Weave Net Pods running in host PID namespace can be used to escalate other Kubernetes vulnerabilities

Vulnerability

Weave Net is an open-source virtual networking solution for Docker containers that connects them across multiple hosts. In Kubernetes deployments before version 2.8.0, the DaemonSet manifest that runs a Weave Net pod on every node sets hostPID: true and privileged: true, along with excessive host volume mounts [2]. The hostPID setting is unnecessary for the pod's intended network management functions and grants the container the ability to see all host processes and write anywhere in the host filesystem [1][2].

Exploitation

An attacker must first achieve code execution inside the Weave Net pod through another vulnerability (e.g., a bug in a Kubernetes component) or a cluster misconfiguration. No such companion vulnerability was known at the time of disclosure, and there were no reported instances of exploitation [2]. Once inside the pod, the attacker can leverage the existing hostPID: true and privileged access—along with mounted host volumes—to escape the container and gain full control over the underlying node [2].

Impact

A successful attacker can take over any host in the Kubernetes cluster, achieving complete compromise of the node's operating system. This enables arbitrary code execution at the host level, full access to all containers on that host, and the ability to disrupt or observe cluster-wide network operations [2]. The privilege escalation is from container-level access to root-level host access.

Mitigation

Weave Net 2.8.0, released on the same day as the advisory (2021-01-20), removes hostPID: true and moves CNI plugin installation to an init container, which reduces the attack surface [1][2]. Users who cannot update immediately can edit the hostPID line in their existing DaemonSet manifest from true to false, arrange an alternative method to install CNI plugins (e.g., Ansible), and remove the unnecessary host volume mounts from the manifest [2]. The project later released 2.8.1 to fix a regression, but the security improvements remain [1].