Security Challenges in the IP Multimedia Subsystem (IMS): An In-Depth Analysis

Abstract

The IP Multimedia Subsystem (IMS) stands as a foundational architectural framework within modern telecommunications, designed to facilitate the seamless delivery of a vast array of IP-based multimedia services. These services encompass, but are not limited to, voice over LTE (VoLTE), voice over Wi-Fi (VoWiFi), video conferencing, instant messaging, and rich communication services (RCS). Its sophisticated architecture integrates a multitude of protocols and network elements, orchestrating a complex interplay to enable ubiquitous and seamless communication across diverse access technologies and user platforms. However, the inherent complexity, open design principles, and reliance on various interconnected protocols introduce significant security vulnerabilities that can be systematically exploited by malicious entities. This comprehensive report embarks on a detailed examination of the IMS architecture, elucidating its pivotal role in contemporary telecommunication ecosystems, and meticulously analyzing the specific and pervasive security challenges it confronts. Through an in-depth exploration of these multifaceted aspects, the report aims to unequivocally underscore the critical importance of implementing robust, multi-layered security measures and adopting a proactive posture in safeguarding IMS infrastructures against an ever-evolving landscape of cyber threats.

Many thanks to our sponsor Esdebe who helped us prepare this research report.

1. Introduction

The landscape of telecommunications has undergone a profound and transformative evolution, transitioning decisively from the traditional circuit-switched networks, exemplified by the Public Switched Telephone Network (PSTN), towards sophisticated, packet-switched, IP-based systems. This paradigm shift was primarily driven by the exponential growth in data traffic, the demand for multimedia convergence, and the burgeoning need for a flexible, cost-efficient infrastructure capable of supporting a rich tapestry of services beyond mere voice communication. It was in response to these burgeoning requirements that the IP Multimedia Subsystem (IMS) was conceptualized and subsequently standardized by the 3rd Generation Partnership Project (3GPP), beginning with Release 5 in the early 2000s [1].

IMS was introduced to address the critical need for a standardized, future-proof architectural framework capable of delivering a wide array of real-time and non-real-time multimedia services, including voice, video, and messaging, directly over IP networks. Its design philosophy emphasizes flexibility, scalability, and interoperability, allowing for the rapid deployment of new services and seamless integration across disparate access technologies, ranging from cellular (LTE, 5G) to fixed broadband and Wi-Fi [2]. This capability makes IMS the de facto backbone for services like VoLTE, which has become indispensable for high-quality voice calls in 4G networks, and is set to continue its critical role in 5G deployments [3].

Despite its undeniable architectural advantages and its instrumental role in enabling modern communication services, the intricate design of IMS, characterized by its distributed nature, reliance on numerous interconnected functional entities, and its use of a diverse set of signaling and media protocols, inherently exposes it to a myriad of security threats. The very openness and extensibility that contribute to its flexibility also expand its attack surface. Vulnerabilities can arise at various layers of the architecture, from the user equipment and access network to the core signaling components and application servers. These threats range from overt denial-of-service attacks aimed at disrupting service availability to insidious eavesdropping attempts targeting user confidentiality, and sophisticated identity theft schemes exploiting authentication weaknesses [4].

Understanding these inherent vulnerabilities is not merely an academic exercise; it is an essential prerequisite for developing, implementing, and continually refining effective countermeasures to protect IMS infrastructures. A breach in IMS security can have far-reaching consequences, including significant financial losses for operators due to fraud, erosion of subscriber trust, degradation of service quality, and compromise of sensitive personal or corporate data. Therefore, this report undertakes a comprehensive and detailed analysis of the IMS architecture, its operational protocols, the specific security challenges it confronts, and the array of mitigation strategies available, aiming to provide a holistic understanding of how to fortify this critical telecommunications infrastructure.

Many thanks to our sponsor Esdebe who helped us prepare this research report.

2. IMS Architecture and Components

IMS is meticulously structured as a flexible and scalable environment, adhering to a layered approach that logically separates access, control, and application functionalities. This modular architecture is key to its adaptability and extensibility, allowing it to support a wide range of services and integrate with various network environments. The primary layers and their constituent functional entities are detailed below:

2.1 User Equipment (UE)

The User Equipment (UE) represents the endpoint devices through which subscribers access IMS services. This category is broad, encompassing modern smartphones, feature phones, tablets, personal computers, embedded devices, and various Internet of Things (IoT) devices capable of running an IMS client. The UE is responsible for initiating and terminating communication sessions by interacting with the IMS core network via an access network. Key functionalities of the UE in an IMS context include:

• SIP Client Functionality: The UE hosts a Session Initiation Protocol (SIP) user agent that enables it to register with the IMS network, initiate, receive, and terminate multimedia sessions.
• Media Codecs: It contains the necessary codecs for encoding and decoding various media types, such as voice (e.g., AMR-WB, EVS for VoLTE) and video (e.g., H.264, H.265).
• Security Capabilities: Modern UEs incorporate security features, including capabilities for IPsec and TLS, to secure signaling and media traffic with the network.
• Access Network Interface: It possesses interfaces to connect to various access networks (e.g., LTE/5G radio interface, Wi-Fi).

