CVE-2026-73216 in Coturn
Summary
by MITRE • 08/11/2026
Coturn is a free open source implementation of TURN and STUN Server. Prior to 4.17.0, shutdown_client_connection() in src/server/ns_turn_server.c prematurely calls dec_quota() and releases bandwidth accounting during the first-stage close of a mobility-enabled allocation while preserving the allocation, relay socket, session, and mobility ticket, allowing an authenticated client to bypass --user-quota and --total-quota and exhaust relay ports. This issue is fixed in version 4.17.0.
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Analysis
by VulDB Data Team • 08/11/2026
The vulnerability exists within the coturn TURN and STUN server implementation where a race condition occurs during the shutdown of client connections with mobility support enabled. The flaw specifically manifests in the shutdown_client_connection() function located in src/server/ns_turn_server.c, where the system prematurely executes dec_quota() before completing the full connection termination process. This premature bandwidth accounting release occurs during what is termed the first-stage close of a mobility-enabled allocation, creating a window where the system maintains certain connection components while simultaneously releasing others.
The technical implementation issue stems from improper state management during connection teardown when mobility features are active. When a client connection undergoes shutdown with mobility enabled, the server should maintain consistency in its resource accounting and connection state handling throughout the complete termination sequence. However, the current implementation violates this principle by decrementing the quota immediately upon detecting the first stage of closure while preserving critical allocation resources including relay sockets, session data, and mobility tickets.
This vulnerability directly impacts the server's ability to enforce bandwidth and port quotas as defined by the --user-quota and --total-quota parameters. The flaw allows authenticated clients to effectively bypass these quota mechanisms entirely, enabling them to consume excessive relay ports and bandwidth resources beyond what the system administrator intended to permit. The preservation of allocation state components while releasing accounting information creates a scenario where a client can repeatedly establish connections and consume resources without proper quota enforcement.
The operational impact of this vulnerability extends beyond simple resource exhaustion, as it fundamentally undermines the security model that coturn implements for resource management. Attackers could potentially exhaust available relay ports and bandwidth, leading to denial of service conditions that affect legitimate users. This issue represents a classic case of incomplete state cleanup where the system fails to maintain atomicity in its resource management operations, allowing unauthorized consumption of system resources while appearing to maintain proper connection state.
The vulnerability affects systems configured with mobility support enabled for TURN allocations, making it particularly concerning for deployments that require dynamic client reconnection capabilities. The fix implemented in version 4.17.0 addresses the core issue by ensuring proper sequencing of resource cleanup operations and maintaining consistency between bandwidth accounting and connection state management. This remediation aligns with best practices for resource management in network services and addresses similar patterns found in other networking implementations that have been catalogued under CWE-691 as insufficient resource cleanup or improper resource handling.
The security implications of this flaw are significant as it enables authenticated users to circumvent access controls and resource limits that are fundamental to maintaining server stability and fair resource distribution. This type of vulnerability would typically be categorized under ATT&CK technique T1499 for resource hijacking, where an adversary gains unauthorized access to system resources through manipulation of legitimate administrative controls. The fix ensures that quota enforcement remains consistent throughout the complete connection lifecycle, preventing scenarios where valid authentication credentials can be used to bypass resource limitations designed to protect server capacity and maintain fair usage policies across all authenticated clients.
This vulnerability demonstrates the complexity inherent in managing stateful network services where features such as mobility require careful coordination between different subsystems. The improper sequencing of operations in connection teardown creates a potential attack vector that could be exploited systematically to exhaust server resources, particularly in environments where coturn serves as a critical infrastructure component for NAT traversal and media relay services.