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Assuming “security webhook” means a Kubernetes admission webhook, fail-open means the API server can continue a request when it cannot call the webhook. It does not mean Kubernetes ignores a denial the webhook successfully returns. Kubernetes recommends this approach for some mutating webhooks to keep compliant resources deployable during webhook outages, with validation of the final object state as a backstop. That general guidance does not establish why any unnamed organization chose to ship its own webhook fail-open.
What does fail-open mean for a Kubernetes webhook?
Kubernetes admission webhooks use the failurePolicy setting to decide what happens when the API server cannot obtain a usable response from a webhook. With failurePolicy: Ignore, the API request may continue despite that call error. With failurePolicy: Fail, the request is rejected. The default is Fail. See the Kubernetes failure-policy documentation.
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This setting governs failures to call the webhook, including network failures, timeouts, non-2xx responses, or malformed responses. It does not override a successful policy decision: if the webhook responds and explicitly denies the request, that denial still applies.
Why allow a security webhook to fail open?
A webhook can become unavailable because of a service outage, network problem, or deployment issue. If every call failure blocks admission, even resources that would otherwise comply with policy can be rejected until the webhook recovers. Kubernetes recommends that mutating webhooks can fail open where appropriate, while a validating admission controller checks the final object state. This can preserve deployment availability without treating an unrun mutation as proof that the resource is safe. See Kubernetes Admission Webhook Good Practices and the Kubernetes admission controller documentation.
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The rationale is conditional, not universal: the validating control must actually enforce the security property that the skipped mutation or check would have protected. A fail-open configuration without an effective downstream check can leave a gap during an outage.
Fail-open versus fail-closed
| Question | Fail-open (Ignore) |
Fail-closed (Fail) |
|---|---|---|
| Can an API operation proceed while the webhook call fails? | Yes; the call error is ignored for admission. | No; the request is rejected. |
| What happens to the webhook’s mutation or check? | It may not run, so another control must protect the required final state. | The request is blocked rather than admitted without the webhook’s response. |
| Does a successful explicit denial get ignored? | No. A denial returned by the webhook is still a denial. | No. A denial returned by the webhook is still a denial. |
| What is the principal outage risk? | Policy enforcement may be skipped unless a backstop catches the resulting state. | Compliant requests may be blocked while the webhook is unavailable. |
How timeout settings affect the choice
Kubernetes documents a default admission webhook timeout of 10 seconds and allows configuration from 1 to 30 seconds. When the webhook call times out, the configured failure policy determines whether admission continues or the request is rejected. Because webhooks add latency to API requests, Kubernetes advises that they evaluate quickly. Check the documentation for the Kubernetes version you run before relying on these settings: Webhook timeouts.
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What to verify before shipping fail-open
- Define the outage case. Identify which mutations or checks will not execute when the API server cannot get a usable webhook response.
- Verify the backstop. Confirm that validation checks the final object state and enforces the security properties that matter, rather than assuming the mutation ran.
- Make failures visible. Monitor webhook errors and admission outcomes so fail-open does not turn an outage into a silent policy bypass.
- Limit the scope. Invoke the webhook only for resources that need it. Kubernetes cautions against broad scope and dependency loops that can interfere with the components needed to recover the webhook. See Limit the scope of your webhook.
- Plan recovery. Decide how to identify and reconcile resources admitted while the webhook was unavailable, if missed processing could matter.
Do webhook retries work the same everywhere?
No. “Webhook” describes several systems, and delivery and acknowledgement rules depend on the provider or specification. For example, GitHub recommends that a receiver return a 2XX response within 10 seconds and suggests queueing longer work for asynchronous processing. GitHub says failed deliveries are not automatically redelivered, so recovery may require manual or scheduled action: GitHub webhook best practices and handling failed deliveries. The Standard Webhooks specification recommends retries over multiple days with exponential backoff and jitter. Those examples do not describe Kubernetes admission behavior or create a universal retry guarantee.
Can this explain why a particular organization ships its webhook fail-open?
Not by itself. Kubernetes guidance supports a general availability rationale for appropriately designed mutating admission webhooks, but it cannot verify an unnamed organization’s implementation, threat model, compensating controls, or actual reason for its choice. A claim about that specific design requires confirmation from the responsible engineering or security team.
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