A tool advertised on a cybercriminal forum provides a stealthier method for carrying out fraud or malware attacks by allowing messages to be injected directly into the victim’s inbox. By slipping content in the normal email flow, the utility can help bypass protections that verify messages traveling to their destination mail server. Researchers at Gemini Advisory saw this utility advertised in October, from an actor that also provides a mass email service called GetMailer Pro The tool preys on credential stuffing where passwords are reused on multiple services.
Source: https://www.bleepingcomputer.com/news/security/new-tool-lets-attackers-easily-create-reply-chain-phishing-emails/