The TLS 1.3 0-RTT vulnerability: Why blindly enabling zero-round-trip session resumption exposes state-modifying APIs to duplicate financial transactions.

One of the headline performance features of TLS 1.3 is **0-RTT (Zero Round Trip Time) Resumption**, often marketed as "Early Data". When a returning client reconnects to your server, it uses a previously negotiated session ticket to transmit HTTP request data alongside the initial TLS Client Hello—achieving zero handshake latency and dramatically speeding up mobile connections.

However, enabling 0-RTT without strict Layer-7 application awareness introduces a critical security flaw: **Early Data lacks forward secrecy and is vulnerable to replay attacks**.

1. The Replay Attack Vector: Because early data is encrypted under a static resumption key derived from the previous session, a passive on-path network eavesdropper (in a coffee shop Wi-Fi or compromised transit router) can capture the raw 0-RTT packet and re-transmit it to your server minutes or hours later. 2. Duplicate Non-Idempotent Actions: If the replayed request is a non-idempotent API call—such as ``POST /api/v1/orders/checkout``, ``POST /api/v1/transfers``, or ``POST /api/v1/votes``—your server decrypts and processes the request a second time. Customers are double-billed, account balances are debited twice, or state is corrupted. 3. Standard TLS Defenses Are Bypassed: Standard TLS sequence numbers do not protect 0-RTT data because each replayed packet appears to the server as a new, legitimate handshake resumption.

Hardening TLS 1.3 ingress requires enforcing strict anti-replay boundaries at the reverse proxy layer: • Restrict 0-RTT Strictly to Idempotent Verbs: In HAProxy, Nginx, or Caddy, configure edge rules to reject early data for non-safe HTTP methods. If an incoming 0-RTT request contains ``POST``, ``PUT``, ``PATCH``, or ``DELETE``, the proxy must delay processing until the full 1-RTT handshake completes. • Enforce Anti-Replay Caches: Enable edge anti-replay mechanisms (such as HAProxy ``ssl-max-record`` and ticket replay filters) that record client ticket identifiers and reject duplicated resumption handshakes within the ticket validity window. • Mandate Idempotency Keys in Application APIs: Require clients to include a unique UUID ``Idempotency-Key`` header on all state-modifying requests, deduplicating transactions in Redis before executing business logic. • Conservative Session Ticket Lifetimes: Limit session ticket validity (``ssl-default-bind-options ssl-min-ver TLSv1.3`` and short ticket lifetimes) to shrink the replay window.

Enjoy the speed of modern TLS 1.3 without exposing your financial and transactional endpoints to replay exploits.

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