A SIP client call flood, commonly referred to as a SIP INVITE flood, is a denial-of-service (DoS) attack that targets Session Initiation Protocol (SIP) signaling infrastructure used by Voice over IP (VoIP) and unified communications systems. In this attack, an adversary sends a large volume of SIP INVITE requests toward a SIP server, proxy, or session border controller (SBC) in an attempt to overwhelm call-processing resources and prevent legitimate users from establishing voice or video sessions.
Unlike volumetric network floods that focus primarily on saturating bandwidth, SIP client call floods focus on exhausting signaling-plane resources. Because SIP servers must parse, validate, and maintain state for each incoming call request, even moderate attack volumes can consume CPU, memory, and connection tables. The result is delayed call setup, dropped calls, or complete service unavailability.
SIP call floods are widely used against VoIP service providers, enterprise communications platforms, contact centers, and emergency services environments, where availability is mission-critical.
SIP is a text-based signaling protocol used to establish, modify, and terminate real-time communication sessions such as voice and video calls. At a high level, a typical SIP call setup involves the following sequence:
- A calling client sends an INVITE request to initiate a session.
- The receiving server or proxy responds with provisional messages such as TRYING and RINGING.
- When the callee accepts the call, the server sends an OK response.
- The caller acknowledges with an ACK, and media exchange begins.
SIP infrastructure commonly includes SIP proxies, registrars, application servers, and SBCs. These components must track transaction state, authenticate endpoints, apply routing logic, and enforce policies for every signaling message.
This stateful nature makes SIP signaling highly sensitive to flooding. Even if media traffic never begins, large volumes of INVITE messages can consume server resources long before bandwidth is saturated.
In a SIP client call flood, attackers generate massive numbers of INVITE requests and send them to exposed SIP services. These requests may be:
- Spoofed or randomly generated source IP addresses
- Sent from botnets or compromised hosts
- Replayed or slightly modified to evade simple signatures
Each INVITE forces the target to allocate processing cycles and often create partial call state. As queues fill and CPU utilization rises, legitimate INVITEs are delayed or dropped.
Attackers may tune the flood to remain below obvious volumetric thresholds while still exhausting signaling resources, making detection more difficult than traditional bandwidth-based DDoS attacks.
SIP call floods are frequently combined with other techniques, such as UDP floods, TCP SYN floods, or malformed SIP messages, to increase attack impact and complicate mitigation.
- SIP INVITE floods
- SIP REGISTER floods
- SIP BYE or CANCEL floods
- Malformed SIP message floods
- Mixed-method SIP flood campaigns
Organizations experiencing a SIP flood may observe:
- Sudden spikes in SIP INVITE request rates
- Large numbers of half-open or incomplete call sessions
- Increased call setup latency
- Failed or dropped calls
- High CPU and memory utilization on SIP servers or SBCs
- Abnormal traffic from unfamiliar IP ranges or geographies
Because many SIP floods attempt to mimic legitimate traffic patterns, distinguishing attack traffic from real call attempts requires protocol-aware inspection and behavioral baselining.
The impact of SIP client call floods extends beyond technical disruption:
- Service outages
- Emergency services disruption
- Revenue loss
- Reputation damage
- Operational strain
For organizations that depend on real-time communications, even short outages can have disproportionate consequences.
Telecommunications providers have repeatedly reported SIP signaling floods that caused partial or complete VoIP outages across regional networks. Attack campaigns attributed to botnets and booter-style tools have targeted exposed SIP ports on UDP 5060 and TCP 5060/5061, overwhelming softswitches and SBC clusters.
In several cases, attackers combined SIP INVITE floods with generic UDP floods, masking the signaling attack behind high background noise. These incidents highlight why pure volumetric protection is insufficient for VoIP environments.
- Apply rate limits and connection thresholds
- Enforce authentication for SIP endpoints and registrations
- Harden SIP configurations by disabling unused methods, removing debug features, and tuning timeouts
- Patch SIP servers, SBCs, and VoIP software regularly
- Segment SIP signaling infrastructure
- Network-layer anomaly detection
- Protocol-aware SIP inspection
- Behavioral rate controls
- Upstream filtering and cloud scrubbing
- Continuous monitoring and alerting
Because SIP floods target signaling logic rather than bandwidth alone, defenses must understand SIP behavior and state.
Radware DefensePro provides inline, behavior-based DDoS protection with protocol awareness for SIP traffic. It baselines normal SIP signaling patterns and detects abnormal surges in INVITE requests, malformed messages, and half-open transactions. DefensePro can apply challenge-response techniques, SYN proxying for SIP over TCP, and rate controls to block malicious signaling while allowing legitimate calls to complete.
For large-scale floods that threaten upstream capacity, Radware Cloud DDoS Protection Service delivers high-capacity scrubbing that mitigates attack traffic before it reaches on-prem or cloud-based SIP infrastructure.
Threat Intelligence Subscriptions enrich SIP defenses with real-world attacker context, allowing known malicious sources associated with DDoS and booter activity to be blocked proactively.
As VoIP and unified communications continue to replace legacy telephony, SIP signaling infrastructure will remain a high-value target. Attack tools increasingly incorporate SIP-specific flood capabilities, making protocol-aware defenses essential. Organizations that combine hardened configurations, behavioral DDoS protection, and cloud-based mitigation will be better positioned to sustain reliable voice services under attack.
To learn more about how Radware can safeguard your organization from SIP client call floods and other DDoS attacks, contact us now.