I have received the exact same message today. I've downloaded the .iso again to a separate Ubuntu machine and verified the checksum:
$ echo "b9beac143e36226aa8a0b03fc1cbb5921cff80123866e718aaeba4edb81cfa63 *ubuntu-18.04.3-live-server-amd64.iso" | shasum -a 256 --check
After that, I've extracted the file in question (raid456.ko) and uploaded to virustotal.com:
It was last analyzed a month ago and it was fine. I've reanalyzed it again and it seems that now Microsoft is the only one detecting this as Trojan:Win64/Longage:
I would say new Microsoft Defender signatures triggered a false positive here. Even in the very unlikely event that Ubuntu would have embedded a trojan in .iso, Windows machine itself does not / should not execute Linux (ELF) binaries and there's nothing to worry about on Windows side. However, if that were the case we would, of course, have a whole lot bigger issue to worry about.
I have submitted this file to Microsoft and flagged it as false positive, using this link: https://www.microsoft.com/en-us/wdsi/filesubmission
I'll update this answer when/if I receive a response from Microsoft analyst.
UPDATE: No response from Microsoft yet, but their engine no longer detects this. TrendMicro does now though. The likelihood that this is a false positive is extremely high.
UPDATE 2: I have also submitted the file to TrendMicro yesterday (no reply yet - will not follow up). I consider this case closed. Reply from Microsoft:
We have removed the detection. Please follow the steps below to clear cached detection and obtain the latest malware definitions.
- Open command prompt as administrator and change directory to c:\Program Files\Windows Defender
- Run “MpCmdRun.exe -removedefinitions -dynamicsignatures”
- Run "MpCmdRun.exe -SignatureUpdate"
Alternatively, the latest definition is available for download here: https://www.microsoft.com/en-us/wdsi/definitions
Thank you for contacting Microsoft.