iPerf3 before 3.17, when used with OpenSSL before 3.2.0 as a server with RSA authentication, allows a timing side channel in RSA decryption operations. This side channel could be sufficient for an attacker to recover credential plaintext. It requires the attacker to send a large number of messages for decryption, as described in "Everlasting ROBOT: the Marvin Attack" by Hubert Kario.
Covert timing channels convey information by modulating some aspect of system behavior over time, so that the program receiving the information can observe system behavior and infer protected information.