Add application-controlled security policies for kubeconfig exec plugins

[Copilot]

Untrusted kubeconfigs can launch arbitrary local processes through exec credential plugins. This adds application-controlled authorization without requiring callers to parse and inspect kubeconfig themselves.

• Policy API: Immutable AllowAll (default), DenyAll, and Allowlist policies across configuration builders, including Classic and AOT. Existing API signatures remain available.
• Enforcement: Checks initial execution and token refresh before process launch. Denials throw ExecCredentialPluginDeniedException; kubeconfig cannot override the application policy.
• Allowlist semantics: Match basenames or exact absolute paths, including Windows UNC paths. Empty lists deny all; invalid entries and quoted commands are rejected. Allowlist mode also rejects kubeconfig-supplied environment overrides to prevent command redirection and loader injection.
• Trust boundaries: Basenames permit any matching executable location. Arguments and executable contents are not restricted; the policy is not a sandbox.

var config = KubernetesClientConfiguration.BuildConfigFromConfigFile(
    kubeConfigPath,
    execCredentialPluginPolicy: ExecCredentialPluginPolicy.DenyAll);

// Or explicitly approve credential plugins:
var config = KubernetesClientConfiguration.BuildConfigFromConfigFile(
    kubeConfigPath,
    execCredentialPluginPolicy: ExecCredentialPluginPolicy.Allowlist(
        "kubelogin",
        @"C:\Program Files\Contoso\trusted-plugin.exe"));

• Fixes Add security policy support for kubeconfig exec credential plugins #1918

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[kubernetes-prow]

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[Copilot]

Copilot review overview

🟡 Changes recommended

Kubeconfig-controlled environment variables can bypass allowlisted command restrictions and still enable arbitrary code execution.

Review effort: Balanced
Findings: 1

Open (1)

• Allowlist ignores kubeconfig-controlled environment during execution · New

[Copilot]

Addressed in the latest commit: Allowlist mode now rejects all kubeconfig-supplied environment overrides before process creation and again at the launch boundary; AllowAll retains existing behavior. Tests cover PATH, LD_PRELOAD, DYLD_INSERT_LIBRARIES, and ordinary overrides during configuration loading, direct execution, and refresh. Documentation explains trusted application-inherited environment settings, basename resolution limitations, and that approved plugin arguments/executable behavior are not sandboxed.

[Copilot]

Copilot review overview

🔵 Needs a closer look

Security-sensitive process execution changes span multiple target variants, while the PR explicitly indicates build and security validation remain incomplete.

Review effort: Balanced
Findings: 1

Open (1)

• Allowlist ignores kubeconfig-controlled environment during execution