CVE-2026-32241: Flannel vulnerable to cross-node remote code execution via extension backend BackendData injection

Vulnerability summary

CVE-2026-32241 is a high (CVSS 8.8) vulnerability in flannel-io flannel. EPSS estimates a 2.76% chance of exploitation in the next 30 days, higher than 86.2% of scored vulnerabilities.

Timeline

1. 27-03-2026
   PublishedVulnerability first disclosed.
2. 31-03-2026
   Last modifiedVulnerability information updated.

Description

Flannel is a network fabric for containers, designed for Kubernetes. The Flannel project includes an experimental Extension backend that allows users to easily prototype new backend types. In versions of Flannel prior to 0.28.2, this Extension backend is vulnerable to a command injection that allows an attacker who can set Kubernetes Node annotations to achieve root-level arbitrary command execution on every flannel node in the cluster. The Extension backend's SubnetAddCommand and SubnetRemoveCommand receive attacker-controlled data via stdin (from the `flannel.alpha.coreos.com/backend-data` Node annotation). The content of this annotation is unmarshalled and piped directly to a shell command without checks. Kubernetes clusters using Flannel with the Extension backend are affected by this vulnerability. Other backends such as vxlan and wireguard are unaffected. The vulnerability is fixed in version v0.28.2. As a workaround, use Flannel with another backend such as vxlan or wireguard.

• CWE-77Improper Neutralization of Special Elements used in a CommandThe product constructs all or part of a command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended command when it is sent to a downstream component.