2.2 Access Network

The Access Network provides the connectivity infrastructure that links the User Equipment to the IMS core. IMS is access-agnostic, meaning it can operate over virtually any IP-capable access technology. This versatility is a core strength of IMS. Common access network types include:

• LTE/5G New Radio (NR): For mobile broadband access, UEs connect to the evolved packet core (EPC) in LTE networks or the 5G core (5GC) in 5G networks. The Packet Data Network Gateway (PGW) in LTE or the User Plane Function (UPF) in 5G connects the UE’s IP session to the IMS network. VoLTE is a prime example where IMS relies entirely on LTE as the access technology.
• Wi-Fi: For voice over Wi-Fi (VoWiFi) or other IMS services over Wi-Fi, UEs typically connect to a Wi-Fi Access Point (AP), which then routes traffic through an Evolved Packet Data Gateway (ePDG) to the EPC/5GC and subsequently to IMS. The ePDG secures the connection using IPsec tunnels [5].
• Fixed Broadband: Technologies like DSL, Cable, Fiber Optic (GPON/EPON), and Ethernet provide fixed-line access. These networks route IP traffic to the IMS core, often via a Broadband Remote Access Server (BRAS) or similar aggregation device.
• Other Access Technologies: IMS can also integrate with older technologies like WiMAX or even PSTN via specific gateways, although these are less common in modern deployments focusing on all-IP.

2.3 Core Network Functional Entities

The IMS Core Network is the central nervous system of the architecture, comprising a sophisticated set of functional entities responsible for session control, subscriber management, routing, and service delivery. These entities communicate using various IP-based protocols, primarily SIP and Diameter.

• Proxy Call Session Control Function (P-CSCF):

  • First Point of Contact: The P-CSCF is the initial contact point for the User Equipment (UE) within the IMS domain. All SIP signaling messages from the UE first pass through the P-CSCF [6].
  • SIP Message Validation: It performs syntax checking and validation of SIP messages, ensuring they conform to IMS specifications.
  • QoS Enforcement: The P-CSCF interacts with the Policy and Charging Rules Function (PCRF) via the Rx interface to request and enforce Quality of Service (QoS) parameters for media streams, ensuring appropriate bandwidth allocation.
  • NAT Traversal: It assists UEs behind Network Address Translators (NATs) by modifying SIP messages to allow successful session establishment.
  • Topology Hiding: While primarily handled by Session Border Controllers (SBCs), the P-CSCF can contribute to hiding the internal network topology from external networks.
  • Lawful Interception: It facilitates lawful interception of communication sessions, in coordination with other IMS entities.
  • Security Gateway: It acts as an IPsec client/server for securing communication with the UE, establishing an IPsec security association for signaling messages.

• Interrogating Call Session Control Function (I-CSCF):

  • Entry Point to Home Network: The I-CSCF serves as the entry point into the IMS network for incoming SIP requests. It is the first IMS element encountered by SIP messages originating from other IMS networks or the PSTN.
  • S-CSCF Selection: For incoming mobile-terminated calls or initial registrations, the I-CSCF queries the Home Subscriber Server (HSS) via the Diameter Cx interface to determine which Serving Call Session Control Function (S-CSCF) is currently serving the user, or to select an S-CSCF for a new registration [7].
  • Topology Hiding: The I-CSCF helps hide the internal network topology by acting as a SIP proxy, forwarding requests without revealing the specific S-CSCF address.

• Serving Call Session Control Function (S-CSCF):

  • Central Control Point: The S-CSCF is arguably the most critical and complex entity in the IMS core, serving as the central control point for multimedia sessions within its serving area. It maintains session state for all active sessions it controls [8].
  • Registration Management: It handles user registration, authenticating users by interacting with the HSS and registering their location information.
  • Session Management: The S-CSCF performs call routing, applies service logic, and manages the lifecycle of sessions (setup, modification, termination).
  • Subscriber Profile Management: It retrieves and caches subscriber profiles from the HSS via the Diameter Cx interface, which include information about services subscribed, authentication keys, and routing preferences.
  • Application Server Interaction: The S-CSCF interacts with Application Servers (AS) via the ISC (IMS Service Control) interface to invoke value-added services based on the subscriber’s profile and service triggers.
  • Lawful Interception: It coordinates lawful interception activities for sessions it controls.

• Home Subscriber Server (HSS):

  • Central Database: The HSS is the authoritative, centralized database for subscriber profiles, authentication and authorization information, and service-specific data [9]. It is a highly robust and redundant component.
  • Subscriber Data: It stores various data, including subscriber IDs (IMSI, MSISDN, SIP URI), authentication vectors (for IMS AKA), service profiles (which IMS services a user is subscribed to), and user location information (which S-CSCF is currently serving the user).
  • Interactions: It interacts with the I-CSCF and S-CSCF over the Diameter Cx interface, and with Application Servers over the Diameter Sh interface for user profile data retrieval and updates.
  • Authentication and Authorization: The HSS is central to the IMS authentication and authorization process, providing security parameters to the S-CSCF for authenticating UEs.

• Subscription Locator Function (SLF):

  • In large-scale IMS deployments where multiple HSS instances are used, the SLF is a crucial functional entity. It acts as a routing agent, providing the address of the HSS that holds the subscriber data for a specific user to the I-CSCF or S-CSCF. This prevents a single HSS from becoming a bottleneck and facilitates load balancing and geographical distribution of subscriber data.

• Breakout Gateway Control Function (BGCF):

  • The BGCF is responsible for determining the next hop for calls originating in IMS but destined for the Public Switched Telephone Network (PSTN) [10]. It performs number analysis to select an appropriate Media Gateway Control Function (MGCF) within its own network or in a peering network, which will then handle the interworking with the PSTN.

• Media Gateway Control Function (MGCF):

  • The MGCF acts as a signaling gateway between the IMS domain (using SIP) and the circuit-switched PSTN domain (using traditional signaling protocols like SS7’s ISUP) [11]. It translates SIP messages to ISUP messages and vice versa, controlling the associated Media Gateway (MGW).

