CVE-2026-61682

kcp is a Kubernetes-like control plane for form-factors and use-cases beyond Kubernetes and container workloads. Prior to 0.31.4 and 0.32.2, the kcp front-proxy does not remove inbound X-Remote-User, X-Remote-Group, or X-Remote-Extra-* identity headers before forwarding requests to shards. Any authenticated tenant can inject X-Remote-Group: system:masters, authorization.kcp.io/warrant, authentication.kcp.io/scopes, or a group used for per-workspace required-group gating, and the shard trusts these values as authenticated identity assertions. This allows cross-workspace impersonation, authorization bypass, and arbitrary reading, writing, or deletion of resources, secrets, RBAC data, APIExports, APIBindings, and LogicalClusters. This issue is fixed in versions 0.31.4 and 0.32.2.

kcp is a Kubernetes-like control plane for form-factors and use-cases beyond Kubernetes and container workloads. Prior to 0.31.4 and 0.32.2, the kcp front-proxy does not remove inbound X-Remote-User, X-Remote-Group, or X-Remote-Extra-* identity headers before forwarding requests to shards. Any authenticated tenant can inject X-Remote-Group: system:masters, authorization.kcp.io/warrant, authentication.kcp.io/scopes, or a group used for per-workspace required-group gating, and the shard trusts these values as authenticated identity assertions. This allows cross-workspace impersonation, authorization bypass, and arbitrary reading, writing, or deletion of resources, secrets, RBAC data, APIExports, APIBindings, and LogicalClusters. This issue is fixed in versions 0.31.4 and 0.32.2.

Summary

The kcp front-proxy fails to strip client-supplied identity headers before forwarding requests to shards. Any authenticated tenant can inject their own X-Remote-Group and X-Remote-Extra-* headers, which the shard trusts as a verified identity assertion — allowing a low-privilege user to escalate to cluster administrator (system:masters) and read, write, or delete resources in any workspace on the shard. This is a complete multi-tenant isolation and authorization bypass.

Impact

In a sharded kcp deployment, external clients reach shards through the front-proxy, which authenticates the client and then forwards the resulting identity to the shard using Kubernetes request-header authentication (X-Remote-User / X-Remote-Group / X-Remote-Extra-*). The shard trusts these headers because they arrive over the front-proxy's mutually-authenticated connection.

Because the front-proxy appended its identity headers instead of replacing them — and never removed any copies the client sent — an authenticated attacker could smuggle forged identity headers through to the shard. With this, an attacker holding any ordinary credential (client certificate, OIDC token, or service-account token) and no special privileges could:

• assert X-Remote-Group: system:masters and act as cluster super-user, bypassing the entire kcp authorizer chain in every workspace on the shard;
• forge authorization.kcp.io/warrant to assume an arbitrary user/group identity via kcp's delegated-identity mechanism;
• forge authentication.kcp.io/scopes to escape the cluster-scoping that confines service-account and impersonated identities to their origin workspace;
• satisfy per-workspace required-group gating by injecting the required group.

The result is arbitrary read/write/delete access to any tenant's resources, secrets, RBAC, APIExports/APIBindings, and LogicalClusters — a cross-workspace access break and authorizer bypass across the proxy's trust boundary.

Patches

Fixed in v0.31.4, 0.32.2. The front-proxy and the shard's in-process local-proxy now unconditionally remove any inbound X-Remote-* identity headers before stamping the authenticated identity, so no client-supplied value can be forwarded to a shard.

Operators should upgrade to a patched release. No configuration changes are required after upgrading.

Workarounds

There is no complete workaround other than upgrading. Deployments that terminate client connections at an external proxy capable of stripping X-Remote-User, X-Remote-Group, and all X-Remote-Extra-* headers from inbound requests before they reach the kcp front-proxy can mitigate exposure in the interim.

