Hello Kubernetes Community,
A security issue was discovered in kube-apiserver that allows an aggregated API server to redirect client traffic to any URL. This could lead to the client performing unexpected actions as well as forwarding the client's API server credentials to third parties.
All Kubernetes clusters with the following versions that are running aggregated API servers are impacted. To identify if you have aggregated API servers configured, run the following command:
- kube-apiserver v1.25.0
- kube-apiserver v1.24.0 - v1.24.4
- kube-apiserver v1.23.0 - v1.23.10
- kube-apiserver v1.22.0 - v1.22.13
- kube-apiserver <= v1.21.14
How do I mitigate this vulnerability?
Aside from upgrading, no direct mitigation is available.
Aggregated API servers are a trusted part of the Kubernetes control plane, and configuring them is a privileged administrative operation. Ensure that only trusted cluster administrators are allowed to create or modify
APIService configuration, and follow security best practices with any aggregated API servers that may be in use.
- kube-apiserver v1.25.1
- kube-apiserver v1.24.5
- kube-apiserver v1.23.11
- kube-apiserver v1.22.14
Fix impact: The fix blocks all 3XX responses from aggregated API servers by default. This may disrupt an aggregated API server that relies on redirects as part of its normal function. If all current and future aggregated API servers are considered trustworthy and redirect functionality is required, set the
--aggregator-reject-forwarding-redirect Kubernetes API server flag to
false to restore the previous behavior.
Kubernetes audit log events indicate the HTTP status code sent to the client via the
responseStatus.code field. This can be used to detect if an aggregated API server is redirecting clients.
This vulnerability was reported by Nicolas Joly & Weinong Wang @weinong from Microsoft.
The issue was fixed and coordinated by Di Jin @jindijamie @enj @liggitt @lavalamp @deads2k and @puerco.
Mo Khan on behalf of the Kubernetes Security Response Committee