GHSA-8rvj-mm4h-c258 - cert-manager: Direct ACME Challenge resources can bypass Issuer DNS01 solver policy and use ClusterIssuer DNS credentials

## Summary `Challenge` resources under `acme.cert-manager.io` can be created directly by namespace users. On create, cert-manager performs no admission validation tying the Challenge to an `Order`, an owner reference, or an Issuer-selected solver. The ACME Challenge controller then reconciles the direct resource and uses `Challenge.spec.solver` as the DNS01 provider configuration. When the direct Challenge references a `ClusterIssuer`, cert-manager derives the DNS provider secret namespace and ambient-credential policy from the `ClusterIssuer` reference, but the DNS provider configuration itself is attacker-controlled through `Challenge.spec.solver`. This lets a namespace user bypass `Issuer`/`ClusterIssuer` solver selectors such as `dnsZones`, `dnsNames`, and `matchLabels`, and make the cert-manager controller use ClusterIssuer DNS credentials for attacker-selected provider settings and DNS names. In the acme-dns case, this becomes credential disclosure because the attacker-controlled solver host receives the `X-Api-User` and `X-Api-Key` headers loaded by cert-manager from the ClusterIssuer resource namespace. ## Attack preconditions - The attacker can create `challenges.acme.cert-manager.io` resources in at least one namespace. - The cert-manager controller is running and reconciles ACME `Challenge` resources cluster-wide. - A usable `ClusterIssuer` exists in the cluster. - The referenced `ClusterIssuer` has a DNS01 solver that uses a Secret in the cluster resource namespace, or relies on ambient credentials enabled for `ClusterIssuer`. - The cluster is multi-tenant or otherwise separates namespace users from platform-owned Issuer/ClusterIssuer and DNS credential administration. ## Severity Rationale: a tenant with namespace-level ACME Challenge create permissions, including permissions granted by cert-manager's aggregate edit role, can cause a cluster-scoped controller to use DNS credentials or ambient cloud credentials outside the Issuer's intended solver