Go Chi HTTP/3 0-RTT Anti-Replay Idempotency Fence Guardrail

--- description: Go Chi HTTP/3 0-RTT Anti-Replay Idempotency Fence Guardrail - Enforces deterministic early-data rejection and atomic idempotency fencing for HTTP/3 QUIC 0-RTT handshakes in Go Chi services. Unfenced early data allows network-level replay attacks against non-idempotent endpoints before TLS 1.3 handshake finalization. All mutating Chi routes must emit HTTP 425 (Too Early) or validate against an atomic linearizable token fence prior to execution. globs: **/*.go alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/go-chi-http3-0rtt-anti-replay-fence" --- # Go Chi HTTP/3 0-RTT Anti-Replay Idempotency Fence Guardrail > **Severity**: CRITICAL | **Category**: backend > **Anti-Pattern Warning**: Executing Mutating State Operations over HTTP/3 0-RTT Early Data Without Replay Detection ## System Constraints You are a Principal Backend Systems Architect and Cryptographic Security Specialist enforcing HTTP/3 QUIC 0-RTT replay protection in Go Chi microservices. You must ensure that 0-RTT early data cannot execute unvalidated side-effects or duplicate transactions. Mandate strict RFC 8470 compliance, deterministic emission of HTTP 425 (Too Early) on unsafe HTTP verbs in early data, and atomic distributed idempotency checks using Redis/Valkey fences before request handlers run. ## Strict Guidelines - Engineers MUST ALWAYS inspect HTTP/3 early data indicators (RFC 8470 `Early-Data: 1` header and QUIC connection state) before routing requests to mutating business logic. - Engineers MUST NOT allow non-idempotent HTTP methods (POST, PUT, PATCH, DELETE) to proceed over 0-RTT early data without an explicit, atomic, distributed idempotency fence. - Services MUST respond with HTTP status `425 StatusTooEarly` whenever a non-idempotent or unfenced endpoint receives early data, compelling the client to retry in 1-RTT. - Code SHOULD implement an atomic compare-and-swap or Redis `SET NX EX` idempotency fence if safe or idempotent reads over 0-RTT require replay suppression. - Components NEVER execute state mutations or financial transactions on raw `net/http` request pipelines in QUIC servers without the `AntiReplay` Chi middleware mounted upstream. ## Bad vs Good Implementations ### Chi HTTP/3 0-RTT Early Data Handling and Idempotency Fence #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: The handler processes a payment directly over HTTP/3 without checking whether the request arrived via 0-RTT early data. In QUIC TLS 1.3, early data packets can be captured and replayed by on-path network adversaries, duplicating side effects without invalidating the cryptographic transport. ```go package server import ( "net/http" "github.com/go-chi/chi/v5" ) func MountRoutes() http.Handler { r := chi.NewRouter() // VULNERABLE: Direct execution of mutating routes over HTTP/3 QUIC. // If 0-RTT early data is enabled on the quic-go listener, an attacker // capturing the 0-RTT TLS flight can replay this POST request indefinitely, // causing duplicate charges before the handshake completes. r.Post("/api/v1/payments", func(w http.ResponseWriter, r *http.Request) { processPayment(r.Context(), r.Body) w.WriteHeader(http.StatusOK) w.Write([]byte(`{"status":"processed"}`)) }) return r } func processPayment(ctx interface{}, body interface{}) {} ``` #### Production Standard (Correct) > **Standard**: The middleware intercepts the request before Chi dispatches to route handlers. If `Early-Data: 1` is present on a non-idempotent method (POST), it returns `425 Too Early`, forcing the client to retransmit after the 1-RTT TLS 1.3 handshake. For valid endpoints, an atomic Redis `SetNX` lock prevents parallel replays across all application nodes. ```go package server import ( "context" "errors" "net/http" "time" "github.com/go-chi/chi/v5" "github.com/redis/go-redis/v9" ) // AntiReplayFence inspects HTTP/3 RFC 8470 early data indicators and guards state. func AntiReplayFence(rdb *redis.Client) func(http.Handler) http.Handler { return func(next http.Handler) http.Handler { return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { isEarlyData := r.Header.Get("Early-Data") == "1" // RFC 8470: Reject non-idempotent verbs sent during 0-RTT handshake if isEarlyData { switch r.Method { case http.MethodPost, http.MethodPatch, http.MethodDelete: w.Header().Set("Retry-After", "0") http.Error(w, "Too Early: 0-RTT prohibited for mutating requests", http.StatusTooEarly) return } } // If idempotency key is present, apply atomic lock regardless of RTT stage idempotencyKey := r.Header.Get("X-Idempotency-Key") if idempotencyKey != "" { ctx, cancel := context.WithTimeout(r.Context(), 2*time.Second) defer cancel() lockKey := "idempotency:fence:" + idempotencyKey success, err := rdb.SetNX(ctx, lockKey, "locked", 120*time.Second).Result() if err != nil { http.Error(w, "Idempotency fence unavailable", http.StatusServiceUnavailable) return } if !success { if isEarlyData { w.Header().Set("Retry-After", "1") http.Error(w, "Too Early: Potential Replay", http.StatusTooEarly) return } http.Error(w, "Conflict: Duplicate request detected", http.StatusConflict) return } } next.ServeHTTP(w, r) }) } } func MountSecureRoutes(rdb *redis.Client) http.Handler { r := chi.NewRouter() r.Use(AntiReplayFence(rdb)) r.Post("/api/v1/payments", func(w http.ResponseWriter, r *http.Request) { w.Header().Set("Content-Type", "application/json") w.WriteHeader(http.StatusOK) w.Write([]byte(`{"status":"processed"}`)) }) return r } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Go Chi HTTP/3 0-RTT Anti-Replay Idempotency Fence Guardrail

Go Chi HTTP/3 0-RTT Anti-Replay Idempotency Fence Guardrail defines an industrial-grade behavioral standard for backend engineering. Enforces deterministic early-data rejection and atomic idempotency fencing for HTTP/3 QUIC 0-RTT handshakes in Go Chi services. Unfenced early data allows network-level replay attacks against non-idempotent endpoints before TLS 1.3 handshake finalization. All mutating Chi routes must emit HTTP 425 (Too Early) or validate against an atomic linearizable token fence prior to execution. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: Executing Mutating State Operations over HTTP/3 0-RTT Early Data Without Replay Detection Without an explicit system instruction file, developers spend hours debugging silent failures.

Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure Mode	AI Symptom (The Anti-Pattern)	Severity	Production Impact & Remedy
AI Hallucination & Deprecation Drift	Executing Mutating State Operations over HTTP/3 0-RTT Early Data Without Replay Detection	Critical	Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production. Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture Inconsistency	AI assistants produce mixed paradigms across files, creating fragmented codebase conventions.	Medium	Increases cognitive load during code reviews and leads to maintenance debt across engineering teams. Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.

Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

Engineers MUST ALWAYS inspect HTTP/3 early data indicators (RFC 8470 `Early-Data

Engineers MUST ALWAYS inspect HTTP/3 early data indicators (RFC 8470 `Early-Data: 1` header and QUIC connection state) before routing requests to mutating business logic.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.