Frost-k8s: A FROST-Based Threshold Signing Proxy for Kubernetes Service Account Tokens

Kubernetes service account token signing relies on a single private key, creating a critical single point of failure: one compromise enables forgery of any token cluster-wide. We present frost-k8s, the first open-source implementation—to our knowledge—combining the FROST threshold signature protocol (RFC 9591) with the Kubernetes ExternalJWTSigner gRPC interface (KEP-740, stable April 2026). Our prototype requires zero modifications to Kubernetes core and produces standard JWTs accepted by all existing clients. During implementation we identified and characterized two previously undescribed engineering defects in KEP-740 integration: a payload encoding discrepancy and a signature format incompatibility between Go's crypto/ecdsa and the go-jose verification library. Both defects cause silent verification failures that are not surfaced as errors by the Kubernetes apiserver. On a single-node Minikube environment, warm-path signing latency overhead is ~3% at sequential load, rising to ~167% at 50 concurrent requests due to sequential signer communication. The prototype tolerates two of five signer failures and recovers in 84ms after signer restart. We characterize six prototype limitations and identify distributed key generation as the primary remaining research challenge.