• Media Gateway (MGW):

  • The MGW is responsible for converting media streams between the IMS IP-based network and the circuit-switched PSTN. It performs transcoding if different codecs are used and handles the actual voice and data transport across the network boundary.

• Application Servers (AS):

  • Application Servers are external entities that provide value-added services to IMS subscribers. They interact with the S-CSCF via the ISC (IMS Service Control) interface (a SIP-based interface) and often with the HSS via the Diameter Sh interface (for subscriber profile access).
  • Types of AS:
    • SIP Application Servers: Directly process SIP messages to provide services like voicemail, conferencing, presence, and instant messaging.
    • OSA Parlay Gateway: Provides an open API (Application Programming Interface) for third-party applications to access network capabilities and subscriber information, facilitating the creation of new services.
    • IP Multimedia Service Switching Function (IM-SSF): Allows legacy GSM/UMTS services (like SMS) to be provided over the IMS framework.
  • Examples of Services: Voicemail, Push-to-Talk (PoC), Rich Communication Services (RCS) which include enhanced messaging, group chat, video calls, and file sharing; conferencing bridges, location-based services, and emergency calling capabilities.

• Session Border Controller (SBC):

  • While not a mandatory 3GPP-defined IMS component, the SBC is ubiquitous and critical in virtually all commercial IMS deployments, particularly at network borders. Its primary roles include:
    • Security: Acting as a firewall, protecting the IMS core from external threats, hiding internal network topology, and preventing DoS attacks.
    • Interoperability: Handling protocol interworking (e.g., different SIP dialects), NAT traversal, and addressing various network impairments.
    • QoS Enforcement: Managing bandwidth and prioritizing traffic.
    • Lawful Interception: Facilitating lawful interception at the network edge.

2.4 Policy and Charging Control (PCC) Framework

While distinct from the core IMS entities, the PCC framework is an integral part of the overall IMS ecosystem, particularly for Quality of Service (QoS) and charging. It comprises:

• Policy and Charging Rules Function (PCRF): The PCRF is a functional entity that provides policy control decisions and charging rules based on subscriber profiles, service requirements, and network conditions. It interacts with the P-CSCF (Rx interface) to authorize and control media flows, ensuring that adequate QoS is provisioned for IMS sessions [12].
• Online/Offline Charging Systems: IMS integrates with these systems for real-time (online) and post-session (offline) billing and accounting of services consumed by subscribers.

This intricate and interconnected architecture provides the flexibility and power necessary for modern multimedia communication, yet each interconnection point and functional entity represents a potential point of vulnerability.

Many thanks to our sponsor Esdebe who helped us prepare this research report.

3. Signaling Protocols in IMS

IMS relies on a sophisticated suite of signaling and media protocols to manage the establishment, modification, and termination of communication sessions, as well as the transport of multimedia data. Understanding these protocols is fundamental to appreciating the architectural and security complexities of IMS.

3.1 Session Initiation Protocol (SIP)

SIP is the cornerstone signaling protocol for IMS, serving as the primary mechanism for initiating, maintaining, and terminating real-time multimedia sessions. It is an application-layer control protocol that is independent of the underlying transport layer, typically running over UDP for reliability-tolerant scenarios or TCP for more critical signaling [13].

• Request/Response Model: SIP operates on a request/response model, similar to HTTP. Key SIP methods (requests) include:
  • INVITE: Initiates a session.
  • ACK: Confirms the final response to an INVITE.
  • BYE: Terminates an existing session.
  • CANCEL: Terminates a pending session establishment attempt.
  • REGISTER: Registers a user’s location with the IMS network (S-CSCF via P-CSCF).
  • SUBSCRIBE/NOTIFY: Used for presence and event notification services.
  • OPTIONS: Queries a server for its capabilities.
  • MESSAGE: Used for instant messaging.
• Text-Based Nature: SIP messages are largely human-readable, making them flexible but also potentially vulnerable to manipulation if not properly secured.
• Extensibility: SIP is highly extensible through the addition of new headers and methods, allowing it to adapt to new services and features.
• SIP URIs: Users are identified by SIP Uniform Resource Identifiers (URIs), which function similarly to email addresses (e.g., ‘sip:[email protected]’).
• Session Description Protocol (SDP) Integration: SIP carries SDP within its message body to describe the media characteristics of a session. SDP defines parameters such as media types (audio, video), codecs (e.g., G.711, H.264), transport protocols (e.g., RTP/RTCP), and connection information (IP addresses, port numbers).

3.2 Diameter Protocol

Diameter is a robust authentication, authorization, and accounting (AAA) protocol that serves as a successor to RADIUS. In IMS, Diameter is extensively used for communication between various core network elements, providing critical security and policy enforcement capabilities [14]. Unlike RADIUS, Diameter supports reliable transport (over TCP or SCTP), has a larger address space, and is highly extensible.

Key Diameter interfaces in IMS include:

• Cx Interface: Used between the I-CSCF/S-CSCF and the HSS for subscriber data management, authentication, and authorization during registration and session setup.
• Dx Interface: Used between the I-CSCF and the SLF to locate the correct HSS for a specific subscriber when multiple HSS instances are deployed.
• Sh Interface: Used between Application Servers (AS) and the HSS to access and update subscriber profile information, enabling the AS to tailor services to specific users.
• Rx Interface: Used between the P-CSCF and the Policy and Charging Rules Function (PCRF) to request and enforce QoS policies for media sessions.
• Ro/Rf Interfaces: Used for online (Ro) and offline (Rf) charging respectively, connecting IMS entities to charging systems.

