The Lucky13 article on Our Results;
... attacks can be mounted by a standard man-in-the-middle (MITM) attacker who sees only ciphertext and can inject ciphertexts of his own composition into the network.
Definition of passive and active MTIM from sll.com
Passive MITM: In a passive MITM attack attackers ‘tap’ the communication, capturing information in transit without changing it.
Active MITM: If attackers attempt to modify or tamper with the information itself they are committing an active MITM attack. A very common way to launch a MITM attack is by creating a fake node on a publicly-available computer network, such as a coffee shop's WiFi network.
Injection is an attempt to modify therefore it is an active MITM attack.
on the discussion;
We reiterate that the attacks are ciphertext-only, and so can be carried out by the standard MITM attacker, without a chosen-plaintext capability.
Chosen plaintext attack means; the attacker obtains encryption of plaintexts of his choice. So, in Luck13, the attacker only used ciphertext that they can obtain and injected them into the network.