CVE-2026-61672 in Capsule
Summary
by MITRE • 09/18/2026
Capsule is a multi-tenancy and policy-based framework for Kubernetes. Prior to 0.13.7, ForbiddenListSpec.ExactMatch in pkg/api/forbidden_list.go sorts denied metadata keys case-insensitively and then uses sort.SearchStrings, which assumes byte-order sorting. When an administrator's forbidden list mixes capitalized and lowercase keys or otherwise has different case-insensitive and byte ordering, the binary search can return false for a key that is present. An authenticated tenant owner can then pass the missed key through api.ValidateForbidden and bypass configured namespace, Service, or delegated node metadata restrictions, potentially influencing cluster policies, network exposure, or scheduling outside the tenant boundary. Uniformly lowercase lists whose two orderings coincide are not affected. This issue is fixed in version 0.13.7.
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Analysis
by VulDB Data Team • 09/18/2026
The vulnerability identified in Capsule versions prior to 0.13.7 represents a critical logic error within its multi-tenancy and policy enforcement framework, specifically affecting the mechanism used to validate forbidden metadata keys on Kubernetes resources. Capsile serves as an operator that enables fine-grained access control and resource isolation for tenants within shared clusters by enforcing policies through admission webhooks and validation routines. The core of this flaw resides in the implementation of ForbiddenListSpec.ExactMatch found in pkg/api/forbidden_list.go, which is responsible for determining whether a specific metadata key provided by a user matches an entry in the administrator-defined forbidden list. This function relies on Go standard library functions to perform lookups, but it does so using inconsistent sorting methodologies that create a discrepancy between how keys are stored and how they are searched.
The technical root cause of this vulnerability is a mismatch between case-insensitive comparison logic and byte-order binary search assumptions. When an administrator configures the forbidden list with metadata keys that contain mixed casing or where the alphabetical order differs from the lexicographical byte order, the system first sorts these denied keys using a case-insensitive comparator to establish their position in the data structure. However, subsequent lookups are performed using sort.SearchStrings, which operates on raw bytes and assumes standard ASCII byte-order sorting rather than locale-aware or case-insensitive ordering. In many character sets, particularly those involving uppercase letters with different code points than lowercase ones, these two orderings do not align perfectly. Consequently, the binary search algorithm may fail to locate a key that is technically present in the forbidden list because it looks for the item at an index determined by byte-order logic rather than the case-insensitive position where the item was actually stored during initialization.
This implementation flaw leads directly to a security bypass with significant operational impact on cluster integrity and tenant isolation. An authenticated user who holds ownership of a namespace or has permissions to create resources within that namespace can exploit this discrepancy by crafting requests that include metadata keys which are present in the forbidden list but fall into the gap between case-insensitive and byte-order sorting sequences. By passing such a key through api.ValidateForbidden, the validation routine incorrectly returns true for allowed status, effectively bypassing the configured restrictions on namespace labels, Service specifications, or delegated node metadata assignments. This allows tenant owners to inject unauthorized metadata that could influence cluster-wide policies, expose services beyond intended network boundaries, or affect pod scheduling decisions in ways that violate the multi-tenancy model established by the administrator.
The scope of this vulnerability is limited to configurations where the forbidden list contains keys with mixed casing or characters whose case-insensitive sort order diverges from their byte-order sort order. Administrators who enforce uniformly lowercase lists are not affected, as both sorting methods yield identical results in that specific scenario. However, given the flexibility required for real-world Kubernetes environments where metadata conventions vary widely, this edge case poses a tangible risk to security posture. The vulnerability essentially undermines the principle of least privilege by allowing tenants to circumvent administrative controls designed to prevent them from setting certain labels or annotations that could be used for malicious purposes such as lateral movement, policy evasion, or resource exhaustion attacks targeting specific nodes or services.
To mitigate this issue and restore proper enforcement of security policies, organizations running Capsule must upgrade immediately to version 0.13.7 or later, where the sorting logic has been corrected to ensure consistency between storage and lookup operations. Until an upgrade is performed, administrators should audit their forbidden list configurations to ensure all metadata keys are strictly lowercase, thereby eliminating the discrepancy between case-insensitive and byte-order sorting that triggers the bug. Additionally, it is advisable to implement rigorous monitoring of admission webhook rejections and successes to detect any anomalous patterns that might indicate exploitation attempts while patching efforts are underway. This vulnerability aligns with CWE-20 Improper Input Validation as the system fails to correctly validate input against a defined set of constraints due to internal logic errors, and it relates to ATT&CK technique T1659 Misuse of Cloud Infrastructure Features by allowing tenants to manipulate cluster resources in ways that bypass intended isolation controls.