O an attacker may try to exploit ip address and port

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mechanism then works correctly. o An attacker may try to exploit IP address and port re-assignment as follows: he repeatedly attaches to the network hoping to be assigned the IP address or port of another user who dropped off without deregistering in IMS. If this indeed happens then any non-registration message sent by the attacker would be accepted by the IP address check mechanism in the P-CSCF as coming from the previous user. The attacker does not attempt to register in IMS as he would not be able to send a correct SIP Digest response. This possibility of attack seems difficult to exploit, but again, the likelihood for success depends on the environment. 3GPP 3GPP TS 33.203 V12.67.0 (2014-0609) 97 Release 12
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Annex R (normative): NASS-IMS-bundled authentication R.1 Overview The main objectives and requirements on NASS-IMS-bundled authentication is that it shall be possible to gain access to IMS based on successful access level (NASS, cf. ETSI ES 282 004 [36]) authentication (see requirements for Early Deployments in ETSI TS 187 001 [37]). In practice this is achieved by associating an IMS identity with a fixed specific location from where it is authorized to access from. When registering to the IMS subsystem, the location of where the UE is accessing from is verified by the NASS (which also handles the authentication / authorization) and if the NASS location is equal to the provisioned location, the UE is authorized to access IMS. It is assumed that there exist a strong relationship between the access network and the IMS network, and that the NASS location of the UE can be provisioned in the user profile of the HSS. R.2 Use Cases and Limitations The main use case for NASS-IMS-bundled authentication is to provide access to the IMS network for legacy equipment that cannot support the IMS access security (see clause 6.1). This is also reflected by the requirements in ETSI TS 187 001 [37] (see clause 4.2, Early Deployments ), which requires the possibility to link NASS and IMS authentication so that it is possible to reuse the authentication of the NASS to gain access to IMS. It is the responsibility of the end user to ensure the protection between the entity providing access level authentication and the entity including the IMS application. NASS-IMS-bundled authentication has a number of deployment requirements which restricts its usage for general usage. This includes: The access network provides sufficient means to assure the IMS layer that a specific UE/user is connecting from a specific location. The access network provides sufficient means for confidentiality and integrity of the signalling communication. The access network is providing anti-IP spoofing mechanisms. Nomadicity (and roaming) is not possible as the user is fixed to a specific location and the access network and IMS network need to be tightly coupled.
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