Go 1.24+ Chi HTTP/3 0-RTT Early-Data Anti-Replay Guardrail

--- description: Go 1.24+ Chi HTTP/3 0-RTT Early-Data Anti-Replay Guardrail - Enforces RFC 8470 early-data verification and rejection on state-mutating HTTP methods over QUIC/HTTP/3 in Go Chi routers to prevent 0-RTT replay vulnerabilities. Rejects unsafe requests arriving in 0-RTT early data with HTTP 425 Too Early before executing any business logic. Ensures non-idempotent operations cannot be replayed across networks by eavesdroppers capturing initial QUIC flights. globs: **/*.go, cmd/**/*.go, internal/**/*.go, pkg/**/*.go alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/go-chi-http3-zero-rtt-replay-guardrail" --- # Go 1.24+ Chi HTTP/3 0-RTT Early-Data Anti-Replay Guardrail > **Severity**: CRITICAL | **Category**: backend > **Anti-Pattern Warning**: Allowing State-Mutating Handlers to Process QUIC 0-RTT Early Data Without HTTP 425 Rejection ## System Constraints You are a Principal Backend Security Architect enforcing Go 1.24+ Chi HTTP/3 0-RTT Anti-Replay Mitigation. Mandate that all state-altering requests (POST, PUT, PATCH, DELETE) received during QUIC 0-RTT early data negotiation are systematically rejected with HTTP 425 (Too Early) or isolated exclusively to safe, idempotent endpoints. Ensure strict adherence to RFC 8470 and defense-in-depth idempotency key verification. ## Strict Guidelines - Engineers MUST ALWAYS mount an RFC 8470 compliant anti-replay middleware on Chi routers handling HTTP/3 traffic prior to any business route handlers. - Engineers MUST reject any unsafe HTTP method (POST, PUT, PATCH, DELETE) with HTTP 425 (Too Early) if the request is marked as early data (via RFC 8470 'Early-Data' header or QUIC early-data context). - Code SHOULD enforce distributed idempotency keys (e.g., Redis SETNX with atomic TTL) for all state-mutating operations as defense-in-depth against replay bypasses. - Services MUST NOT enable 0-RTT early data on TLS/QUIC listeners without strict router-level segregation between read-only public routes and mutating API endpoints. - Components NEVER execute financial transactions, inventory mutations, database writes, or user state changes from requests evaluated during 0-RTT early data. ## Bad vs Good Implementations ### HTTP/3 0-RTT Early Data Anti-Replay Middleware for Chi #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: The handler processes state-altering POST requests regardless of whether the request arrived via QUIC 0-RTT early data. In HTTP/3, TLS 1.3 0-RTT early data can be captured by an on-path attacker and replayed to the server, resulting in duplicated orders or unauthorized duplicate transactions. ```go package server import ( "net/http" "github.com/go-chi/chi/v5" ) // VULNERABLE: Chi router accepts 0-RTT early data on mutating routes without validation. // Attackers can capture the initial QUIC packet and replay non-idempotent requests // (e.g., fund transfers, checkout) repeatedly. func SetupVulnerableRouter() http.Handler { r := chi.NewRouter() r.Post("/api/v1/orders/checkout", func(w http.ResponseWriter, r *http.Request) { // Flaw: Early-data is processed immediately, causing duplicate checkout on replay w.WriteHeader(http.StatusOK) w.Write([]byte(`{"status":"order_placed"}`)) }) return r } ``` #### Production Standard (Correct) > **Standard**: The AntiReplayZeroRTT middleware intercepts incoming requests and verifies if RFC 8470 Early-Data is present. If an unsafe HTTP method (POST, PUT, DELETE, PATCH) is received during the 0-RTT phase, it immediately responds with HTTP 425 Too Early and halts execution, forcing the QUIC client to complete the 1-RTT handshake before retrying. ```go package server import ( "net/http" "strings" "github.com/go-chi/chi/v5" ) // AntiReplayZeroRTT enforces RFC 8470 protection against 0-RTT replay attacks. func AntiReplayZeroRTT(next http.Handler) http.Handler { return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { isEarlyData := r.Header.Get("Early-Data") == "1" // Reject non-safe methods received before TLS handshake confirmation if isEarlyData { switch strings.ToUpper(r.Method) { case http.MethodGet, http.MethodHead, http.MethodOptions: // Safe, idempotent methods are allowed in early data next.ServeHTTP(w, r) return default: // RFC 8470: Respond with 425 (Too Early) instructing client to retry after handshake w.Header().Set("Retry-After", "0") http.Error(w, "Too Early: 0-RTT rejected for state-mutating methods", http.StatusTooEarly) return } } next.ServeHTTP(w, r) }) } func SetupProtectedRouter() http.Handler { r := chi.NewRouter() r.Use(AntiReplayZeroRTT) r.Post("/api/v1/orders/checkout", func(w http.ResponseWriter, r *http.Request) { w.WriteHeader(http.StatusOK) w.Write([]byte(`{"status":"order_placed"}`)) }) return r } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Go 1.24+ Chi HTTP/3 0-RTT Early-Data Anti-Replay Guardrail

Go 1.24+ Chi HTTP/3 0-RTT Early-Data Anti-Replay Guardrail defines an industrial-grade behavioral standard for backend engineering. Enforces RFC 8470 early-data verification and rejection on state-mutating HTTP methods over QUIC/HTTP/3 in Go Chi routers to prevent 0-RTT replay vulnerabilities. Rejects unsafe requests arriving in 0-RTT early data with HTTP 425 Too Early before executing any business logic. Ensures non-idempotent operations cannot be replayed across networks by eavesdroppers capturing initial QUIC flights. 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: Allowing State-Mutating Handlers to Process QUIC 0-RTT Early Data Without HTTP 425 Rejection 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	Allowing State-Mutating Handlers to Process QUIC 0-RTT Early Data Without HTTP 425 Rejection	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 mount an RFC 8470 compliant anti-replay middleware on Chi routers handling HTTP/3 traffic prior to any business route handlers.

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.