While most phishing campaigns are fairly simplistic in nature and easy to spot (they usually involve a legitimate-looking email, often with a malicious attachment or link embedded in the text), a spam campaign we observed in September indicates attackers are angling towards a more sophisticated form of phishing. The campaign uses hijacked email accounts to send malware as part of or as a response to an existing email thread

https://blog.trendmicro.com/trendlabs-security-intelligence/phishing-campaign-uses-hijacked-emails-to-deliver-ursnif-by-replying-to-ongoing-threads/