Earlier this week a major vulnerability in Kubernetes was made public by its maintainers. It was originally caught as a bug by Darren Shepherd and was later marked as a critical vulnerability and assigned CVE-2018-1002105. Its implications were clearly laid out in its Github issue page by Kubernetes developer Jordan Liggitt. The bug was fixed and new versions were tagged for all supported Kubernetes releases.
Many technology news sites published articles with warnings, and cloud providers followed with their own updates and mitigations (Google, Azure, AWS). At Twistlock, our CTO John Morello authored an excellent post with all the relevant details and Twistlock platform mitigations.
Since then I’ve seen multiple tools and scripts released and many commercial companies addressed this vulnerability in dedicated updates. The level of visibility this vulnerability attracted was much greater than with what I’ve seen with any similar vulnerability. In all fairness though, it is not common to find critical security bugs in Go code. This provided me with enough motivation to dig in further and produce a working exploit that would prove that the vulnerability really is as critical as suggested.
Let’s break down the current Github issue description:
The source of the bug is the Kubernetes API server, aka kube-apiserver. For those unfamiliar with it, the API server provides the REST API endpoint through which Kubernetes operations are made. It is most commonly used through
Access to the API server is secured with strong authentication and authorization mechanisms. Both mechanisms are vital to any Kubernetes cluster configuration, and authentication is also enforced internally between system pods. In essence, Authentication is used for securely identifying and trusting users (or vice versa, for trusting the API server), usually with TLS certificates, while the Authorization strategy determines who can do what. RBAC is commonly used for that purpose.
The bug allows an attacker who can send a legitimate, authorized request to the API server to bypass the authorization logic in any sequenced request. In other words, escalate privileges to that of any user. That’s pretty bad, let’s figure out how that happened.
By looking at the fixing pull request we learn that the faulting code was in the
The handler only serves as a proxy however. It passes all valid requests to an API aggregation layer which routes each request to its appropriate underlying server. This aggregation layer allows adding extensions to the Kubernetes API endpoint. The actual API servers run as servers in system pods and the requests from the aggregation layer are routed to them.
Now to the logical flaw in the handler code. When an HTTP request with HTTP Upgrade headers was sent, given it was an authorized request to the requested object, it would be passed to the aggregation server by the handler. Even if the returned response would not contain HTTP return code 101 (Switching Protocols), the response would be sent to the client and the proxy connection to the internal server would still be established, just as it would have if the connection really was upgraded to use WebSockets.
The problem is that the proxy connection does not enforce the authorization scheme on any of the subsequent requests. It uses its TLS certificate to authenticate against the internal server, and forwards all requests as they come.
An attacker could send an authorized request to the API server with HTTP Upgrade headers and then continue sending any HTTP requests on the same TCP socket, bypassing authorization control.
The fix involved checking the response code from the response of the internal server, and prevent the creation of a proxy connection if no protocol upgrade actually took place.
Possible attack vectors
To successfully establish an attack the attacker must be authorized to perform some request to the API server. Unfortunately, the default Kubernetes configuration actually helps the attacker in this scenario.
By default, the RBAC configuration allows any user to perform discovery calls against any aggregated API server. These discovery calls allow any user or pod with access to the API to check for the availability of aggregated API servers. It is also used internally by the node kubelet to perform health checks of the API server.
Anonymous requests are likewise enabled by default, allowing anyone with access to the API endpoint to send valid unauthenticated requests. Specifically, discovery requests can be performed anonymously with no user identification. So with the default configuration, an attacker with access to the API server, from outside or from inside a pod, could use this vulnerability to escalate privileges to any user in the scope of an aggregated API server, without any additional prerequisites.
In order for a request to pass through the vulnerable logic, it needs to either be targeted at an aggregated API server, or be one of
One of the questions I was immediately concerned with is if there are any aggregated API servers installed by default on Kubernetes clusters. Turns out that since Kubernetes 1.8, metrics-server is deployed by default when the kube-up.sh script is used. This Metrics Server API facilitates the
The metrics server signs up its aggregated API under
Now that we have a better understanding of the vulnerability, let’s see an exploit in action. I wrote a super simple Ruby script that exploits the vulnerability against the metrics server, and leaks information on pods from all namespaces in the cluster. It can be executed from any pod and should work assuming
In the exploit code, you may notice the second request includes the
One tricky part of the exploitation was finding a user that is always configured to have access to the metrics server. There was no such user, but after digging a little in the documentation, I found that by using the format
I looked at my ClusterRoles configuration and decided to use
Proof of concept
In the following demo, I ran the PoC code inside a fresh nginx pod, and successfully got back a JSON list including all the pods in my cluster.
Liggitt did a great job in the issue explaining how to easily determine if you configuration was affected.
In the issue page he also lists potential mitigations for every scenario, but none can really be applied without breaking anything else in the cluster. My recommendation is to update the cluster to one of the fixed Kubernetes releases.
To Twistlock customers, I would also like to add that besides identifying the CVE in the vulnerability monitor, and having a compliance check to disable anonymous authorization (both mentioned in John’s post), the Twistlock platform should also detect traffic between pods that shouldn’t talk to each other. So in case a pod access the API server and is not expected to, Twistlock would alert on this event in the radar.
I hope this post cleared some of the fog around this vulnerability. Feel free to reach me out with any questions you may have through email or at @TwistlockLabs. Happy holidays!