My six year-old WPF executable is suddenly being detected as MSIL/AgentTesla.FU!MTB by Windows Defender
Solution 1:
Get a second opinion by scanning it with other engines, such as with VirusTotal.
Upload it for analysis by Microsoft security intelligence. Microsoft has an interest in keeping you both as a developer and Windows user.
This specific variant was cataloged by Microsoft in January 2021. AgentTesla is a remote access trojan that has evolved, so unfortunately it seems your thing got flagged as a false positive in a recent detection update.