{"id":"GHSA-c8w2-fgvx-vhv4","summary":"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","details":"# Summary\n\nThe 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.\n\n## Impact \n\nIn 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.\n\nBecause 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:\n\n- assert `X-Remote-Group: system:masters` and act as cluster super-user, bypassing the entire kcp authorizer chain in every workspace on the shard;\n- forge `authorization.kcp.io/warrant` to assume an arbitrary user/group identity via kcp's delegated-identity mechanism;\n- forge `authentication.kcp.io/scopes` to escape the cluster-scoping that confines service-account and impersonated identities to their origin workspace;\n- satisfy per-workspace required-group gating by injecting the required group.\n\nThe 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.\n\n\n# Patches\nFixed 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.\n\nOperators should upgrade to a patched release. No configuration changes are required after upgrading.\n\n# Workarounds\n\nThere 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.\n\nCredit to [5ud0er](https://github.com/5ud0er) / Tarmo Technologies.","aliases":["CVE-2026-61682"],"modified":"2026-09-18T17:30:11.696775983Z","published":"2026-09-18T17:15:49Z","database_specific":{"severity":"CRITICAL","github_reviewed":true,"github_reviewed_at":"2026-09-18T17:15:49Z","nvd_published_at":null,"cwe_ids":["CWE-290","CWE-302","CWE-348"]},"references":[{"type":"WEB","url":"https://github.com/kcp-dev/kcp/security/advisories/GHSA-c8w2-fgvx-vhv4"},{"type":"WEB","url":"https://github.com/kcp-dev/kcp/commit/7437cdcfec8f927d1a9bf1b2dd1e075d038e27ca"},{"type":"PACKAGE","url":"https://github.com/kcp-dev/kcp"},{"type":"WEB","url":"https://github.com/kcp-dev/kcp/releases/tag/v0.31.4"},{"type":"WEB","url":"https://github.com/kcp-dev/kcp/releases/tag/v0.32.2"}],"affected":[{"package":{"name":"github.com/kcp-dev/kcp","ecosystem":"Go","purl":"pkg:golang/github.com/kcp-dev/kcp"},"ranges":[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"0.31.4"}]}],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/09/GHSA-c8w2-fgvx-vhv4/GHSA-c8w2-fgvx-vhv4.json"}},{"package":{"name":"github.com/kcp-dev/kcp","ecosystem":"Go","purl":"pkg:golang/github.com/kcp-dev/kcp"},"ranges":[{"type":"SEMVER","events":[{"introduced":"0.32.0"},{"fixed":"0.32.2"}]}],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/09/GHSA-c8w2-fgvx-vhv4/GHSA-c8w2-fgvx-vhv4.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H"}]}