CMP CA proxy checking whether the second NF is registered with the NRF and determining that the second NF is registered with the NRF; In response to the CMP CA proxy determining that the second NF is registered with the NRF, the CMP CA proxy checks whether the second CMP certificate request includes the NRF-issued access token associated with the second NF, and determines that the second CMP certificate request includes the NRF-issued access token associated with the second NF; 6. The method of claim 1, further comprising: allowing processing of the second CMP certificate request by the CMP CA proxy in response to the CMP CA proxy determining that the second CMP certificate request includes the NRF-issued access token associated with the second NF.

12. 1. A system for preventing unauthorized use of a Certificate Management Protocol (CMP) client identity private key and a CMP public key certificate associated with a Network Function (NF), comprising: a CMP certificate authority (CA) proxy including at least one processor and memory; a CMP certificate request validator implemented by the at least one processor, the validator receiving a first CMP certificate request for updating a security certificate associated with a first NF, the CMP certificate request including a public key certificate associated with the first NF and protected by a CMP client identity private key associated with the first NF; The validator checks whether the first NF is registered with a NF Repository Function (NRF) and determines that the first NF is registered with the NRF; in response to determining that the first NF is registered with the NRF, checks whether the first CMP certificate request includes an NRF-issued access token for the first NF and determines that the CMP certificate request does not include the NRF-issued access token for the first NF; and in response to determining that the first CMP certificate request does not include the NRF-issued access token for the first NF, performs a network security action with respect to the first CMP certificate request.

13. 13. The system of claim 12, wherein the first CMP certificate request originates from a hacker masquerading as the first NF.

14. 14. The system of claim 13, wherein when performing a network security action with respect to the first CMP certificate request, the CMP certificate request validator is configured to block the first CMP certificate request from reaching a CMP CA, thereby preventing the CMP CA from providing the security certificate to the hacker.

15. The system of claims 12 to 14, wherein the CMP CA proxy comprises a component of the NRF or is separate from the NRF.

16. The system of any of claims 12 to 15, wherein the CMP CA proxy is configured to receive the first CMP certificate request by intercepting the first CMP certificate request.

17. The system of any of claims 12 to 16, wherein the security credentials include a Transport Layer Security (TLS) or Client Credential Assertion (CCA) certificate of the first NF.

18. The system of any one of claims 12 to 17, wherein the NRF-issued access token for the first NF comprises an OAuth 2.0 access token issued to the first NF by the NRF.

19. The CMP certificate request validator: configured to receive a second CMP certificate request for updating a security certificate associated with a second NF, the request including a public key certificate associated with the second NF and protected by a CMP client identity private key associated with the second NF; checking whether the second NF is registered with the NRF and determining that the second NF is not registered with the NRF; The system of any one of claims 12 to 18, configured to allow processing of the second CMP certificate request in response to determining that the second NF is not registered with the NRF.

20. a second NF sending a second CMP certificate request to the CMP CA proxy to update a second security certificate associated with the second NF, the second CMP certificate request including a public key certificate associated with the second NF, protected by a CMP client identity private key associated with the second NF, and including an NRF-issued access token associated with the second NF; and the CMP certificate request validator: receiving the second CMP certificate request; checking whether the second NF is registered with the NRF and determining that the second NF is registered with the NRF; In response to determining that the second NF is registered with the NRF, checking whether the second CMP certificate request includes the NRF-issued access token associated with the second NF, and determining that the second CMP certificate request includes the NRF-issued access token associated with the second NF; and, in response to determining that the second CMP certificate request includes the NRF-issued access token associated with the second NF, permitting processing of the second CMP certificate request by forwarding the second CMP certificate request to the CMP CA.

21. A non-transitory computer-readable medium having stored thereon executable instructions that, when executed by a processor of a computer, control the computer to perform steps, the steps comprising: a Certificate Management Protocol (CMP) Certificate Authority (CA) proxy receiving a first CMP certificate request for updating a security certificate associated with a first Network Function (NF), the CMP certificate request including a public key certificate associated with the first NF and protected by a CMP client identity private key associated with the first NF; the CMP CA proxy checking whether the first NF is registered with an NF Repository Function (NRF) and determining that the first NF is registered with the NRF; In response to the CMP CA proxy determining that the first NF is registered with the NRF, the CMP CA proxy checks whether the first CMP certificate request includes an NRF-issued access token for the first NF, and determines that the CMP certificate request does not include the NRF-issued access token for the first NF; and performing a network security action with respect to the first CMP certificate request, in response to the CMP CA proxy determining that the first CMP certificate request does not include the NRF-issued access token for the first NF.

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