CVE-2026-61682: 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

# 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](https://github.com/5ud0er) / Tarmo Technologies.