CVE-2016-6595 in Docker
Summary
by MITRE
** DISPUTED ** The SwarmKit toolkit 1.12.0 for Docker allows remote authenticated users to cause a denial of service (prevention of cluster joins) via a long sequence of join and quit actions. NOTE: the vendor disputes this issue, stating that this sequence is not "removing the state that is left by old nodes. At some point the manager obviously stops being able to accept new nodes, since it runs out of memory. Given that both for Docker swarm and for Docker Swarmkit nodes are *required* to provide a secret token (it's actually the only mode of operation), this means that no adversary can simply join nodes and exhaust manager resources. We can't do anything about a manager running out of memory and not being able to add new legitimate nodes to the system. This is merely a resource provisioning issue, and definitely not a CVE worthy vulnerability."
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Analysis
by VulDB Data Team • 08/06/2024
The vulnerability identified as CVE-2016-6595 pertains to the SwarmKit toolkit version 1.12.0 within Docker's ecosystem, specifically targeting the cluster management functionality. This issue involves a denial of service condition that occurs when authenticated remote users exploit a particular pattern of cluster operations. The flaw manifests through a sequence of join and quit actions that, when executed repeatedly, can prevent new nodes from successfully joining the cluster. While the vendor has disputed the classification of this as a security vulnerability, the technical implications remain significant for system administrators and security professionals managing Docker swarm environments.
The technical mechanism behind this vulnerability involves the accumulation of state information within the SwarmKit manager nodes during the join and quit operations. When users repeatedly perform these actions without proper cleanup of the underlying state data, the system's memory consumption increases progressively. This behavior aligns with CWE-400, which categorizes resource exhaustion vulnerabilities, and demonstrates how improper state management can lead to system instability. The SwarmKit implementation appears to lack adequate garbage collection or state cleanup mechanisms for nodes that have disconnected, causing the manager to eventually reach resource limits and reject new legitimate node requests.
From an operational perspective, this vulnerability represents a significant concern for organizations relying on Docker swarm clusters for container orchestration. The denial of service condition directly impacts cluster availability and can disrupt critical applications that depend on seamless node management. Attackers with authenticated access can exploit this weakness to systematically degrade cluster performance until new node joins become impossible, effectively creating a service disruption that can last until manual intervention occurs. The vulnerability's impact is particularly concerning in environments where cluster scalability and node mobility are essential requirements, as it undermines the fundamental reliability of the swarm management system.
The vendor's response indicates that they consider this issue to be primarily a resource provisioning problem rather than a security vulnerability, given that the attack requires authenticated access and relies on legitimate operational patterns. However, this classification overlooks the potential for this weakness to be exploited in combination with other attack vectors or to serve as a stepping stone for more sophisticated attacks. The denial of service capability represents a valid security concern under ATT&CK framework's T1499 technique, which covers denial of service attacks. Organizations should consider implementing monitoring solutions to detect unusual patterns of node join and quit operations, as well as establishing proper resource limits and memory management policies to prevent legitimate cluster operations from being disrupted by this vulnerability.
While the vendor disputes the CVE classification, security practitioners should treat this issue as a legitimate operational concern that requires attention. The underlying problem demonstrates inadequate state management within the SwarmKit architecture and highlights the importance of proper resource monitoring and management in container orchestration systems. Organizations should implement proactive measures including regular cluster maintenance, automated cleanup of disconnected nodes, and capacity planning to ensure sufficient resources are maintained for legitimate cluster operations. This vulnerability serves as a reminder of the critical importance of proper state management and resource handling in distributed systems, particularly those handling authentication and authorization processes where legitimate users can inadvertently or maliciously cause operational disruptions through valid but excessive system interactions.