CVE-2026-61794 in Capsule
Summary
by MITRE • 09/18/2026
Capsule is a multi-tenancy and policy-based framework for Kubernetes. From 0.13.0 until 0.13.7, the Tenant update validation in internal/webhook/tenant/validation/forbidden_annotations_regex.go compiles ForbiddenLabels.Regex for both the labels and annotations checks instead of validating ForbiddenAnnotations.Regex. An administrator who can update a Tenant can therefore persist a malformed ForbiddenAnnotations.Regex while leaving the labels expression valid. Namespace creation or update later passes the stored expression through pkg/api/forbidden_list.go, where regexp.MustCompile can panic during forbidden metadata validation and deny namespace operations for the affected tenant. This issue is fixed in version 0.13.7.
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Analysis
by VulDB Data Team • 09/18/2026
The vulnerability identified within the Capsule Kubernetes framework stems from a logic error in the Tenant update validation mechanism present in versions ranging from 0.13.0 to 0.13.6, with resolution occurring in version 0.13.7. Capsule serves as a multi-tenancy and policy-based management layer for Kubernetes clusters, allowing administrators to define strict policies regarding resource usage and access control. The core of this flaw resides in the internal webhook validation logic located at internal/webhook/tenant/validation/forbidden_annotations_regex.go. When an administrator attempts to update a Tenant configuration, the system is designed to validate two distinct sets of regular expressions: ForbiddenLabels.Regex for label restrictions and ForbiddenAnnotations.Regex for annotation restrictions. However, due to a coding oversight, the validation process incorrectly compiles the ForbiddenLabels.Regex pattern for both checks. This means that while the labels expression undergoes proper syntax verification, the annotations expression is validated against an entirely different regular expression pattern intended for labels.
This discrepancy creates a specific attack vector where an administrator possessing permissions to update Tenant configurations can persistently store a malformed or syntactically invalid ForbiddenAnnotations.Regex value. Because the validation step incorrectly applies the label regex rules to the annotation field, it fails to detect errors that would be caught if the correct annotation-specific regex were used. Consequently, the system accepts and saves this erroneous configuration without raising an error during the update operation. The labels expression remains valid under its own stricter or different constraints, masking the fact that the annotations expression is fundamentally broken according to standard regular expression syntax rules expected by the Go runtime environment.
The operational impact of this vulnerability manifests when subsequent namespace creation or update operations are attempted for the affected tenant. During these operations, the system invokes pkg/api/forbidden_list.go to perform forbidden metadata validation against the stored policies. This function attempts to compile and execute the ForbiddenAnnotations.Regex using regexp.MustCompile. Since the previously saved expression was malformed according to standard regex syntax but passed the incorrect label-based validation check, the compilation process fails catastrophically. In Go, a failure in regexp.MustCompile triggers a panic rather than returning an error value. This unhandled panic causes the control plane components handling these requests to crash or become unavailable for that specific tenant context.
The primary consequence of this denial-of-service condition is the disruption of namespace operations for the targeted tenant. Administrators and automated systems attempting to create, update, or manage namespaces associated with the compromised Tenant configuration will encounter failures due to the underlying application panic. This effectively renders the multi-tenancy controls unusable for that specific scope, potentially blocking critical deployment workflows and disrupting service availability within the Kubernetes cluster. The vulnerability highlights a significant reliability issue in policy enforcement mechanisms where input validation errors lead directly to system instability rather than graceful error handling or rejection of invalid inputs.
From a security architecture perspective, this flaw aligns with CWE-20 Improper Input Validation, as the application fails to adequately validate user-supplied data before processing it. Furthermore, because the vulnerability allows an authenticated administrator to trigger a denial-of-service condition through configuration manipulation, it relates to CWE-754: Improper Check for Unusual or Exceptional Conditions in software that leads to crashes. In terms of adversarial tactics, this scenario reflects aspects of ATT&CK technique T1098: Account Manipulation if the attacker uses legitimate administrative credentials to degrade service availability, although strictly speaking it is a configuration-induced stability issue rather than malicious account modification.
To mitigate this vulnerability and prevent recurrence, organizations running Capsule must ensure they are operating on version 0.13.7 or later where the validation logic has been corrected to properly distinguish between label and annotation regex patterns. For environments that cannot immediately upgrade, administrators should exercise extreme caution when configuring Tenant policies, ensuring that any regular expressions defined for forbidden annotations adhere strictly to valid syntax standards. Implementing additional monitoring around webhook admission controller logs can help detect panic events early, allowing for rapid remediation before widespread service disruption occurs. Regular auditing of configuration drift and automated testing of policy definitions against known regex patterns are recommended practices to maintain the integrity of multi-tenancy controls in Kubernetes environments.