3.3 Real-Time Transport Protocol (RTP) and RTP Control Protocol (RTCP)

RTP and RTCP are fundamental for the efficient and reliable delivery of real-time multimedia data streams, such as voice and video, over IP networks.

• RTP (Real-Time Transport Protocol): RTP carries the actual media payloads. It provides mechanisms for:

  • Payload Type Identification: Specifies the type of media being carried (e.g., G.711 audio, H.264 video).
  • Sequence Numbering: Detects packet loss and reorders packets at the receiver.
  • Timestamping: Synchronizes media streams and compensates for jitter.
  • Source Identification: Identifies the sender of the media stream.
    RTP itself does not guarantee QoS or reliable delivery; it relies on underlying transport protocols (typically UDP) and network infrastructure for these.

• RTCP (RTP Control Protocol): RTCP works in conjunction with RTP to provide out-of-band control information and quality feedback for the media stream [15]. It enables participants in a session to:

  • Monitor QoS: Report on packet loss, jitter, and round-trip delay.
  • Synchronize Streams: Helps synchronize different media streams (e.g., audio and video) from the same source.
  • Identify Participants: Provides canonical names for participants in a session.
    RTCP reports are crucial for adaptive media applications to adjust their sending rates or codecs based on network conditions.

3.4 Other Relevant Protocols

While SIP, Diameter, and RTP/RTCP are central, IMS relies on several other protocols for its full functionality:

• DNS (Domain Name System): Critical for name resolution and service discovery within the IMS network, enabling CSCFs and other entities to locate each other.
• TCP/UDP: These are the underlying transport protocols. SIP can run over TCP or UDP, Diameter typically over TCP or SCTP, and RTP/RTCP predominantly over UDP.
• HTTP/XML: Used by some Application Servers (e.g., for provisioning, configuration, or interaction with web-based services).
• SCTP (Stream Control Transmission Protocol): A reliable, message-oriented transport protocol that provides features useful for signaling, such as multi-homing and multi-streaming. Diameter often leverages SCTP for enhanced reliability.

This complex web of protocols, while enabling rich multimedia experiences, also presents a substantial attack surface, as vulnerabilities in any one of these protocols or their interactions can be leveraged to compromise the entire IMS system.

Many thanks to our sponsor Esdebe who helped us prepare this research report.

4. Security Challenges in IMS

The inherent complexity, distributed nature, and reliance on multiple protocols within the IMS architecture introduce a broad spectrum of security vulnerabilities. These challenges stem from the need for openness and interoperability, which can conflict with stringent security requirements. Malicious actors can exploit these weaknesses to compromise service availability, confidentiality, integrity, and accountability within the IMS domain. The primary security challenges are detailed below:

4.1 Denial of Service (DoS) and Distributed Denial of Service (DDoS) Attacks

DoS and DDoS attacks aim to make IMS services unavailable to legitimate users by overwhelming network resources or specific IMS components. The impact can range from service degradation to complete outages, affecting mission-critical voice and data communication.

• Mechanism: Attackers flood IMS components with excessive legitimate or malformed signaling messages, or massive media traffic. This consumes CPU cycles, memory, network bandwidth, and session capacities, leading to resource exhaustion.
• Targeted Components:
  • S-CSCF: Highly vulnerable due to its role in managing registrations and sessions. Flooding the S-CSCF with REGISTER requests (registration floods) or INVITE requests (call floods) can exhaust its processing power and memory, preventing legitimate users from registering or initiating calls. An S-CSCF failure can bring down a large portion of a mobile operator’s VoLTE service [16].
  • HSS: The HSS can be targeted with excessive authentication or subscriber profile queries, leading to database overload and delays in user registration and service activation.
  • P-CSCF: Can be overwhelmed by a flood of signaling messages from multiple UEs or an external network, impacting its ability to proxy SIP messages and enforce QoS policies.
  • Application Servers (AS): Vulnerable to floods of service-specific requests, disrupting value-added services like voicemail or conferencing.
  • Media Gateways (MGW): Can be targeted by RTP floods, leading to congestion and degraded media quality or complete media path failure.
• Specific Attack Types:
  • SIP Message Floods: Malicious INVITE, REGISTER, SUBSCRIBE, MESSAGE, or OPTIONS requests sent at high rates to specific CSCFs [17]. These can be legitimate requests, just overwhelming in volume.
  • Malformed SIP Messages: Crafted SIP messages that exploit parsing vulnerabilities in IMS entities, potentially causing crashes or unexpected behavior. This can lead to a localized DoS.
  • RTP Floods (Media DoS): Directly targeting the media plane by sending a massive volume of dummy RTP packets to overwhelm media gateways or UEs, leading to voice/video call degradation or drops. This often occurs after successful session establishment.
  • Signaling Storms: Accidental or malicious events (e.g., misconfigured UEs, rapidly fluctuating network conditions, or malicious scripts) can cause a surge in legitimate-looking signaling traffic (e.g., re-registrations, location updates), leading to network congestion and component overload. This is particularly relevant in inter-network scenarios or with misbehaving IoT devices [18].
• Impact: Service unavailability, increased call setup delays, dropped calls, degradation of voice and video quality, and revenue loss for operators.

4.2 Session Hijacking and Manipulation

Session hijacking involves an attacker taking control of an existing communication session, while manipulation involves altering the content or flow of that session. These attacks compromise the integrity and authenticity of communications.

