TS-2026-018

Vulnerability from tailscale - Published: Thu, 12 Nov 2026 00:00:00 GMT

Description: An X-Forwarded-For fallback in the Tailscale Kubernetes operator's API server proxy allowed a host without a Tailscale identity to impersonate any tailnet node.

What happened?

The Tailscale Kubernetes operator's API server proxy authenticates requests from tailnet clients to your cluster's API server. It resolves the caller's Tailscale identity from the source address of their connection and forwards that identity to kube-apiserver using Kubernetes impersonation headers.

In deployments that run the proxy as a dedicated ProxyGroup with spec.type: kube-apiserver (or standalone via k8s-proxy), the proxy fell back to the client-supplied X-Forwarded-For header when the source address lookup failed. A host reached over a subnet route but not itself enrolled in your tailnet has no Tailscale identity, so its source address lookup always fails and the fallback applies. Such a host could set X-Forwarded-For to any node's Tailscale IP address and have its requests processed by kube-apiserver as that node.

Tailscale now identifies callers solely from the connection's source address and no longer accepts X-Forwarded-For.

This vulnerability is fixed in Tailscale version 1.104.7 or newer.

What was the impact?

A host on an advertised subnet with no Tailscale identity of its own could make requests to your Kubernetes API server as any tailnet node, including nodes with full cluster administrator access.

Who was affected?

This affects tailnets running the API server proxy as a dedicated ProxyGroup with spec.type: kube-apiserver (or standalone via k8s-proxy) where the proxy's ProxyClass sets spec.tailscale.acceptRoutes to true and an ACL grant allows connections from hosts behind subnet routes. The operator's in-process API server proxy is not affected. Clients enrolled in your tailnet authenticate through their real source addresses and cannot trigger this fallback.

What do I need to do?

If you use the Tailscale Kubernetes operator's dedicated API server proxy, update your deployment to Tailscale version 1.104.7 or newer. If hosts on your subnets could previously reach it, review your Kubernetes audit logs for unexpected impersonated identities.

Credits

We would like to thank Emilio Mendoza for reporting this issue.

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