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Hacking Kubernetes

Running cloud native workloads on Kubernetes can be challenging: keeping them secure is even more so. Kubernetes’ complexity offers malicious in-house users and external attackers alike a large assortment of attack vectors. In this book, Andrew Martin and Michael Hausenblas review Kubernetes defaults and threat models and shows how to protect against attacks.

The book is published and available via O’Reilly or Amazon.

Coverage

• Chapter 1: we set the scene, introducing our main antagonist and also what threat modelling is.
• Chapter 2: where we focuses on pods, from configurations to attacks to defenses.
• Chapter 3: we switch gears and dive deep into sandboxing and isolation techniques (KVM, gVisor, Firecracker, Kata).
• Chapter 4: covers supply chain attacks and what you can do to detect and mitigate them.
• Chapter 5: where we review networking defaults and how to secure your cluster and workload traffic incl. service meshes and eBPF.
• Chapter 6: we shift our focus on the persistency aspects, looking at filesystems, volumes, and sensitive information at rest.
• Chapter 7: covers the topic of running workloads for multi-tenants in a cluster and what can go wrong with this.
• Chapter 8: we review different kinds of policies in use, discuss access control—specifically RBAC—and generic policy solutions such as OPA.
• Chapter 9: we cover the question what you can do if, despite controls put in place, someone manages to break (intrusion detection system, etc.).
• Chapter 10: a somewhat special one, in that it doesn’t focus on tooling but on the human aspects, in the context of public cloud as well as on-prem environments.

About the authors

Based on our combined 10+ years of hands-on experience designing, running, attacking, and defending Kubernetes-based workloads and clusters, we want to equip you, the cloud native security practitioner, with what you need to be successful in your job.

We both have served in different companies and roles, gave training sessions, and published material from tooling to blog posts as well as have shared lessons learned on the topic in various public speaking engagements. Much of what motivates us here and the examples we use are rooted in experiences we made in our day-to-day jobs and/or saw at customers.

Notable CVEs

Unless noted, these CVEs are patched, and are here to serve only as a historical reference. See also @rasene’s HackMD.

• CVE-2017-1002101 - Subpath volume mount mishander. Containers using subpath volume mounts with any volume type (including nonprivileged pods subject to file permissions) can access files/directories outside of the volume including the host’s filesystem.

• CVE-2017-1002102 - Downward API host filesystem delete. Containers using a Secret, ConfigMap, projected or downwardAPI volume can trigger deletion of arbitrary files/directories from the nodes where they are running.

• CVE-2017-5638 - (Non-Kubernetes) Apache Struts invalid Content-Type header parsing failure, allowing arbitrary code execution. The bug in the Jakarta Multipart parser registered the input as OGNL code, converted it to an executable, and moved it to the server’s temporary directory.

• CVE-2018-1002105 - API server websocket TLS tunnel error mishandling. Incorrect error response handling of proxied upgrade requests in the kube-apiserver allowed specially crafted requests to establish a connection through the Kubernetes API server to backend servers. Subsequent arbitrary requests over the same connection transit directly to the backend authenticated with the Kubernetes API server’s TLS credentials.

• CVE-2019-16884 - runc hostile image AppArmor bypass. Allows AppArmor restriction bypass because libcontainer/rootfs_linux.go incorrectly checks mount targets, and thus a malicious Docker image can mount over a /proc directory.

• CVE-2019-5736 - runc /proc/self/exe. runc allows attackers to overwrite the host runc binary (and consequently obtain host root access) by leveraging the ability to execute a command as root within one of these types of containers: (1) a new container with an attacker-controlled image, or (2) an existing container, to which the attacker previously had write access, that can be attached with docker exec. This occurs because of file-descriptor mishandling, related to /proc/self/exe.

• CVE-2019-11249 - kubectl cp scp reverse write. To copy files from a container Kubernetes runs tar inside the container to create a Tar archive, and copies it over the network where kubectl unpacks it on the user’s machine. If the tar binary in the container is malicious, it could run any code and output unexpected malicious results. An attacker could use this to write files to any path on the user’s machine when kubectl cp is called, limited only by the system permissions of the local user.

• CVE-2018-18264 - Kubernetes Dashboard before v1.10.1 allows attackers to bypass authentication and use Dashboard’s ServiceAccount for reading Secrets within the cluster.

• CVE-2019-1002100 - API Server JSON patch Denial of Service. Users that are authorized to make HTTP PATCH requests to the Kubernetes API Server can send a specially crafted patch of type ``json-patch’’ (e.g., kubectl patch --type json or "Content-Type: application/json-patch+json") that consumes excessive resources while processing.

• CVE-2018-1002100 - Original kubectl cp. The kubectl cp command insecurely handles tar data returned from the container and can be caused to overwrite arbitrary local files.

• CVE-2019-1002101 - Similar to CVE-2019-11249, but extended in that the untar function can both create and follow symbolic links.

• CVE-2019-11245 - mustRunAsNonRoot: true bypass. Containers for pods that do not specify an explicit runAsUser attempt to run as uid 0 (root) on container restart, or if the image was previously pulled to the node

• CVE-2019-11247 - Cluster RBAC mishandler. The Kubernetes kube-apiserver mistakenly allows access to a cluster-scoped custom resource if the request is made as if the resource were namespaced. Authorizations for the resource accessed in this manner are enforced using roles and role bindings within the namespace meaning that a user with access only to a resource in one namespace could create, view, update, or delete the cluster-scoped resource (according to their namespace role privileges).

• CVE-2019-11248 - kubelet /debug/pprof information disclosure and denial of service. The debugging endpoint /debug/pprof is exposed over the unauthenticated kubelet healthz healthcheck endpoint port, which can potentially leak sensitive information such as internal Kubelet memory addresses and configuration or for limited denial of service.

• CVE-2019-11250 - Side channel information disclosure. The Kubernetes client-go library logs request headers at verbosity levels of 7 or higher. This can disclose credentials to unauthorized users via logs or command output. Kubernetes components (such as kube-apiserver) which make use of basic or bearer token authentication and run at high verbosity levels are affected.

• CVE-2020-8558 - kube-proxy unexpectedly makes localhost-bound host services available on the network.

• CVE-2020-14386 - Integer overflow from raw packet on the ``loopback’’ (or localhost) network interface. Removing this with sysctl -w kernel.unprivileged_userns_clone=0 or denying CAP_NET_RAW protects unpatched kernels from exploitation.

• CVE-2021-22555 - Linux Netfilter local privilege escalation flaw. When processing setsockopt IPT_SO_SET_REPLACE (or IP6T_SO_SET_REPLACE) a local user may exploit memory corruption to gain privileges or cause a DoS via a user namespace. A kernel compiled with CONFIG_USER_NS and CONFIG_NET_NS allows an unprivileged user to elevate privileges.

• CVE-2021-25740 (unpatched) - Endpoint and EndpointSlice permissions allow cross-Namespace forwarding. users to send network traffic to locations they would otherwise not have access to via a confused deputy attack.

• CVE-2021-31440 - Incorrect bounds calculation in the Linux kernel eBPF verifier. By bypassing the verifier, this can exploit out-of-bounds kernel access to escape, and the original proof of concept set UID and GID to 0 and gained CAP_SYS_MODULE to load an arbitrary kernel outside the container.

• CVE-2021-25741 - Symlink exchange can allow host filesystem access. A user may be able to create a container with subpath volume mounts to access files and directories outside of the volume, including on the host filesystem.