• Mechanism: Attackers intercept SIP signaling or RTP media streams, typically by positioning themselves as a Man-in-the-Middle (MITM). Once intercepted, they can modify message content, redirect calls, inject false data, or even terminate sessions.
• Techniques:
  • MITM Attacks: Achieved through various network-layer exploits such as ARP spoofing, DNS spoofing, BGP hijacking, or by compromising network devices. Once positioned, the attacker can decrypt (if weak encryption is used) or simply modify unencrypted SIP/RTP traffic.
  • SIP Header Manipulation: Modifying SIP headers (e.g., Via, Record-Route, Contact, From, To headers) to redirect calls to an attacker-controlled endpoint or to inject malicious instructions.
  • SDP Modification: Altering the SDP part of a SIP message to change media parameters (e.g., codecs, IP addresses, port numbers) to redirect media streams to an attacker, degrade quality, or introduce incompatible parameters [19].
  • Session Teardown: Injecting BYE messages to prematurely terminate legitimate calls.
• Impact: Unauthorized call termination, eavesdropping on private conversations, call redirection to fraudulent numbers, injection of malicious content, billing fraud, and disruption of critical communications.

4.3 Eavesdropping and Data Interception (Confidentiality)

Eavesdropping involves passively listening to communication channels to gain unauthorized access to sensitive information, compromising confidentiality.

• Mechanism: Without robust encryption mechanisms, both SIP signaling and RTP media streams can be intercepted by anyone with access to the network path. This is particularly true over shared or untrusted networks.
• Vulnerable Points:
  • Access Network: Wireless access networks (LTE, Wi-Fi) are inherently prone to passive listening if traffic is not adequately encrypted at the air interface and transport layers.
  • Interconnection Points: Traffic exchanged between different IMS operators or between IMS and other networks (e.g., PSTN gateways) might traverse unprotected transit networks.
  • Internal Network Links: Even within an operator’s core network, if inter-IMS component communication is not encrypted, an insider or compromised internal system can intercept traffic.
• Tools: Readily available network sniffing tools like Wireshark can capture and analyze unprotected IMS traffic, revealing call details (caller/callee IDs, timestamps, session duration), SIP message content (which can include sensitive user-specific headers), and even the raw voice/video streams [20].
• Impact: Disclosure of sensitive user data (call content, signaling information, user location, service usage patterns, presence information), violation of privacy, corporate espionage, and potential for further attacks based on intercepted information.

4.4 Unauthorized Access and Identity Theft (Authentication & Authorization)

These threats involve gaining illicit access to IMS services or impersonating legitimate users, leading to fraudulent activities and privacy breaches.

• Mechanism: Exploiting weaknesses in authentication protocols, compromising user credentials, or bypassing authorization checks.
• Vulnerabilities in IMS Authentication and Key Agreement (AKA) Protocol: While IMS AKA (based on 3GPP AKA) is designed for strong mutual authentication between the UE and the network, it is not without potential vulnerabilities if not implemented or enhanced correctly:
  • Replay Attacks: If session parameters are not adequately protected, attackers might replay authentication messages.
  • Side-Channel Attacks: Information leakage through timing or power consumption during cryptographic operations can potentially compromise keys.
  • Weak Key Derivation: If the key derivation functions are not robust, an attacker might deduce session keys.
  • Lack of Strong User Identity Verification: While the SIM/USIM provides strong network authentication, the user’s identity to the service might still be vulnerable to social engineering or phishing attacks [21].
• Credential Stuffing/Brute Force: If IMS services rely on user-chosen passwords (e.g., for accessing application servers), these can be vulnerable to brute-force attacks or credential stuffing if users reuse passwords compromised elsewhere.
• Subscriber Profile Alteration: Attackers, if gaining unauthorized access to the HSS or an AS, could modify subscriber profiles, change service entitlements, or redirect calls/messages.
• Fraudulent Activities: Impersonating legitimate users allows attackers to make fraudulent calls, access premium services, or send unauthorized messages, leading to significant financial losses for operators and subscribers.
• Impact: Billing fraud, unauthorized service usage, identity impersonation, privacy violations, and disruption of legitimate services.

4.5 Protocol Fuzzing and Application Layer Attacks

These attacks target the robustness and logical integrity of IMS components by exploiting vulnerabilities in protocol implementations or application logic.

• Mechanism: Attackers send malformed, oversized, or unexpected inputs to IMS components (P-CSCF, S-CSCF, AS). These inputs are designed to trigger parsing errors, buffer overflows, memory leaks, or logical flaws within the software implementations of IMS protocols like SIP or Diameter [22].
• Fuzzing: Automated techniques are used to generate vast numbers of invalid, unexpected, or random data inputs to test the robustness of a system. When a component crashes or behaves unexpectedly, it indicates a potential vulnerability.
• Application Logic Attacks: These go beyond simple protocol violations and target the specific logic of an application server or IMS service. Examples include:
  • SIP Header Injection: Injecting unauthorized or malicious headers that the application might process incorrectly.
  • SQL Injection/Cross-Site Scripting (XSS): If Application Servers have web interfaces or interact with databases, they can be vulnerable to standard web application attacks if not securely developed.
  • Service Chaining Exploits: Exploiting vulnerabilities in how different application servers or services interact, allowing an attacker to bypass security controls or gain unauthorized access to data.
• Impact: Component crashes (leading to DoS), privilege escalation, unauthorized access to system resources or sensitive data, information leakage, and potentially remote code execution on compromised servers. This type of attack is particularly insidious as it targets the fundamental stability and security of the IMS software stack.

4.6 Signaling Interconnection and Roaming Threats

IMS networks must interwork with other networks, including other IMS operators, traditional PSTN, and global roaming networks. These interconnection points are often significant security weak links.

