The vulnerability CVE-2025-31133 affects the opencontainers runc tool, a widely used CLI utility for spawning and running containers compliant with the OCI specification. The flaw is rooted in improper validation of the bind-mount source inode when masking the container's /dev/null device. Specifically, runc versions 1.2.7 and below, 1.3.0-rc.1 through 1.3.1, and 1.4.0-rc.1 through 1.4.0-rc.2 fail to verify that the bind-mounted /dev/null inside the container corresponds to a genuine /dev/null inode on the host. This inadequate verification allows attackers to exploit symbolic link following (CWE-61) and race conditions (CWE-363) to mount arbitrary files or directories inside the container or host namespace. Such arbitrary mount gadgets can be leveraged to disclose sensitive host information, cause denial of service by interfering with critical host resources, escape container isolation boundaries, or bypass security features like maskedPaths designed to hide sensitive host paths from containers. The attack vector requires local access with low privileges and some user interaction, but the impact on confidentiality, integrity, and availability is high. The CVSS 4.0 vector (AV:L/AC:L/AT:P/PR:L/UI:A/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H) reflects these factors, indicating a high-severity vulnerability with complex scope and significant consequences. Although no active exploits are currently known, the widespread use of runc in container orchestration platforms (e.g., Kubernetes) makes this a critical issue. The vulnerability is addressed in runc versions 1.2.8, 1.3.3, and 1.4.0-rc.3 and later, where proper inode verification and symlink handling have been implemented to prevent such attacks.

For European organizations, this vulnerability poses a substantial risk to containerized environments, which are extensively used across industries such as finance, manufacturing, telecommunications, and government services. Exploitation can lead to container escape, allowing attackers to gain unauthorized access to the host system, potentially compromising sensitive data and critical infrastructure. Host denial of service could disrupt business operations and service availability, impacting compliance with regulations like GDPR that mandate data protection and system integrity. The ability to bypass maskedPaths undermines security controls designed to isolate containers from sensitive host files, increasing the attack surface. Given the reliance on container orchestration platforms that use runc as the default runtime, the vulnerability could affect cloud service providers, managed hosting environments, and private data centers across Europe. The high severity and potential for privilege escalation make this a critical concern for organizations with multi-tenant or hybrid cloud deployments. Failure to patch could result in data breaches, operational downtime, and reputational damage.

European organizations should immediately identify all systems running vulnerable versions of runc by auditing container runtimes and orchestration platforms. Upgrade runc to version 1.2.8, 1.3.3, 1.4.0-rc.3, or later to ensure the vulnerability is patched. Implement strict access controls and monitoring on hosts running containers to detect unusual mount operations or symlink manipulations. Employ container security tools that enforce runtime policies preventing unauthorized bind mounts or device masking. Regularly review and harden container configurations, especially around maskedPaths and device access. Use security frameworks like SELinux or AppArmor to restrict container privileges and limit the impact of potential escapes. Conduct penetration testing focused on container escape vectors to validate defenses. Finally, maintain up-to-date incident response plans that include container-specific threat scenarios.