runc container breakout vulnerabilities: A technical overview

The vulnerabilities Exploitation scenarios and threat model Kubernetes and cloud native implications Affected versions and patches Mitigations The bigger picture: Secure-by-default configurations Credits Posted on November 28, 2025 by Matteo Bisi, DevSecOps Team Leader at ReeVo and CNCF KCD Organizer CNCF projects highlighted in this post A set of high-severity vulnerabilities in runc were publicly disclosed in November 2025, allowing for full container breakouts. Runc is the cornerstone of containerization on Linux, serving as the default low-level container runtime for industry-standard tools like Docker, Podman, and Kubernetes. Its ubiquity means that a vulnerability in runc has far-reaching implications for the entire cloud-native ecosystem. This post summarizes the vulnerabilities, the affected versions, and the recommended actions to mitigate them. Three vulnerabilities were discovered, all related to bypassing runc’s restrictions on writing to arbitrary /proc files. These vulnerabilities can be exploited by starting containers with custom mount configurations, including those defined in Dockerfiles (RUN –mount=…). CVE-2025-31133: “container escape via ‘masked path’ abuse due to mount race conditions” CVSS: 7.3 Description: This vulnerability exploits an issue with how masked paths are implemented. An attacker can replace /dev/null with a symlink to a procfs file (e. g. , /proc/sys/kernel/core_pattern or /proc/sysrq-trigger). Runc will then bind-mount the symlink target read-write, leading to a container breakout or a host crash. CVE-2025-52565: “container escape with malicious config due to /dev/console mount and related races” CVSS: 7.3 Description: Similar to the previous CVE, this vulnerability exploits a flaw in /dev/console bind-mounts.