Capsule: CapsuleConfiguration NodeMetadata regex fields lack webhook validation, allowing MustCompile panic on all Node admission requests

### Summary `CapsuleConfiguration.Spec.NodeMetadata.ForbiddenLabels.Regex` and `ForbiddenAnnotations.Regex` are never validated by any admission webhook. A Cluster Admin can persist a malformed regex to etcd without being blocked. Once stored, every Node `CREATE`, `UPDATE`, or `PATCH` request triggers `regexp.MustCompile()` in `pkg/api/forbidden_list.go:36`, which **panics** and crashes the node admission webhook — causing a cluster-wide Denial of Service for all Node operations. ### Root cause `internal/webhook/tenant/validation/` contains dedicated regex validators for every Tenant regex field (hostname, storageclass, ingressclass, containerregistry, etc.). `internal/webhook/cfg/` contains **no regex validator at all** — only `owners.go`, `serviceaccount.go`, and `warnings.go`. The downstream consumer `internal/webhook/node/user_metadata.go` calls: ```go // line 131 matched = forbiddenLabels.RegexMatch(label) // line 150 matched = forbiddenAnnotations.RegexMatch(annotation) ``` Which routes to `pkg/api/forbidden_list.go:36`: ```go func (in ForbiddenListSpec) RegexMatch(value string) (ok bool) { if len(in.Regex) > 0 { ok = regexp.MustCompile(in.Regex).MatchString(value) // ← panics on invalid regex } return ok } ``` Unlike `regexp.Compile`, `regexp.MustCompile` panics instead of returning an error. Since no webhook validates the `CapsuleConfiguration` regex fields before storage, a malformed value reaches `MustCompile` on every Node admission request. ### Comparison with existing CVEs `GHSA-f94q-w3w8-cj67` and `GHSA-gxjc-74v5-3vx3` affect individual Tenant fields — their validators existed but checked the wrong field. This issue is different: **no validator exists at all** for `CapsuleConfiguration` regex fields, and the blast radius is cluster-wide (all Nodes), not scoped to one tenant. ### PoC ```go package main import ( "fmt" "regexp" ) type ForbiddenListSpec struct{ Regex string } // Exact copy of pkg/api/forbidden_list.go:34-38 func (in ForbiddenListSpec) RegexMatch(value string) bool { if len(in.Regex) > 0 { return regexp.MustCompile(in.Regex).MatchString(value) } return false } func main() { // 1. cfg webhook has no validator → invalid regex stored in etcd // (no webhook in internal/webhook/cfg/ checks regex fields) // 2. Stored malformed regex loaded from CapsuleConfiguration forbidden := ForbiddenListSpec{Regex: `[invalid-regex(`} // 3. node/user_metadata.go:131 called on every Node admission request defer func() { if r := recover(); r != nil { fmt.Printf("PANIC: %v\n", r) // Output: PANIC: regexp: Compile(`[invalid-regex(`): error parsing regexp: missing closing ] } }() forbidden.RegexMatch("kubernetes.io/hostname") } ``` Expected output: ``` PANIC: regexp: Compile(`[invalid-regex(`): error parsing regexp: missing closing ]: `[invalid-regex(` ``` ### Fix Add a `node_metadata_regex.go` handler to `internal/webhook/cfg/` following the same pattern as `forbidden_annotations_regex.go`: ```go package cfg import ( "context" "regexp" "sigs.k8s.io/controller-runtime/pkg/client" "sigs.k8s.io/controller-runtime/pkg/webhook/admission" capsulev1beta2 "github.com/projectcapsule/capsule/api/v1beta2" ad "github.com/projectcapsule/capsule/pkg/runtime/admission" "github.com/projectcapsule/capsule/pkg/runtime/events" "github.com/projectcapsule/capsule/pkg/runtime/handlers" ) type nodeMetadataRegexHandler struct{} func NodeMetadataRegexHandler() handlers.TypedHandler[*capsulev1beta2.CapsuleConfiguration] { return &nodeMetadataRegexHandler{} } func (h *nodeMetadataRegexHandler) OnCreate( _ client.Client, _ client.Reader, cfg *capsulev1beta2.CapsuleConfiguration, _ admission.Decoder, _ events.EventRecorder, ) handlers.Func { return func(_ context.Context, req admission.Request) *admission.Response { return h.validate(cfg, req) } } func (h *nodeMetadataRegexHandler) OnDelete( client.Client, client.Reader, *capsulev1beta2.CapsuleConfiguration, admission.Decoder, events.EventRecorder, ) handlers.Func { return func(context.Context, admission.Request) *admission.Response { return nil } } func (h *nodeMetadataRegexHandler) OnUpdate( _ client.Client, _ client.Reader, cfg *capsulev1beta2.CapsuleConfiguration, _ *capsulev1beta2.CapsuleConfiguration, _ admission.Decoder, _ events.EventRecorder, ) handlers.Func { return func(_ context.Context, req admission.Request) *admission.Response { return h.validate(cfg, req) } } func (h *nodeMetadataRegexHandler) validate(cfg *capsulev1beta2.CapsuleConfiguration, req admission.Request) *admission.Response { if cfg.Spec.NodeMetadata == nil { return nil } expressions := map[string]string{ "labels": cfg.Spec.NodeMetadata.ForbiddenLabels.Regex, "annotations": cfg.Spec.NodeMetadata.ForbiddenAnnotations.Regex, } for scope, expression := range expressions { if expression == "" { continue } if _, err := regexp.Compile(expression); err != nil { return ad.Denyf( "unable to compile regex %q for forbidden %s: %v", expression, scope, err, ) } } return nil } ``` ``` Step 2: Register the handler in cmd/controller/main.go: route.ConfigValidation( cfgvalidation.Handler(cfg, cfgvalidation.WarningHandler(), cfgvalidation.ServiceAccountHandler(), cfgvalidation.OwnerHandler(), cfgvalidation.NodeMetadataRegexHandler(), // ← ADD THIS LINE ), ), ``` ### Impact A Cluster Admin (or compromised admin account) can update CapsuleConfiguration with a malformed NodeMetadata regex (e.g., `[invalid-regex(`). The update is accepted without validation and persisted to etcd. Once stored, every subsequent Node admission request triggers `regexp.MustCompile()` with the invalid pattern, causing the Capsule node webhook to panic. **Affected operations (cluster-wide):** - Node labeling, annotations, and taints (`kubectl label/annotate/taint node`) - Cluster autoscaler operations (cannot register or remove nodes) - Cloud provider node lifecycle management (metadata sync, status updates) - Node maintenance workflows (cordon, drain, uncordon) **Severity:** This is a **cluster-wide Denial of Service** affecting all Node infrastructure operations. Unlike tenant-scoped CVEs (GHSA-f94q-w3w8-cj67, GHSA-gxjc-74v5-3vx3) that impact only Ingress or Namespace operations within a single tenant, this vulnerability blocks the entire cluster's ability to manage nodes. The cluster cannot scale, perform maintenance, or process any node metadata changes until a Cluster Admin manually corrects the CapsuleConfiguration—requiring direct kubectl access with valid YAML.

Source published

31 Jul 2026 16:53:25 UTC

Source modified

18 Aug 2026 15:10:36 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:N/I:N/A:H

Publisher rating · MODERATE