• SS7/Diameter Interworking: Interconnections with legacy SS7 networks (via MGCF) or Diameter routing via IPX/GRX providers for roaming can expose IMS to vulnerabilities inherent in these external networks. SS7 vulnerabilities, for instance, can lead to subscriber location tracking, call interception, and message manipulation [23]. Similarly, poorly secured Diameter routing can facilitate DoS or information leakage.
• Inter-Operator Peering: Lack of strong security agreements, differing security postures, or inadequate trust models between peered IMS networks can allow attacks to propagate across network boundaries or facilitate fraudulent activity.
• Roaming Fraud: Exploiting authentication or routing weaknesses during international roaming can lead to fraudulent call generation or unauthorized service usage, with costs potentially passed on to the victim operator.
• Impact: Global DoS, widespread fraud, privacy breaches across network borders, and reputational damage for operators.

4.7 Malicious Application Servers (AS) and API Abuse

The extensible nature of IMS means that new services are introduced through Application Servers, often developed by third parties. If an AS is compromised or maliciously designed, it poses a significant threat.

• Compromised AS: An attacker gaining control of a legitimate AS can leverage its extensive privileges (e.g., access to subscriber profiles via Sh interface, ability to initiate/terminate calls via ISC interface) to conduct large-scale fraud, data breaches, or service disruption.
• Rogue AS: A maliciously designed AS could surreptitiously collect user data, initiate unauthorized calls/messages, or degrade service quality.
• API Abuse: The interfaces (ISC, Sh) between the S-CSCF, HSS, and AS provide powerful capabilities. If these APIs are not properly secured and monitored, an attacker (internal or external) could abuse them to bypass core IMS security controls.
• Impact: Mass data exfiltration, widespread service hijacking, billing fraud, privacy violations, and potential for launching further attacks against other IMS components.

4.8 Insider Threats

Even with robust external defenses, an insider (an employee, contractor, or anyone with authorized access) can pose a significant threat.

• Mechanism: Abuse of legitimate access privileges, misconfiguration of IMS components (accidental or intentional), data exfiltration, or introduction of malware into the IMS infrastructure.
• Impact: Data integrity compromise, widespread service disruption (e.g., by taking down critical IMS functions), confidentiality breaches, and circumvention of standard security controls. Insider threats are particularly challenging to detect and mitigate due to their privileged access.

4.9 Vulnerabilities in Underlying Infrastructure

IMS relies on a robust IP network infrastructure (routers, switches, firewalls, DNS servers). Vulnerabilities in these underlying components can indirectly impact IMS security.

• Network Equipment Compromise: Exploiting weaknesses in network devices can lead to traffic redirection, interception, or DoS against IMS traffic.
• DNS Attacks: DNS spoofing or cache poisoning can redirect SIP signaling to malicious servers, facilitating MITM attacks or unauthorized registration.
• Operating System/Hypervisor Vulnerabilities: If IMS components run on compromised operating systems or virtualized environments, the entire IMS instance can be affected.
• Impact: All IMS services are at risk if the foundational network infrastructure is compromised, leading to widespread and severe security incidents.

These multifaceted security challenges necessitate a holistic and layered approach to IMS security, combining strong cryptographic protocols, robust authentication, continuous monitoring, and proactive vulnerability management.

Many thanks to our sponsor Esdebe who helped us prepare this research report.

5. Security Measures and Countermeasures

Mitigating the complex security risks associated with IMS requires a comprehensive and multi-layered approach, integrating technological solutions, robust processes, and trained personnel. Effective countermeasures aim to protect the confidentiality, integrity, availability, and authenticity of IMS services and user data.

5.1 Encryption and Secure Communication

Encryption is fundamental to ensuring the confidentiality and integrity of both signaling and media streams within the IMS domain. Implementing strong cryptographic protocols across various interfaces is paramount.

• IPsec (Internet Protocol Security): IPsec is a suite of protocols that provides cryptographic security at the IP layer. It is widely used to secure communication between IMS components, particularly between the UE and the P-CSCF, and potentially between core IMS elements:
  • UE-P-CSCF Security: For mobile access (e.g., VoLTE), an IPsec security association (SA) is typically established between the UE and the P-CSCF. This tunnel encrypts and authenticates all SIP signaling traffic, preventing eavesdropping and tampering of signaling messages [24]. Both Authentication Header (AH) for integrity and Encapsulating Security Payload (ESP) for confidentiality and integrity are utilized, often in tunnel mode.
  • Core Network Security: IPsec can also be deployed to secure logical links between IMS core components (e.g., S-CSCF to HSS, S-CSCF to AS) or between different operators’ IMS networks, providing hop-by-hop security.
• TLS (Transport Layer Security) and DTLS (Datagram Transport Layer Security):
  • SIP over TLS: SIP signaling can be secured using TLS, particularly over TCP connections. This is commonly applied to protect communication between the P-CSCF and other CSCFs, between CSCFs and Application Servers (via ISC interface), and for inter-operator SIP peering. TLS provides mutual authentication, data confidentiality, and integrity for the SIP messages [25].
  • Diameter over TLS/DTLS: Diameter, used for AAA purposes, is often secured using TLS over TCP or DTLS over SCTP (Stream Control Transmission Protocol). This protects sensitive subscriber authentication and profile data exchanged between the HSS, CSCFs, and Application Servers.
• SRTP (Secure Real-time Transport Protocol): While RTP carries media (voice, video), SRTP provides cryptographic protection for these streams. SRTP encrypts the media payload, authenticates packets, and protects against replay attacks. It is crucial for ensuring the confidentiality of conversations and video feeds. Key exchange for SRTP sessions can be managed through various mechanisms, such as Session Description Protocol Security Descriptions (SDES) transported over encrypted SIP, or more robust protocols like MIKEY (Multimedia Internet KEYing) [26].
• Key Management: The effectiveness of encryption relies heavily on robust key management practices, including secure key generation, distribution, storage, and rotation. Poor key management can undermine even the strongest cryptographic algorithms.

