Microsoft is warning Azure cloud users that a Chinese controlled botnet is engaging in “highly evasive” password spraying. Not sure about the “highly evasive” part; the techniques seem basically what you get in a distributed password-guessing attack:

“Any threat actor using the CovertNetwork-1658 infrastructure could conduct password spraying campaigns at a larger scale and greatly increase the likelihood of successful credential compromise and initial access to multiple organizations in a short amount of time,” Microsoft officials wrote. “This scale, combined with quick operational turnover of compromised credentials between CovertNetwork-1658 and Chinese threat actors, allows for the potential of account compromises across multiple sectors and geographic regions.”

Some of the characteristics that make detection difficult are:

The use of compromised SOHO IP addresses
The use of a rotating set of IP addresses at any given time. The threat actors had thousands of available IP addresses at their disposal. The average uptime for a CovertNetwork-1658 node is approximately 90 days.
The low-volume password spray process; for example, monitoring for multiple failed sign-in attempts from one IP address or to one account will not detect this activity.