TLS 1.3 offers a feature called 0-RTT (zero round trip time) Resumption mode, in an effort to enhance performance.
When a browser successfully completes a TLS handshake with a server for the first time, both the client and the server can store a pre-shared encryption key locally. This is known as the resumption master secret.
If the browser establishes a connection with the server again at a later time, it can use this resumption key to send encrypted application data in its first message to the server, without having to perform the handshake a second time.
However, 0-RTT resumption has a caveat; resumption data require no interaction from the server, which means that an attacker can capture encrypted 0-RTT data and re-send them to the server, or replay them.
In the case the server is misconfigured, it can potentially accept replayed requests as valid; essentially, allowing the attackers to perform unsanctioned actions.
The solution to this issue is ensuring that all 0-RTT requests are idempotent.
Idempotent requests can be safely used as 0-RTT requests, since replaying them will have no effect. A quick rule of thumb would be to only use GET requests with 0-RTT resumption.