Flannel, a Kubernetes network fabric, includes an experimental Extension backend that accepts attacker-controlled data through the Node annotation flannel.alpha.coreos.com/backend-data. The backend’s SubnetAddCommand and SubnetRemoveCommand read this annotation and pipe its contents directly into a shell without validation, creating a command‑injection vulnerability (CWE‑77). An attacker who can write this annotation may inject arbitrary shell commands and execute them with root privileges on every Flannel node that uses the Extension backend, leading to full system compromise.

The flaw affects all Flannel releases before v0.28.2 when the Extension backend is enabled. It is limited to flannel‑io:flannel and only the Extension backend; other supported backends such as vxlan and wireguard are not impacted.

The CVSS score of 7.5 signals moderate to high severity, while the EPSS score of less than 1% indicates a low likelihood of exploitation in the wild. The vulnerability is not listed in the CISA KEV catalog. Exploitation requires the ability to set Kubernetes Node annotations, which typically implies cluster‑level permissions; an attacker with such access can add a malicious annotation to any node, trigger command execution, and gain root shell on the node. Although the probability of an exploit appears low, the potential impact is severe, warranting immediate attention.