5.2 Robust Authentication and Authorization Mechanisms

Strong authentication and granular authorization are critical to prevent unauthorized access and identity theft, ensuring that only legitimate users and network elements can access IMS services.

• Enhanced IMS Authentication and Key Agreement (AKA): The IMS AKA protocol, based on 3GPP AKA, provides mutual authentication between the UE and the IMS network. Enhancements to the baseline AKA are continuously developed to address identified vulnerabilities (e.g., susceptibility to certain types of denial-of-service attacks or privacy concerns if not implemented carefully). These enhancements may include:
  • Identity-Based Cryptography: Integrating identity-based cryptography can simplify key management and improve security and performance [27].
  • Stronger Challenge-Response Mechanisms: Ensuring the cryptographic strength of the challenge-response process to resist various forms of attacks.
  • Privacy Enhancements: Protecting user identity during the authentication process.
• Centralized AAA (Diameter): Leveraging the Diameter protocol with proper security configurations is essential for centralized authentication, authorization, and accounting. All access attempts and service requests should be verified against the subscriber profiles stored in the HSS.
• Multi-factor Authentication (MFA): For administrative access to IMS network elements and management systems, MFA should be enforced to prevent unauthorized configuration changes or access due to compromised credentials.
• Granular Authorization Policies: Implementing strict authorization policies that define what services each user is permitted to access and what actions each network element is allowed to perform. This is typically managed via the HSS and enforced by CSCFs and AS.

5.3 Intrusion Detection and Prevention Systems (IDPS) and Security Monitoring

Proactive monitoring and the deployment of IDPS are crucial for detecting and responding to malicious activities in real-time, providing an active defense layer against evolving threats.

• Strategic Deployment: IDPS should be strategically placed at key points within the IMS network: at ingress/egress points (e.g., behind SBCs), on interfaces to external networks (PSTN, roaming partners), and deep within the IMS core (e.g., monitoring traffic to/from S-CSCF, HSS, AS).
• Capabilities:
  • Signature-based Detection: Identifying known attack patterns (e.g., specific malformed SIP messages, DoS flood patterns).
  • Anomaly-based Detection: Baselining normal network behavior and alerting on deviations, which can detect novel or zero-day attacks.
  • Behavioral Analysis: Monitoring user and network element behavior for suspicious patterns indicative of compromise (e.g., a sudden surge in failed registration attempts from a specific source, unusual service requests).
  • Protocol Anomaly Detection: Identifying deviations from standard protocol specifications (SIP, Diameter, RTP) that might indicate a fuzzing attack or session manipulation attempt.
• Security Information and Event Management (SIEM) Systems: Integrating IDPS alerts, firewall logs, and other security events into a centralized SIEM system allows for real-time correlation, comprehensive visibility, and rapid incident response. This facilitates a holistic view of the security posture and helps identify complex, multi-stage attacks.
• Traffic Shaping and Rate Limiting: Implementing traffic shaping and rate-limiting mechanisms on network devices and at the P-CSCF/S-CSCF can mitigate the impact of DoS/DDoS attacks by controlling the volume of signaling and media traffic allowed into the core network.

5.4 Regular Security Audits, Vulnerability Assessments, and Penetration Testing

Proactive security testing is essential to identify and address weaknesses before they can be exploited by attackers.

• Periodic Security Audits: Conducting regular, independent audits of the IMS infrastructure, including network configurations, software code, security policies, and operational procedures, to ensure compliance with best practices and regulatory requirements.
• Vulnerability Assessments: Systematically scanning IMS components and network devices for known vulnerabilities (e.g., outdated software versions, misconfigurations, unpatched systems). This should cover all layers, from operating systems to specific IMS applications.
• Penetration Testing (Pen-testing): Simulating real-world attacks against the IMS network to identify exploitable weaknesses. This includes black-box (no prior knowledge) and white-box (with full knowledge) testing, targeting specific IMS protocols (SIP, Diameter fuzzing), application servers, and inter-network interfaces.
• Red Teaming: Engaging in full-scope, realistic attack simulations, mimicking the tactics, techniques, and procedures (TTPs) of sophisticated threat actors, to test the effectiveness of both technical controls and the security operations center’s response capabilities.
• Compliance with Industry Standards: Adhering to security guidelines and best practices published by organizations like 3GPP and GSMA, which provide specific recommendations for securing mobile networks including IMS components.

5.5 Session Border Controllers (SBCs)

SBCs play an indispensable role in securing IMS deployments, acting as a crucial first line of defense at the network edge and at interconnection points with other operators.

• Topology Hiding: SBCs conceal the internal network topology by acting as a single point of contact for external networks, preventing attackers from mapping the internal structure of the IMS core.
• DoS/DDoS Protection: SBCs perform rate limiting, connection policing, and malformed packet filtering at the network border, preventing signaling floods and other DoS attacks from reaching core IMS entities.
• Network Address Translation (NAT) and Firewall Functionality: SBCs facilitate NAT traversal for UEs and implement advanced firewall rules to filter unauthorized traffic.
• Protocol Normalization: SBCs can normalize variations in SIP protocol implementations between different vendors or networks, preventing protocol-specific attacks and ensuring interoperability.
• Admission Control: They can enforce call admission control policies, limiting the number of simultaneous sessions to prevent resource exhaustion.
• Interworking: SBCs handle interworking between different signaling protocols or network types, ensuring secure and seamless communication while maintaining security boundaries.

