A downgrade attack is a type of man-in-the-middle attack that forces two hosts to use an older or weaker version of the TLS protocol or its parameters. The attacker does this by replacing or deleting the STARTTLS command or exploiting the compatibility features of the protocol. The purpose of the attack is to create a pathway for enabling a cryptographic attack that would not be possible in case of a connection that is encrypted over the latest version of TLS protocol. The IOC shows that most client connections are renegotiated after establishing the connections, which could indicate that an entity is performing downgrade attacks on path by interfering with the initial handshake and making the client and server agree on a lower version of TLS or a weaker cipher suite. Verified References:
* https://en.wikipedia.org/wiki/Downgrade_attack
* https://crypto.stackexchange.com/questions/10493/why-is-tls-susceptible-to-protocol-downgrade-attacks
* https://venafi.com/blog/preventing-downgrade-attacks/