XTLS/Xray-core · error

replay detected

Error message

replay detected

What it means

Error "replay detected" thrown in XTLS/Xray-core.

Source

Thrown at proxy/vless/encryption/server.go:224

		ticket, err := nfsAEAD.Open(nil, nil, encryptedTicket, nil)
		if err != nil {
			return nil, err
		}
		i.RWLock.RLock()
		s := i.Sessions[[16]byte(ticket)]
		i.RWLock.RUnlock()
		if s == nil {
			noises := make([]byte, crypto.RandBetween(1279, 2279)) // matches 1-RTT's server hello length for "random", though it is not important, just for example
			var err error
			for err == nil {
				rand.Read(noises)
				_, err = DecodeHeader(noises)
			}
			conn.Write(noises) // make client do new handshake
			return nil, errors.New("expired ticket")
		}
		if _, loaded := s.NfsKeys.LoadOrStore([32]byte(nfsKey), true); loaded { // prevents bad client also
			return nil, errors.New("replay detected")
		}
		c.UnitedKey = append(s.PfsKey, nfsKey...) // the same nfsKey links the upload & download (prevents server -> client's another request)
		c.PreWrite = make([]byte, 16)
		rand.Read(c.PreWrite) // always trust yourself, not the client (also prevents being parsed as TLS thus causing false interruption for "native" and "xorpub")
		c.AEAD = NewAEAD(c.PreWrite, c.UnitedKey, c.UseAES)
		c.PeerAEAD = NewAEAD(encryptedTicket, c.UnitedKey, c.UseAES) // unchangeable ctx (prevents server -> server), and different ctx length for upload / download (prevents client -> client)
		if i.XorMode == 2 {
			c.Conn = NewXorConn(conn, NewCTR(c.UnitedKey, c.PreWrite), NewCTR(c.UnitedKey, iv), 16, 0) // it doesn't matter if the attacker sends client's iv back to the client
		}
		return c, nil
	}

	if length < 1184+32+16 { // client may send more public keys in the future's version
		return nil, errors.New("too short length")
	}
	encryptedPfsPublicKey := make([]byte, length)
	if _, err := io.ReadFull(conn, encryptedPfsPublicKey); err != nil {
		return nil, err

View on GitHub (pinned to 7d214f8b09)

When it happens

Trigger: Thrown at proxy/vless/encryption/server.go:224 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of XTLS/Xray-core@7d214f8b09 (2026-08-15). Data as JSON: /api/errors/68a55877b379b5c5. Report an issue: GitHub.