5.6 Network Segmentation and Isolation

Implementing network segmentation and strict access controls limits the blast radius of a successful breach and prevents unauthorized lateral movement within the IMS infrastructure.

• VLANs/VPNs: Logically segmenting different IMS functional entities (e.g., CSCFs, HSS, AS) into separate Virtual Local Area Networks (VLANs) or Virtual Private Networks (VPNs) reduces the attack surface.
• Firewalls and Access Control Lists (ACLs): Deploying stateful firewalls between different network segments and configuring strict ACLs that only allow necessary ports and protocols between IMS components. This enforces the principle of least privilege for network traffic.
• Demilitarized Zones (DMZs): Placing externally accessible components (like I-CSCFs and SBCs) in DMZs, separating them from the internal IMS core network.

5.7 Secure Software Development and Patch Management

Ensuring the security of the software itself and maintaining it throughout its lifecycle is paramount.

• Secure Coding Practices: For custom Application Servers or any proprietary software within the IMS domain, secure coding guidelines (e.g., OWASP Top 10) should be followed to minimize vulnerabilities.
• Regular Patch Management: Implementing a rigorous patch management process to ensure that all operating systems, IMS software components, and third-party libraries are kept up-to-date with the latest security patches from vendors. Zero-day exploits often target unpatched vulnerabilities.
• Vendor Collaboration: Close collaboration with IMS solution vendors is crucial to receive timely security advisories and patches for their equipment and software.
• Supply Chain Security: Ensuring the security of the entire supply chain, from hardware components to software libraries, to prevent the introduction of backdoors or vulnerabilities at the manufacturing or development stages.

5.8 Disaster Recovery and Business Continuity Planning

While not directly a security measure, robust disaster recovery and business continuity planning are essential to mitigate the impact of successful attacks, ensuring rapid restoration of IMS services.

• Redundancy and High Availability: Implementing redundant IMS components (e.g., active-standby HSS, clustered S-CSCFs) and geographic distribution to ensure high availability even in the event of a localized attack.
• Regular Backups: Performing regular, secure backups of configuration data, subscriber profiles, and system images to facilitate rapid recovery.
• Incident Response Plan: Developing and regularly testing a comprehensive incident response plan to quickly detect, contain, eradicate, and recover from security breaches.

By systematically implementing these multifaceted security measures, operators can significantly enhance the resilience and integrity of their IMS infrastructures, protecting them against the sophisticated and evolving landscape of cyber threats.

Many thanks to our sponsor Esdebe who helped us prepare this research report.

6. Conclusion

The IP Multimedia Subsystem (IMS) has firmly established itself as an indispensable architectural foundation for modern telecommunications networks, serving as the primary enabler for the pervasive delivery of diverse and rich multimedia services over IP. Its pivotal role spans from critical voice communication over LTE (VoLTE) and Wi-Fi (VoWiFi) to advanced offerings like video conferencing, instant messaging, and Rich Communication Services (RCS), underscoring its centrality in connecting individuals and businesses in the digital age. This report has meticulously detailed the layered IMS architecture, elucidated the complex interplay of its functional entities, and examined the critical signaling protocols that orchestrate its operations.

However, the very attributes that lend IMS its power—its complexity, distributed nature, and reliance on an array of interconnected protocols—simultaneously expose it to a broad spectrum of security threats. As demonstrated, these vulnerabilities are not theoretical but represent tangible risks that can compromise the availability, confidentiality, and integrity of communication services. From resource-exhausting Denial of Service attacks and insidious session hijacking to the pervasive threat of eavesdropping and sophisticated identity theft facilitated by authentication weaknesses, the attack surface of IMS is considerable and dynamic.

To safeguard this critical infrastructure against an ever-evolving landscape of cyber threats, a comprehensive, multi-layered, and proactive approach to security is not merely advisable but absolutely essential. The implementation of robust encryption across signaling and media planes using protocols like IPsec, TLS, and SRTP is fundamental for ensuring data confidentiality and integrity. Complementing this, strong and continuously enhanced authentication mechanisms, such as fortified IMS AKA, are vital to prevent unauthorized access and combat identity impersonation. Furthermore, the strategic deployment of advanced Intrusion Detection and Prevention Systems (IDPS), coupled with vigilant security monitoring, provides the capability for real-time threat detection and rapid response.

Beyond technological solutions, organizational resilience is paramount. This includes conducting regular and thorough security audits, vulnerability assessments, and penetration testing to proactively identify and remediate weaknesses. The strategic use of Session Border Controllers (SBCs) at network boundaries serves as a critical first line of defense, offering protection against DoS attacks and enabling secure inter-operator connectivity. Finally, emphasizing secure software development practices, rigorous patch management, and robust disaster recovery planning ensures that the IMS infrastructure remains resilient and can swiftly recover from any security incidents.

In conclusion, the security of IMS is not a static endeavor but an ongoing commitment requiring continuous vigilance, adaptation to emerging threats, and a holistic integration of security into every facet of its design, deployment, and operation. Only through such comprehensive and sustained efforts can the IP Multimedia Subsystem reliably fulfill its promise of secure, ubiquitous, and high-quality multimedia communication in the future of telecommunications.

Many thanks to our sponsor Esdebe who helped us prepare this research report.

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