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How CVE-2026-65834 Works
CVE-2026-65834 is a vulnerability in the Capsule multi-tenancy framework for Kubernetes. It stems from a missing validation step in the configuration admission webhook. Specifically, the `CapsuleConfiguration.Spec.NodeMetadata.ForbiddenLabels.Regex` and `ForbiddenAnnotations.Regex` fields, which define forbidden patterns for node labels and annotations, are never validated before being persisted to etcd.
While other Tenant-specific regex fields have dedicated validators, the `internal/webhook/cfg/` package lacks any regex validation for the `CapsuleConfiguration` resource. This allows a Cluster Admin to store a malformed regular expression (e.g., [invalid-regex() without any error.
The problem manifests during any Node admission request. The `internal/webhook/node/user_metadata.go` file calls the `RegexMatch` method on these forbidden lists. This method, defined in pkg/api/forbidden_list.go:36, uses `regexp.MustCompile()` to compile the stored regex. Unlike regexp.Compile(), `MustCompile()` will panic if the regex is invalid, crashing the node admission webhook.
This is not a tenant-scoped issue, unlike previous CVEs (GHSA-f94q-w3w8-cj67 and GHSA-gxjc-74v5-3vx3). The validator is entirely missing for CapsuleConfiguration, and a successful exploit causes a cluster-wide Denial of Service (DoS), affecting all Node operations. The fix, implemented in version 0.13.8, adds a new `node_metadata_regex.go` handler to validate these regex fields.
DailyCVE Form:
Platform: Capsule
Version: < 0.13.8
Vulnerability : Improper Input Validation
Severity: Medium (CVSS 6.8)
date: 2026-07-30
Prediction: 2026-08-15
What Undercode Say:
Check Capsule version
kubectl get deployment -n capsule-system capsule-controller-manager -o jsonpath='{.spec.template.spec.containers[bash].image}'
Simulate the panic (PoC)
cat <<EOF | go run -
package main
import ( "fmt" "regexp" )
type ForbiddenListSpec struct{ Regex string }
func (in ForbiddenListSpec) RegexMatch(value string) bool {
if len(in.Regex) > 0 {
return regexp.MustCompile(in.Regex).MatchString(value)
}
return false
}
func main() {
forbidden := ForbiddenListSpec{Regex: <code>[invalid-regex(</code>}
defer func() {
if r := recover(); r != nil {
fmt.Printf("PANIC: %v\n", r)
}
}()
forbidden.RegexMatch("kubernetes.io/hostname")
}
EOF
Output: PANIC: regexp: Compile(<code>[invalid-regex(</code>): error parsing regexp: missing closing ]
Exploit:
1. Prerequisite: Attacker has Cluster Admin privileges.
- Action: The attacker updates the `CapsuleConfiguration` resource, injecting a malformed regex into `spec.nodeMetadata.forbiddenLabels.regex` or
spec.nodeMetadata.forbiddenAnnotations.regex. - Outcome: The update is accepted and the invalid regex is stored in etcd.
- Trigger: Any subsequent Node
CREATE,UPDATE, or `PATCH` request (e.g.,kubectl label node) causes the node webhook to panic. - Result: Cluster-wide Denial of Service for all node management operations.
Protection:
- Immediate: Upgrade Capsule to version v0.13.8 or later.
- Mitigation: Restrict write access to `CapsuleConfiguration` to only highly trusted administrators.
- Detection: Monitor Capsule webhook logs for unexpected panics or `regexp: Compile` errors.
Impact:
- Scope: Cluster-wide Denial of Service.
- Affected Operations: All Node-related operations including labeling, tainting, cluster autoscaling, cloud provider sync, and node maintenance (cordon/drain).
- Remediation Effort: Requires a Cluster Admin with direct `kubectl` access to manually correct the
CapsuleConfiguration.
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Sources:
Reported By: github.com
Extra Source Hub:
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