Credit to 5ud0er / Tarmo Technologies.

kcp is a Kubernetes-like control plane for form-factors and use-cases beyond Kubernetes and container workloads. Prior to 0.31.4 and 0.32.2, the kcp front-proxy does not remove inbound X-Remote-User, X-Remote-Group, or X-Remote-Extra-* identity headers before forwarding requests to shards. Any authenticated tenant can inject X-Remote-Group: system:masters, authorization.kcp.io/warrant, authentication.kcp.io/scopes, or a group used for per-workspace required-group gating, and the shard trusts these values as authenticated identity assertions. This allows cross-workspace impersonation, authorization bypass, and arbitrary reading, writing, or deletion of resources, secrets, RBAC data, APIExports, APIBindings, and LogicalClusters. This issue is fixed in versions 0.31.4 and 0.32.2.

kcp is a Kubernetes-like control plane for form-factors and use-cases beyond Kubernetes and container workloads. Prior to 0.31.4 and 0.32.2, the kcp front-proxy does not remove inbound X-Remote-User, X-Remote-Group, or X-Remote-Extra-* identity headers before forwarding requests to shards. Any authenticated tenant can inject X-Remote-Group: system:masters, authorization.kcp.io/warrant, authentication.kcp.io/scopes, or a group used for per-workspace required-group gating, and the shard trusts these values as authenticated identity assertions. This allows cross-workspace impersonation, authorization bypass, and arbitrary reading, writing, or deletion of resources, secrets, RBAC data, APIExports, APIBindings, and LogicalClusters. This issue is fixed in versions 0.31.4 and 0.32.2.

Summary

The kcp front-proxy fails to strip client-supplied identity headers before forwarding requests to shards. Any authenticated tenant can inject their own X-Remote-Group and X-Remote-Extra-* headers, which the shard trusts as a verified identity assertion — allowing a low-privilege user to escalate to cluster administrator (system:masters) and read, write, or delete resources in any workspace on the shard. This is a complete multi-tenant isolation and authorization bypass.

Impact

In a sharded kcp deployment, external clients reach shards through the front-proxy, which authenticates the client and then forwards the resulting identity to the shard using Kubernetes request-header authentication (X-Remote-User / X-Remote-Group / X-Remote-Extra-*). The shard trusts these headers because they arrive over the front-proxy's mutually-authenticated connection.

Because the front-proxy appended its identity headers instead of replacing them — and never removed any copies the client sent — an authenticated attacker could smuggle forged identity headers through to the shard. With this, an attacker holding any ordinary credential (client certificate, OIDC token, or service-account token) and no special privileges could:

• assert X-Remote-Group: system:masters and act as cluster super-user, bypassing the entire kcp authorizer chain in every workspace on the shard;
• forge authorization.kcp.io/warrant to assume an arbitrary user/group identity via kcp's delegated-identity mechanism;
• forge authentication.kcp.io/scopes to escape the cluster-scoping that confines service-account and impersonated identities to their origin workspace;
• satisfy per-workspace required-group gating by injecting the required group.

The result is arbitrary read/write/delete access to any tenant's resources, secrets, RBAC, APIExports/APIBindings, and LogicalClusters — a cross-workspace access break and authorizer bypass across the proxy's trust boundary.

Patches

Fixed in v0.31.4, 0.32.2. The front-proxy and the shard's in-process local-proxy now unconditionally remove any inbound X-Remote-* identity headers before stamping the authenticated identity, so no client-supplied value can be forwarded to a shard.

Operators should upgrade to a patched release. No configuration changes are required after upgrading.

Workarounds

There is no complete workaround other than upgrading. Deployments that terminate client connections at an external proxy capable of stripping X-Remote-User, X-Remote-Group, and all X-Remote-Extra-* headers from inbound requests before they reach the kcp front-proxy can mitigate exposure in the interim.

Credit to 5ud0er / Tarmo Technologies.

kcp front-proxy does not strip inbound X-Remote-* identity headers, allowing any authenticated client to inject groups/warrants and impersonate system:masters in any workspace in github.com/kcp-dev/kcp