CVE-2015-3629 in Libcontainer
Summary
by MITRE
Libcontainer 1.6.0, as used in Docker Engine, allows local users to escape containerization ("mount namespace breakout") and write to arbitrary file on the host system via a symlink attack in an image when respawning a container.
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Analysis
by VulDB Data Team • 05/17/2022
The vulnerability identified as CVE-2015-3629 represents a critical container escape flaw in libcontainer version 1.6.0, which was integral to the Docker Engine's containerization capabilities. This issue stems from improper handling of mount namespaces and symbolic link resolution during container respawn operations, creating a fundamental security bypass that undermines the core isolation principles of containerized environments. The vulnerability specifically affects systems where Docker containers are managed using libcontainer, making it particularly relevant to organizations relying on containerized infrastructure for application deployment and isolation.
The technical flaw manifests through a sophisticated symlink attack vector that exploits the way libcontainer manages mount namespaces when containers are restarted or respawned. During this process, the container runtime fails to properly validate or sanitize symbolic links that may have been embedded within container images, allowing malicious actors to manipulate the mount namespace in such a way that they can traverse beyond the designated container boundaries. This occurs because the system does not adequately verify the target paths of symbolic links before executing mount operations, enabling attackers to create malicious symlinks that point to sensitive host filesystem locations.
The operational impact of this vulnerability extends far beyond simple privilege escalation, as it fundamentally compromises the security model that containerization is designed to provide. An attacker with local access to a containerized environment can leverage this flaw to gain write access to arbitrary files on the host system, potentially leading to complete system compromise. This breakout capability allows adversaries to modify critical system files, install backdoors, exfiltrate sensitive data, or establish persistent access to the underlying host infrastructure. The vulnerability is particularly dangerous in multi-tenant environments where container isolation is crucial for security separation between different applications or users.
Mitigation strategies for CVE-2015-3629 require immediate patching of affected Docker Engine installations to versions that include the corrected libcontainer implementation. Organizations should also implement additional operational controls such as restrictive container image management policies that prevent the execution of containers with potentially malicious symlinks. The use of read-only root filesystems for containers, combined with proper user namespace mapping and capability dropping, can significantly reduce the attack surface. From a defensive perspective, this vulnerability aligns with CWE-22 (Improper Limitation of a Pathname to a Restricted Directory) and represents a technique categorized under ATT&CK tactic T1068 (Exploitation for Privilege Escalation) and technique T1496 (Resource Hijacking). System administrators should also consider implementing monitoring solutions that can detect anomalous mount operations or symlink creation patterns that may indicate exploitation attempts. Regular security audits of container images and runtime configurations remain essential for maintaining defense in depth against similar namespace-based escape vulnerabilities.