{"id":"CVE-2026-61682","aliases":["GHSA-c8w2-fgvx-vhv4","GO-2026-6517"],"url":"https://o3.security/vulnerability/CVE-2026-61682","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.","published":"2026-09-18T16:11:46.443Z","modified":"2026-10-28T02:30:57.205245604Z","cvss":{"score":9.9,"severity":"CRITICAL","vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H"},"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"Go","name":"github.com/kcp-dev/kcp","fixedVersion":"0.31.4"},{"ecosystem":"Go","name":"github.com/kcp-dev/kcp","fixedVersion":"0.32.2"}],"fix":{"url":"https://github.com/kcp-dev/kcp/commit/7437cdcfec8f927d1a9bf1b2dd1e075d038e27ca","label":"kcp-dev/kcp@7437cdc"},"references":[{"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"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/61xxx/CVE-2026-61682.json"},{"type":"ADVISORY","url":"https://github.com/kcp-dev/kcp/security/advisories/GHSA-c8w2-fgvx-vhv4"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-61682"},{"type":"FIX","url":"https://github.com/kcp-dev/kcp/commit/7437cdcfec8f927d1a9bf1b2dd1e075d038e27ca"},{"type":"FIX","url":"https://github.com/kcp-dev/kcp/commit/f913ee890fb2fd9fa78e50c43474b078bfb6aeff"},{"type":"PACKAGE","url":"https://github.com/kcp-dev/kcp"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T02:30:57.205245604Z"}}