Pod Security Standards
The Pod Security Standards define three different policies to broadly cover the security spectrum. These policies are cumulative and range from highly-permissive to highly-restrictive. This guide outlines the requirements of each policy.
| Profile | Description |
|---|---|
| Privileged | Unrestricted policy, providing the widest possible level of permissions. This policy allows for known privilege escalations. |
| Baseline | Minimally restrictive policy which prevents known privilege escalations. Allows the default (minimally specified) Pod configuration. |
| Restricted | Heavily restricted policy, following current Pod hardening best practices. |
Profile Details
Privileged
The Privileged policy is purposely-open, and entirely unrestricted. This type of policy is typically aimed at system- and infrastructure-level workloads managed by privileged, trusted users.
The Privileged policy is defined by an absence of restrictions. Allow-by-default mechanisms (such as gatekeeper) may be Privileged by default. In contrast, for a deny-by-default mechanism (such as Pod Security Policy) the Privileged policy should disable all restrictions.
Baseline
The Baseline policy is aimed at ease of adoption for common containerized workloads while preventing known privilege escalations. This policy is targeted at application operators and developers of non-critical applications. The following listed controls should be enforced/disallowed:
*) indicate all elements in a list. For example,
spec.containers[*].securityContext refers to the Security Context object for all defined
containers. If any of the listed containers fails to meet the requirements, the entire pod will
fail validation.
| Control | Policy |
|---|---|
| HostProcess |
Windows pods offer the ability to run HostProcess containers which enables privileged access to the Windows node. Privileged access to the host is disallowed in the baseline policy.
FEATURE STATE:
Kubernetes v1.23 [beta]
Restricted Fields
Allowed Values
|
| Host Namespaces |
Sharing the host namespaces must be disallowed. Restricted Fields
Allowed Values
|
| Privileged Containers |
Privileged Pods disable most security mechanisms and must be disallowed. Restricted Fields
Allowed Values
|
| Capabilities |
Adding additional capabilities beyond those listed below must be disallowed. Restricted Fields
Allowed Values
|
| HostPath Volumes |
HostPath volumes must be forbidden. Restricted Fields
Allowed Values
|
| Host Ports |
HostPorts should be disallowed, or at minimum restricted to a known list. Restricted Fields
Allowed Values
|
| AppArmor |
On supported hosts, the Restricted Fields
Allowed Values
|
| SELinux |
Setting the SELinux type is restricted, and setting a custom SELinux user or role option is forbidden. Restricted Fields
Allowed Values
Restricted Fields
Allowed Values
|
/proc Mount Type |
The default Restricted Fields
Allowed Values
|
| Seccomp |
Seccomp profile must not be explicitly set to Restricted Fields
Allowed Values
|
| Sysctls |
Sysctls can disable security mechanisms or affect all containers on a host, and should be disallowed except for an allowed "safe" subset. A sysctl is considered safe if it is namespaced in the container or the Pod, and it is isolated from other Pods or processes on the same Node. Restricted Fields
Allowed Values
|
Restricted
The Restricted policy is aimed at enforcing current Pod hardening best practices, at the expense of some compatibility. It is targeted at operators and developers of security-critical applications, as well as lower-trust users. The following listed controls should be enforced/disallowed:
*) indicate all elements in a list. For example,
spec.containers[*].securityContext refers to the Security Context object for all defined
containers. If any of the listed containers fails to meet the requirements, the entire pod will
fail validation.
| Control | Policy |
| Everything from the baseline profile. | |
| Volume Types |
The restricted policy only permits the following volume types. Restricted Fields
Allowed Values Every item in thespec.volumes[*] list must set one of the following fields to a non-null value:
|
| Privilege Escalation (v1.8+) |
Privilege escalation (such as via set-user-ID or set-group-ID file mode) should not be allowed. This is Linux only policy in v1.25+ Restricted Fields
Allowed Values
|
| Running as Non-root |
Containers must be required to run as non-root users. Restricted Fields
Allowed Values
nil if the pod-level
spec.securityContext.runAsNonRoot is set to true.
|
| Running as Non-root user (v1.23+) |
Containers must not set runAsUser to 0 Restricted Fields
Allowed Values
|
| Seccomp (v1.19+) |
Seccomp profile must be explicitly set to one of the allowed values. Both the Restricted Fields
Allowed Values
nil if the pod-level
spec.securityContext.seccompProfile.type field is set appropriately.
Conversely, the pod-level field may be undefined/nil if _all_ container-
level fields are set.
|
| Capabilities (v1.22+) |
Containers must drop Restricted Fields
Allowed Values
Restricted Fields
Allowed Values
|
Policy Instantiation
Decoupling policy definition from policy instantiation allows for a common understanding and consistent language of policies across clusters, independent of the underlying enforcement mechanism.
As mechanisms mature, they will be defined below on a per-policy basis. The methods of enforcement of individual policies are not defined here.
Pod Security Admission Controller
Alternatives
Other alternatives for enforcing policies are being developed in the Kubernetes ecosystem, such as:
Pod OS field
Kubernetes lets you use nodes that run either Linux or Windows. You can mix both kinds of
node in one cluster.
Windows in Kubernetes has some limitations and differentiators from Linux-based
workloads. Specifically, many of the Pod securityContext fields
have no effect on Windows.
Restricted Pod Security Standard changes
Another important change, made in Kubernetes v1.25 is that the restricted Pod security
has been updated to use the pod.spec.os.name field. Based on the OS name, certain policies that are specific
to a particular OS can be relaxed for the other OS.
OS-specific policy controls
Restrictions on the following controls are only required if .spec.os.name is not windows:
- Privilege Escalation
- Seccomp
- Linux Capabilities
FAQ
Why isn't there a profile between privileged and baseline?
The three profiles defined here have a clear linear progression from most secure (restricted) to least secure (privileged), and cover a broad set of workloads. Privileges required above the baseline policy are typically very application specific, so we do not offer a standard profile in this niche. This is not to say that the privileged profile should always be used in this case, but that policies in this space need to be defined on a case-by-case basis.
SIG Auth may reconsider this position in the future, should a clear need for other profiles arise.
What's the difference between a security profile and a security context?
Security Contexts configure Pods and Containers at runtime. Security contexts are defined as part of the Pod and container specifications in the Pod manifest, and represent parameters to the container runtime.
Security profiles are control plane mechanisms to enforce specific settings in the Security Context, as well as other related parameters outside the Security Context. As of July 2021, Pod Security Policies are deprecated in favor of the built-in Pod Security Admission Controller.
What about sandboxed Pods?
There is not currently an API standard that controls whether a Pod is considered sandboxed or not. Sandbox Pods may be identified by the use of a sandboxed runtime (such as gVisor or Kata Containers), but there is no standard definition of what a sandboxed runtime is.
The protections necessary for sandboxed workloads can differ from others. For example, the need to restrict privileged permissions is lessened when the workload is isolated from the underlying kernel. This allows for workloads requiring heightened permissions to still be isolated.
Additionally, the protection of sandboxed workloads is highly dependent on the method of sandboxing. As such, no single recommended profile is recommended for all sandboxed workloads.
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