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Do not generally permit containers to be run with the hostPID flag set to true.
A container running in the host's process ID (PID) namespace can inspect processes running outside the container. If the container also has access to ptrace capabilities, this can be used to escalate privileges outside of the container. There should be at least one admission control policy defined which does not permit containers to share the host PID namespace. If you need to run containers which require hostPID, this should be defined in a separate policy and you should carefully check to ensure that only limited service accounts and users are given permission to use that policy.
Note
Note
By default, there are no restrictions on the creation of hostPID containers.

Impact

Pods defined with spec.hostPID: true will not be permitted unless they are run under a specific policy.

Audit

List the policies in use for each namespace in the cluster, ensuring that each policy disallows the admission of hostPID containers.
Search for the hostPID flag. In the YAML output, look for the hostPID setting under the spec section to check if it is set to true.
kubectl get pods --all-namespaces -o json | jq -r '.items[] | 
select(.spec.hostPID == true) | 
"\(.metadata.namespace)/\(.metadata.name)"'
OR
kubectl get pods --all-namespaces -o json | jq -r '.items[] | 
select(.spec.hostPID == true) | select(.metadata.namespace != 
"kube-system" and .metadata.namespace != "gatekeeper-system" and 
.metadata.namespace != "azure-arc" and .metadata.namespace != 
"azure-extensions-usage-system") | "\(.metadata.name) 
\(.metadata.namespace)"'
When creating a Pod Security Policy, ["kube-system", "gatekeeper-system", "azure-arc", "azure-extensions-usage-system"] namespaces are excluded by default.
The command retrieves all pods across all namespaces in JSON format, then uses jq to filter out those with the hostPID set to true and finally formats the output to show the namespace and name of each matching pod.

Remediation

Add policies to each namespace in the cluster that has user workloads to restrict the admission of hostPID containers.
Pod Security Policies and Assignments can be found by searching for Policies in the Azure Portal. A detailed step-by-step guide can be found here: https://learn.microsoft.com/en-us/azure/governance/policy/concepts/policy-for-kubernetes