CVE-2020-8557 in kublet
Summary
by MITRE
The Kubernetes kubelet component in versions 1.1-1.16.12, 1.17.0-1.17.8 and 1.18.0-1.18.5 do not account for disk usage by a pod which writes to its own /etc/hosts file. The /etc/hosts file mounted in a pod by kubelet is not included by the kubelet eviction manager when calculating ephemeral storage usage by a pod. If a pod writes a large amount of data to the /etc/hosts file, it could fill the storage space of the node and cause the node to fail.
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Analysis
by VulDB Data Team • 11/05/2020
The vulnerability described in CVE-2020-8557 represents a critical storage consumption issue within the Kubernetes kubelet component that affects multiple versions across the 1.1 through 1.18 release lines. This flaw specifically targets the eviction manager's inability to accurately account for disk usage when pods modify their own /etc/hosts file, creating a potential for node-level storage exhaustion that can lead to system-wide failures. The issue stems from the kubelet's failure to track the ephemeral storage consumption of the /etc/hosts file that is mounted into pods, which constitutes a fundamental oversight in how the system monitors and manages storage resources at the node level.
The technical implementation of this vulnerability occurs through the kubelet's eviction manager mechanism which is responsible for monitoring and enforcing storage limits on pods to prevent node-level resource exhaustion. When a pod writes data to its mounted /etc/hosts file, the kubelet does not include this file's disk usage in its calculations of ephemeral storage consumption for that pod. This oversight means that the eviction manager cannot properly determine when a pod has consumed excessive storage resources and may fail to trigger appropriate eviction actions. The root cause aligns with CWE-16: Improper Configuration, specifically in how the system manages resource accounting and monitoring.
The operational impact of this vulnerability extends beyond simple storage consumption concerns to potentially compromise entire node operations and cluster stability. An attacker or malicious pod could deliberately write large amounts of data to the /etc/hosts file, causing the node's storage to fill completely and resulting in node failure or unresponsiveness. This scenario can lead to cascading failures across the cluster as the affected node becomes unavailable, disrupting pod scheduling and potentially causing service outages. The vulnerability creates a persistent denial-of-service condition that can be exploited to destabilize cluster operations, with implications that align with ATT&CK technique T1499.001: Network Denial of Service and T1566.001: Phishing.
Mitigation strategies for this vulnerability require both immediate patching of affected Kubernetes versions and implementation of additional monitoring controls. Organizations should prioritize upgrading to Kubernetes versions 1.16.13, 1.17.9, or 1.18.6 where this issue has been resolved through proper accounting of /etc/hosts file usage in eviction calculations. Additionally, cluster administrators should implement enhanced monitoring of node storage usage with alerts for unusual patterns, particularly when pods are identified as writing excessive data to mounted volumes. The solution involves ensuring that the kubelet's eviction manager properly includes all mounted file system usage in its calculations, which aligns with security best practices outlined in the Kubernetes security guidelines and represents a fundamental requirement for proper resource management in containerized environments.