CVE-2026-68555 in Coturn
Summary
by MITRE • 08/20/2026
Coturn is a free open source implementation of TURN and STUN Server. In 4.15.0, an authenticated TURN user can repeatedly resume one allocation from fresh UDP 5-tuples without completing a handoff when the server enables --mobility. mobile_begin_transition() in src/server/ns_turn_server.c disarms each new session's allocation timeout and overwrites the allocation's single mobile_pending_resume link, leaving earlier pending sessions unreachable by the cleanup path, while copy_auth_parameters() ignores inc_quota() failure. The attacker can therefore retain unbounded server-side sessions and exhaust process memory even when --user-quota=1 is configured. This issue is fixed in version 4.16.0.
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Analysis
by VulDB Data Team • 08/20/2026
The vulnerability identified in Coturn versions prior to 4.16.0 represents a critical resource exhaustion flaw stemming from improper handling of mobility features within the TURN protocol implementation. Coturn serves as an open-source reference for both STUN and TURN servers, facilitating network address translation traversal by allowing clients to establish relayed communication channels. The specific defect resides in the server-side logic governing mobile transitions, particularly when the --mobility flag is enabled on the server instance. This feature allows a client to maintain connectivity while changing network interfaces or IP addresses, but the implementation contains logical errors that allow an authenticated attacker to manipulate session states for malicious purposes.
The technical root cause involves two distinct failures in the source code handling of allocation lifecycles and quota enforcement. First, the function mobile_begin_transition() fails to properly manage the timeout mechanisms associated with TURN allocations. When a user initiates a transition by resuming an allocation from fresh UDP five-tuples without completing a full handoff sequence, this function disarms the allocation timeout for each new session created during the process. Simultaneously it overwrites the mobile_pending_resume link within the allocation structure. This action effectively severs the connection between earlier pending sessions and the server's cleanup path, rendering those orphaned sessions unreachable by standard garbage collection or expiration routines. Consequently, these stale allocations persist in memory indefinitely rather than being terminated after their designated timeout period expires.
Secondly, a separate but compounding issue exists within the copy_auth_parameters() function which handles authentication parameter replication during session establishment. This routine is designed to increment usage counters via inc_quota() to enforce user-specific resource limits configured by administrators through directives such as --user-quota=1. However, the implementation ignores any failure return values from this quota enforcement mechanism. As a result, even when strict quotas are configured to limit each user to a single active session, the server fails to reject or account for additional sessions created via the mobility resume exploit. The combination of disabled timeouts and ignored quota checks allows an authenticated attacker to spawn unbounded numbers of server-side allocations.
The operational impact of this vulnerability is severe resource exhaustion leading to denial of service conditions. By repeatedly resuming allocations from new UDP endpoints without completing handoffs, an attacker can consume unlimited amounts of process memory on the Coturn server. Since these sessions are not tracked by cleanup routines and bypass quota limits, they accumulate until the server's available memory is depleted. This leads to system instability, potential crashes, or complete unavailability of TURN services for legitimate users relying on the infrastructure for real-time communication applications such as VoIP or video conferencing. The attack requires only authenticated access but does not require high privilege levels beyond standard user credentials, making it accessible to a broader range of threat actors who have obtained valid login details.
Mitigation strategies primarily involve upgrading to Coturn version 4.16.0 where these logical flaws in mobility handling and quota enforcement have been corrected. For environments unable to immediately patch the software, administrators should consider disabling the --mobility feature if it is not strictly required for their deployment scenario, thereby removing the attack vector associated with mobile_begin_transition(). Additionally, implementing strict network-level rate limiting on TURN allocation requests can help mitigate the speed at which resources are consumed during an active exploitation attempt. Monitoring server memory usage and alerting on anomalous spikes in concurrent allocations may also provide early detection of such abuse patterns before total service failure occurs.
From a classification perspective, this vulnerability aligns with CWE-400 Uncontrolled Resource Consumption as it allows for the exhaustion of system resources through unbounded allocation creation. It also relates to CWE-755 Improper Handling of Unexpected Condition regarding the ignored quota errors and CWE-613 Insufficient Session Expiration due to the failure to properly timeout orphaned sessions. In terms of adversarial tactics, this exploitation technique corresponds to ATT&CK T1499 Endpoint Denial of Service where an attacker leverages application logic flaws to degrade or destroy system availability rather than compromising confidentiality or integrity directly.